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Sampi

Sampi (modern: ϡ; ancient shapes:,  ) is an archaic letter of the Greek alphabet. It was used as an addition to the classical 24-letter alphabet in some eastern Ionic dialects of ancient Greek in the 6th and 5th centuries BC, to denote some type of a sibilant sound, probably or, and was abandoned when the sound disappeared from Greek.

It later remained in use as a numeral symbol for 900 in the alphabetic ("Milesian") system of Greek numerals. Its modern shape, which resembles a π inclining to the right with a longish curved cross-stroke, developed during its use as a numeric symbol in minuscule handwriting of the Byzantine era.

Its current name, sampi, originally probably meant "san pi", i.e. "like a pi", and is also of medieval origin. The letter's original name in antiquity is not known. It has been proposed that sampi was a continuation of the archaic letter san, which was originally shaped like an M and denoted the sound in some other dialects. Besides san, names that have been proposed for sampi include parakyisma and angma, while other historically attested terms for it are enacosis, sincope, and o charaktir.

Alphabetic sampi
As an alphabetic letter denoting a sibilant sound, sampi (shaped ) was mostly used between the middle of the 6th and the middle of the 5th centuries BC, although some attestations have been dated as early as the 7th century BC. It has been attested in the cities of Miletus, Ephesos, Halikarnassos, Erythrae, Teos (all situated in the region of Ionia in Asia Minor), in the island of Samos, in the Ionian colony of Massilia, and in Kyzikos (situated farther north in Asia Minor, in the region of Mysia). In addition, in the city of Pontic Mesembria, on the Black Sea coast of Thrace, it was used on coins, which were marked with the abbreviation of the city's name, spelled "ΜΕͲΑ".

Sampi occurs in positions where other dialects, including written Ionic, normally have double sigma (σσ), i.e. a long sound. Some other dialects, particularly Attic Greek, have ττ (long ) in the same words (e.g. θάλασσα vs. θάλαττα 'sea', or τέσσαρες vs. τέτταρες 'four'). The sounds in question are all reflexes of the proto-Greek consonant clusters or. It is therefore believed that the local letter sampi was used to denote some kind of intermediate sound during the phonetic change from the earlier plosive clusters towards the later sound, possibly an affricate, forming a triplet with the Greek letters for  and.

Among the earliest known uses of sampi in this function is an abecedarium from Samos dated to the mid-7th century BC. This early attestation already bears witness to its alphabetic position behind omega (i.e. not the position of san), and it shows that its invention cannot have been much later than that of omega itself.

The first known use of alphabetic sampi in writing native Greek words is an inscription found on a silver plate in Ephesus, which has the words "τέͳαρες" ("four") and "τεͳαράϙοντα" ("forty") spelled with sampi (cf. normal spelling Ionic "τέσσαρες/τεσσαράκοντα" vs. Attic τέτταρες/τετταράκοντα). It can be dated between the late 7th century and mid 6th century BC. An inscription from Halicarnassus has the names "Ἁλικαρναͳέ[ω]ν" ("of the Halicarnassians") and the personal names "Ὀαͳαͳιος" and "Π[α]νυάͳιος". All of these names appear to be of non-Greek, local origin, i.e. Carian. On a late 6th century bronze plate from Miletus dedicated to the sanctuary of Athena at Assesos, the spelling "τῇ Ἀθηνάηι τῇ Ἀͳησίηι" ("to Athena of Assessos") has been identified. This is currently the first known instance of alphabetic sampi in Miletus itself, commonly assumed to be the birthplace of the numeral system and thus of the later numeric use of sampi.

It has been suggested that there may be an isolated example of the use of alphabetic sampi in Athens. In a famous painted black figure amphora from c.615 BC, known as the "Nessos amphora", the inscribed name of the eponymous centaur Nessus is rendered in the irregular spelling "ΝΕΤΟΣ" (Νέτος). The expected regular form of the name would have been either Attic "Νέττος" – with a double "τ" – or Ionic "Νέσσος". Traces of corrections that are still visible underneath the painted "Τ" have led to the conjecture that the painter originally wrote Νέͳος, with sampi for the σσ/ττ sound.

Pamphylian sampi
A letter similar to Ionian sampi, but of unknown historical relation with it, existed in the highly deviant local dialect of Pamphylia in southern Asia Minor. It was shaped like. According to Brixhe it probably stood for the sounds, , or. It is found in a few inscriptions in the cities of Aspendos and Perge as well as on local coins. For instance, an inscription from Perge dated to around 400 BC reads: ͶανάGreek Sampi Pamphylian.svgαι Πρειίαι Κλεμύτας Λϝαράμυ Ͷασιρϝο̄τας ἀνέθε̄κε (="Vanássāi Preiíāi Klemútas Lwarámu Vasirwōtas anéthēke", "Klemutas the vasirwotas, son of Lwaramus, dedicated this to the Queen of Perge"). The same title "Queen of Perge", the local title for the goddess Artemis, is found on coin legends: ͶανάGreek Sampi Pamphylian.svgας Πρειιας. As ͶανάGreek Sampi Pamphylian.svgα is known to be the local feminine form of the archaic Greek noun ἄναξ/ϝάναξ, i.e. (w)anax ("king"), it is believed that the letter stood for some type of sibilant reflecting Proto-Greek.

Numeric sampi
In the alphabetic numeral system, which was probably invented in Miletus and is therefore sometimes called the "Milesian" system, there are 27 numeral signs: the first nine letters of the alphabet, from alpha (A) to theta (Θ) stand for the digits 1–9; the next nine, beginning with iota (Ι), stand for the multiples of ten (10, 20, etc. up to 90); and the last nine, beginning with rho (Ρ), stand for the hundreds (100 – 900). For this purpose, the 24 letters of the standard classical Greek alphabet were used with the addition of three archaic or local letters: digamma/wau (Ϝ,, originally denoting the sound ) for "6", koppa (Ϙ, originally denoting the sound ) for "90", and sampi for "900". While digamma and koppa were retained in their original alphabetic positions inherited from Phoenician, the third archaic Phoenician character, san/tsade (Ϻ, denoting an sound), was not used in this way. Instead, sampi was chosen, and added at the end of the system, after omega (800).

From this, it has been concluded that the system must have been invented at a time and place when digamma and koppa were still either in use or at least still remembered as parts of the alphabetic sequence, whereas san had either already been forgotten, or at least was no longer remembered with its original alphabetic position. In the latter case, according to a much debated view, sampi itself may in fact have been regarded as being san, but with a new position in the alphabet.

The dating of the emergence of this system, and with it of numeric sampi, has been the object of much discussion. At the end of the 19th century, authors such as Thompson placed its full development only in the 3rd century BC. Jeffery states that the system as a whole can be traced much further back, into the 6th century BC. An early, though isolated, instance of apparent use of alphabetic Milesian numerals in Athens occurs on a stone inscribed with several columns of two-digit numerals, of unknown meaning, dated from the middle of the 5th century BC.

While the emergence of the system as a whole has thus been given a much earlier dating than was often assumed earlier, actual occurrences of the letter sampi in this context have as yet not been found in any early examples. According to Threatte, the earliest known use of numeric sampi in a stone inscription occurs in an inscription in Magnesia from the 2nd century BC, in a phrase denoting a sum of money ("δραχ(μὰς) ϡʹ) but the exact numeric meaning of this example is disputed. In Athens, the first attestation is only from the beginning of the 2nd century AD, again in an inscription naming sums of money.

Earlier than the attestations in the full function as a numeral are a few instances where sampi was used in Athens as a mark to enumerate sequences of things in a set, along with the 24 other letters of the alphabet, without implying a specific decimal numeral value. For instance, there is a set of 25 metal tokens, each stamped with one of the letters from alpha to sampi, which are dated to the 4th century BC and were probably used as identification marks for judges in the courts of the Athenian democracy.

In papyrus texts from the Ptolemaic period onwards, numeric sampi occurs with some regularity.

At an early stage in the papyri, the numeral sampi was used not only for 900, but, somewhat confusingly, also as a multiplicator for 1000, since a way of marking thousands and their multiples was not yet otherwise provided by the alphabetic system. Writing an alpha over sampi ( or, in a ligature,  ) meant "1×1000", a theta over sampi  meant "9×1000", and so on. In the examples cited by Gardthausen, a slightly modified shape of sampi, with a shorter right stem, is used. This system was later simplified into one where the thousands operator was marked just as a small stroke to the left of the letter (͵α = 1000).

Glyph development
In early stone inscriptions, the shape of sampi, both alphabetic and numeric, is. Square-topped shapes, with the middle vertical stroke either of equal length with the outer ones or longer , are also found in early papyri. This form fits the earliest attested verbal description of the shape of sampi as a numeral sign in the ancient literature, which occurs in a remark in the works of the 2nd-century AD physician Galen. Commenting on the use of certain obscure abbreviations found in earlier manuscripts of Hippocrates, Galen says that one of them "looks like the way some people write the sign for 900", and describes this as "the shape of the letter Π with a vertical line in the middle" ("ὁ τοῦ π γραμμάτος χαρακτὴρ ἔχων ὀρθίαν μέσην γραμμὴν, ὡς ἔνιοι γράφουσι τῶν ἐννεακοσίων χαρακτῆρα").

From the time of the earliest papyri, the square-topped forms of handwritten sampi alternate with variants where the top is rounded or pointed. The rounded form also occurs in stone inscriptions in the Roman era. In the late Roman period, the arrow-shaped or rounded forms are often written with a loop connecting the two lines at the right, leading to the "ace-of-spades" form, or to. These forms, in turn, occasionally have another decorative stroke added on the left. It can be found attached in several different ways, from the top or the bottom. From these shapes, finally, the modern form of sampi emerges, beginning in the 9th century, with the two straight lines becoming more or less parallel.

In medieval western manuscripts describing the Greek alphabet, the arrowhead form is sometimes rendered as.

Origins
Throughout the 19th and 20th centuries, many authors have assumed that sampi was essentially a historical continuation of the archaic letter san (Ϻ), the M-shaped alternative of sigma (Σ) that formed part of the Greek alphabet when it was originally adopted from Phoenician. Archaic san stood in an alphabetic position between pi (Π) and koppa (Ϙ). It dropped out of use in favour of sigma in most dialects by the 7th century BC, but was retained in place of the latter in a number of local alphabets until the 5th century BC. It is generally agreed to be derived from Phoenician tsade.

The hypothetical identification between san and sampi is based on a number of considerations. One is the similarity of the sounds represented by both. San represented either simple or some other, divergent phonetic realization of the common Greek  phoneme. Suggestions for its original sound value have included, , and. The second reason for the assumption is the systematicity in the development of the letter inventory: there were three archaic letters that dropped out of use in alphabetic writing (digamma/wau, koppa, and san), and three extra-alphabetic letters were adopted for the Milesian numeral system, two of them obviously identical with the archaic digamma and koppa; hence, it is easy to assume that the third in the set had the same history. Objections to this account have been related to the fact that sampi did not assume the same position san had had, and to the lack of any obvious relation between the shapes of the two letters and the lack of any intermediate forms linking the two uses.

Among older authorities, Gardthausen and Thompson took the identity between san and sampi for granted. Foat, in a skeptical reassessment of the evidence, came to the conclusion that it was a plausible hypothesis but unprovable. The discussion has continued until the present, while a steady trickle of new archaeological discoveries regarding the relative dating of the various events involved (i.e. the original emergence of the alphabet, the loss of archaic san, the emergence of alphabetic sampi, and the emergence of the numeral system) have continued to affect the data base on which it is founded.

A part of the discussion about the identity of san and sampi has revolved around a difficult and probably corrupted piece of philological commentary by an anonymous scholiast, which has been debated ever since Joseph Justus Scaliger drew attention to it in the mid-17th century. Scaliger's discussion also contains the first known attestation of the name "san pi" (sampi) in the western literature, and the first attempt at explaining it. The passage in question is a scholion on two rare words occurring in the comedies of Aristophanes, koppatias (κοππατίας) and samphoras (σαμφόρας). Both were names for certain breeds of horses, and both were evidently named after the letter used as a branding mark on each: "koppa" and "san" respectively. After explaining this, the anonymous scholiast adds a digression that appears to be meant to further explain the name and function of "san", drawing some kind of link between it and the numeral sign of 900. However, what exactly was meant here is obscure now, because the text was evidently corrupted during transmission and the actual symbols cited in it were probably exchanged. The following is the passage in the reading provided by a modern edition, with problematic words marked:

There is no agreement on what was originally meant by this passage. While Scaliger in the 17th century believed that the scholiast spoke of san as a synonym for sigma, and meant to describe the (modern) shape of sampi (ϡ) as being composed of an inverted lunate sigma and a π, the modern editor D. Holwerda believes the scholiast spoke of the actual M-shaped san and expressed a belief that modern sampi was related to it.

An alternative hypothesis to that of the historical identity between san and sampi is that Ionian sampi may have been a loan from the neighbouring Anatolian language Carian, which formed the local substrate in the Ionian colonies of Asia Minor. This hypothesis is mentioned by Jeffery and has been supported more recently by Genzardi Brixhe suggested that sampi could be related to the Carian letter 25 " ", transcribed as ś. This would fit in with the "plausible, but not provable" hypothesis that the root contained in the Carian-Greek names spelled with sampi, "Πανυασσις" and "Οασσασσις", is identical with a root *uś-/waś- identified elsewhere in Carian, which contains the Carian ś sound spelled with. Adiego follows this with the hypothesis that both the Carian letter and sampi could ultimately go back to Greek Ζ. Like the san–sampi hypothesis, the Carian hypothesis remains an open and controversial issue, especially since the knowledge of Carian itself is still fragmentary and developing.

While the origin of sampi continues to be debated, the identity between the alphabetic Ionian sampi and the numeral for 900 has rarely been in doubt, although in the older literature it was sometimes mentioned only tentatively, An isolated position was expressed in the early 20th century by Jannaris, who – without mentioning the alphabetic use of Ionian  – proposed that the shape of numeric sampi was derived from a juxtaposition of three "T"s, i.e. 3×300=900. (He also rejected the historical identity of the other two numerals, stigma (6) and koppa (90), with their apparent alphabetic predecessors.) Today, the link between alphabetic and numeral sampi is universally accepted.

Names
Despite all uncertainties, authors who subscribe to the hypothesis of a historical link between ancient san and sampi also often continue to use the name san for the latter. Benedict Einarson hypothesizes that it was in fact called *ssan, with the special quality of the sibilant sound it had as Ionian. This opinion has been rejected as phonologically impossible by Soldati, who points out that the sound only ever occurred in the middle of words and therefore could not have been used in the beginning of its own name.

As for the name sampi itself, it is generally agreed today that it is of late origin and not the original name of the character in either its ancient alphabetic or its numeral function. Babiniotis describes it as "medieval", while Jannaris places its emergence "after the thirteenth century". However, the precise time of its emergence in Greek is not documented.

The name is already attested in manuscript copies of an Old Church Slavonic text describing the development of the alphabet, the treatise On Letters ascribed to the 9th-century monk Hrabar, which was written first in Glagolitic and later transmitted in the Cyrillic script. In one medieval Cyrillic group of manuscripts of this text, probably going back to a marginal note in an earlier Glagolitic version, the letter names "sampi" ("") and "koppa" ("") are used for the Greek numerals. Witnesses of this textual variant exist from c.1200, but its archetype can be dated to before 1000 AD.

The first reference to the name sampi in the western literature occurs in a 17th-century work, Scaliger's discussion of the Aristophanes scholion regarding the word samphoras (see above). Some modern authors, taking Scaliger's reference as the first known use and unaware of earlier attestations, have claimed that the name itself only originated in the 17th century and/or that Scaliger himself invented it. A related term was used shortly after Scaliger by the French author Montfaucon, who called the sign "ἀντίσιγμα πῖ" (antisigma-pi), "because the Greeks regarded it as being composed of an inverted sigma, which is called ἀντίσιγμα, and from πῖ" ("Graeci putarunt ex inverso sigma, quod ἀντίσιγμα vocatur, et ex πῖ compositum esse").

The etymology of sampi has given rise to much speculation. The only element all authors agree on is that the -pi refers to the letter π, but about the rest accounts differ depending on each author's stance on the question of the historical identity between sampi and san.

According to the original suggestion by Scaliger, san-pi means "written like a san and a pi together". Here, "san" refers not to the archaic letter san (i.e. Ϻ) itself, but to "san" as a mere synonym of "sigma", referring to the outer curve of the modern ϡ as resembling an inverted lunate sigma. This reading is problematic because it fits the shape only of the modern (late Byzantine) sampi but presupposes active use of an archaic nomenclature that had long since lost currency by the time that shape emerged. According to a second hypothesis, san-pi would originally have meant "the san that stands next to pi in the alphabet". This proposal thus presupposes the historical identity between sampi and ancient san (Ϻ), which indeed stood behind Π. However, this account too is problematic as it implies a very early date of the emergence of the name, since after the archaic period the original position of san was apparently no longer remembered, and the whole point of the use of sampi in the numeral system is that it stands somewhere else. Yet a different hypothesis interprets san-pi in the sense of "the san that resembles pi". This is usually taken as referring to the modern ϡ shape, presupposing that the name san alone had persisted from antiquity until the time the sign took that modern shape. None of these hypotheses has wide support today. The most commonly accepted explanation of the name today is that san pi (σὰν πῖ) simply means "like a pi", where the word san is unrelated to any letter name but simply the modern Greek preposition σαν ("like", from ancient Greek ὡς ἂν).

In the absence of a proper name, there are indications that various generic terms were used in Byzantine times to refer to the sign. Thus, the 15th-century Greek mathematician Nikolaos Rabdas referred to the three numerals for 6, 90 and 900 as "τὸ ἐπίσημον", "τὸ ἀνώνυμον σημεῖον" ("the nameless sign", i.e. koppa), and "ὁ καλούμενος χαρακτήρ" ("the so-called charaktir", i.e. just "the character"), respectively. The term "ἐπίσημον" (episēmon, literally "outstanding") is today used properly as a generic cover term for all three extra-alphabetic numeral signs, but was used specifically to refer to 6 (i.e. digamma/stigma).

In some early medieval Latin documents from western Europe, there are descriptions of the contemporary Greek numeral system which imply that sampi was known simply by the Greek word for its numeric value, ἐννεακόσια (enneakosia, "nine hundred"). Thus, in De loquela per gestum digitorum, a didactic text about arithmetics attributed to the Venerable Bede, the three Greek numerals for 6, 90 and 900 are called "episimon", "cophe" and "enneacosis" respectively. the latter two being evidently corrupted versions of koppa and enneakosia. An anonymous 9th-century manuscript from Rheinau Abbey has epistmon [sic], kophe, and ennakose. Similarly, the Psalterium Cusanum, a 9th or 10th century bilingual Greek—Latin manuscript, has episimôn, enacôse and cophê respectively (with the latter two names mistakenly interchanged for each other.) Another medieval manuscript has the same words distorted somewhat more, as psima, coppo and enacos, Other, similar versions of the name include enacosin and niacusin, or the curious corruption sincope,

A curious name for sampi that occurs in one Greek source is "παρακύϊσμα" (parakyisma). It occurs in a scholion to Dionysius Thrax, where the three numerals are referred to as "τὸ δίγαμμα καὶ τὸ κόππα καὶ τό καλούμενον παρακύϊσμα". The obscure word ("… the so-called parakyisma") literally means "a spurious pregnancy", from "παρα-" and the verb "κυέω" "to be pregnant". The term has been used and accepted as possibly authentic by Jannaris, Uhlhorn and again by Soldati. While Jannaris hypothesizes that it was meant to evoke the oblique, reclining shape of the character, Soldati suggests it was meant to evoke its status as an irregular, out-of-place addition ("un'utile superfetazione"). Einarson, however, argues that the word is probably the product of textual corruption during transmission in the Byzantine period. He suggests that the original reading was similar to that used by Rabdas, "ὁ καλούμενος χαρακτήρ" ("the so-called character"). Another contemporary cover term for the extra-alphabetic numerals would have been "παράσημον" (parasēmon, lit. "extra sign"). A redactor could have written the consonant letters "π-σ-μ" of "παράσημον" over the letters "χ-κτ-ρ" of "χαρακτήρ", as both words happen to share their remaining intermediate letters. The result, mixed together from letters of both words, could have been misread in the next step as "παρακυησμ", and hence, "παρακύϊσμα".

An entirely new proposal has been made by A. Willi, who suggests that the original name of the letter in ancient Greek was angma (ἄγμα). This proposal is based on a passage in a Latin grammarian, Varro, who uses this name for what he calls a "25th letter" of the alphabet. Varro himself is clearly not referring to sampi, but is using angma to refer to the ng sound in words like angelus. However, Varro ascribes the use of the name angma to an ancient Ionian Greek author, Ion of Chios. Willi conjectures that Varro misunderstood Ion, believing the name angma referred to the sound because that sound happened to occur in the name itself. However, Ion, in speaking of a "25th letter of the alphabet", meant not just a different pronunciation of some other letters but an actual written letter in its own right, namely sampi. According to Willi's hypothesis, the name angma would have been derived from the verbal root *ank-, "to bend, curve", and referred to a "crooked object", used because of the hook-like shape of the letter.

In other scripts

In the Greco–Iberian alphabet, used during the 4th century BC in eastern Spain to write the Iberian language (a language unrelated to Greek), sampi was adopted along with the rest of the Ionian Greek alphabet, as an alphabetic character to write a second sibilant sound distinct from sigma. It had the shape, with three vertical lines of equal length. Its pronunciation is uncertain, but it is transliterated as $⟨s⟩$.

The Greek script was also adapted in Hellenistic times to write the Iranian language Bactrian, spoken in today's Afghanistan. Bactrian used an additional letter Ϸ, shaped like the later (unrelated) Germanic letter "thorn" (Þ), to denote its sh sound (š, ). This letter, too, has been hypothesized to be a continuation of Greek sampi, and/or san.

During the first millennium AD, several neighbouring languages whose alphabets were wholly or partly derived from the Greek adopted the structure of the Greek numeral system, and with it, some version or local replacement of sampi.

In Coptic, the sign "Ⳁ" (, which has been described as "the Greek  with a Ρ above" ), was used for 900. Its numeric role was subsequently taken over by the native character Ϣ (shei, ), which is related to the Semitic tsade (and thus, ultimately, cognate with Greek san as well).

The Gothic alphabet adopted sampi in its Roman era form of an upwards-pointing arrow (, 𐍊)

In the Slavic writing system Glagolitic, the letter (tse, ) was used for 900. It too may have been derived from a form of the Hebrew tsade. In Cyrillic, in contrast, the character (small yus, ) was used initially, being the one among the native Cyrillic letters that resembled sampi most closely in shape. However, the letter (tse), the equivalent of the Glagolitic sign, took its place soon later. It has been proposed that sampi was retained in its alternative function of denoting multiplication by thousand, and became the Cyrillic "thousands sign".

In Armenian, the letter Ջ (ǰ) stands for 900, while Ք (kʿ), similar in shape to the Coptic sign, stands for 9000.

Modern use
Together with the other elements of the Greek numeral system, sampi is occasionally still used in Greek today. However, since the system is typically used only to enumerate items in relatively small sets, such as the chapters of a book or the names of rulers in a dynasty, the signs for the higher tens and hundreds, including sampi, are much less frequently found in practice than the lower letters for 1 to 10. One of the few domains where higher numbers including thousands and hundreds are still expressed in the old system in Greece with some regularity is the field of law, because until 1914 laws were numbered in this way. For instance, one law which happens to have sampi in its name and is still in force and relatively often referred to is "Νόμος ͵ΓϠΝʹ/1911" (i.e. Law Number 3950 of 1911), "Περί της εκ των αυτοκινήτων ποινικής και αστικής ευθύνης" ("About penal and civil responsibility arising from the use of automobiles"). However, in informal practice, the letter sampi is often replaced in such instances by a lowercase or uppercase π.

Typography
With the advent of modern printing in the western Renaissance, printers adopted the minuscule version of the numeral sign, ϡ, for their fonts. The typographic realization of Sampi has varied widely throughout its history in print, and a large range of different shapes can still be found in current electronic typesetting. Commonly used forms range from small, π-like shapes to shapes with large swash curves, while the stems can be almost upright  or almost horizontal. More rarely, one can find shapes with the lower end curving outwards, forming an "s" curve.

In its modern use as a numeral (as with the other two episema, stigma and koppa) no difference is normally made in print between an upper case and lower case form; the same character is typically used in both environments. However, occasionally special typographic variants adapted to an upper case style have also been employed in print. The issue of designing such uppercase variants has become more prominent since the decision of Unicode to encode separate character codepoints for uppercase and lowercase sampi. Several different designs are currently found. Older versions of the Unicode charts showed a glyph with a crooked and thicker lower stem. While this form has been adopted in some modern fonts, it has been replaced in more recent versions of Unicode with a simpler glyph, similar to the lowercase forms. Many fonts designed for scholarly use have adopted an upright triangular shape with straight lines and serifs, as proposed by the typographer Yannis Haralambous. Other versions include large curved shapes, or an upright large π-like glyph with a long descending curve.

The epigraphic ancient Ionian sampi is not normally rendered with the modern numeral character in print. In specialized epigraphical or palaeographic academic discussion, it is either represented by a glyph, or by a Latin capital serifed T as a makeshift replacement. As this character has in the past never been supported in normal Greek fonts, there is no typographical tradition for its uppercase and lowercase representation in the style of a normal text font. Since its inclusion in the Unicode character encoding standard, experimental typographical stylizations of a lowercase textual Sampi have been developed. The Unicode reference glyph for "small letter archaic sampi", according to an original draft, was to have looked like the stem of a small τ with a square top at x height, but was changed after consultation with Greek typesetting experts. The glyph shown in current official code charts stands on the baseline and has an ascender slightly higher than cap height, but its stem has no serifs and is slightly curved leftwards like the descender of an ρ or β. Most type designers who have implemented the character in current fonts have chosen to design a glyph either at x height, or with a descender but no ascender, and with the top either square or curved (corresponding to ancient scribal practice. )

Computer encoding

Several codepoints for the encoding of sampi and its variants have been included in Unicode. As they were adopted successively in different versions of Unicode, their coverage in current computer fonts and operating systems is inconsistent as of 2010. U+03E0 ("Greek letter Sampi") was present from version 1.1 (1993) and was originally meant to show the normal modern numeral glyph. The uppercase/lowercase contrast was introduced with version 3.0 (1999). As the existing code point had been technically defined as an uppercase character, the new addition was declared lowercase (U+03E1, "Greek small letter Sampi"). This has led to some inconsistency between fonts, because the glyph that was present at U+03E0 in older fonts is now usually found at U+03E1 in newer ones, while U+03E0 may have a typographically uncommon capital glyph.

New, separate codepoints for ancient epigraphic sampi, also in an uppercase and lowercase variant, were proposed in 2005, and included in the standard with version 5.1 (2008). They are meant to cover both the Ionian and Pamphylian , with the Ionian character serving as the reference glyph. As of 2010, these characters are not yet supported by most current Greek fonts. The Gothic "900" symbol was encoded in version 3.1 (2001), and Coptic sampi in version 4.1 (2005). Codepoints for the related Greek characters san and Bactrian "sho" were added in version 4.0 (2003).

Prior to Unicode, support for sampi in electronic encoding was marginal. No common 8-bit codepage for Greek (such as ISO 8859-7) contained sampi. However, lowercase and uppercase sampi were provided for by the ISO 5428:1984 Greek alphabet coded character set for bibliographic information interchange. In Beta code, lowercase and uppercase sampi are encoded as "#5" and "*#5" respectively. In the LaTeX typesetting system, the "Babel" package allows accessing lowercase and uppercase sampi through the commands "\sampi" and "\Sampi". Non-Unicode (8-bit) fonts for polytonic Greek sometimes contained sampi mapped to arbitrary positions, but usually not as a casing pair. For instance, the "WP Greek Century" font that came with WordPerfect had sampi encoded as 0xFC, while the popular "Wingreek" fonts had it encoded as 0x22. No encoding system prior to Unicode 5.1 catered for archaic epigraphic sampi separately.

title: Sampi


text:
Altocumulus cloud

Altocumulus is a middle-altitude cloud genus that belongs mainly to the  physical category, characterized by globular masses or rolls in layers or patches – the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. However, if the layers become tufted in appearance due to increased airmass instability, then the altocumulus clouds become more purely cumuliform in structure. Like other cumuliform and stratocumuliform clouds, altocumulus signifies convection. A sheet of partially conjoined altocumulus perlucidus is sometimes found preceding a weakening warm front, where the altostratus is starting to fragment, resulting in patches of altocumulus perlucidus between the areas of altostratus. Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds. Towering altocumulus, known as altocumulus castellanus, frequently signals the development of thunderstorms later in the day, as it shows instability and convection in the middle levels of the troposphere, the area where towering cumulus clouds can turn into cumulonimbus. It is therefore one of three warning clouds often recorded by the aviation industry, the other two being towering cumulus and cumulonimbus. Altocumulus generally forms at about 2,000 to 6,100 m above ground level, a similar level to altostratus formations, and satellite photography has revealed that the two types of cloud can create formations that can stretch for thousands of square miles. Extensive altocumulus formations, particularly if they take the form of undulatus are often referred to as altocumulus mackerel sky.

Subtypes

 * Species: Altocumulus has four species.  The stratiformis species (Ac str) is composed of sheets or relatively flat patches of stratocumuliform cloud.  The synoptic coding is determined by the predominant variety or occasionally by the genitus mother cloud. Altocumulus lenticularis (Ac len lenticular cloud) is a lens-shaped middle cloud which can resemble flying saucers and may occasionally be mistaken for "unidentified flying objects". This is formed by uplift usually associated with mountains. but usually with at least some grey shading. It is coded CM4 on the SYNOP weather observation. Grey shading is also seen with altocumulus castellanus (Ac cas), a turreted middle cloud that can achieve significant vertical development and signals increasing air mass instability. It is nevertheless usually classified as middle rather than vertical and is coded CM8. The floccus species (Ac flo) is a tufted middle cloud which is also associated with greater instability. It shares the same code CM8. Chaotic altocumulus, which is typically poorly defined with multiple species or transitional forms arranged in several layers, is coded CM9.
 * Opacity-based varieties: Altocumulus stratiformis has three opacity-based varieties; Translucidus (CM3), perlucidus (CM3 or 7 depending on predominant opacity), and opacus (CM7). Varieties based on opacity are not commonly associated with the species lenticularis, castellanus, or floccus.
 * Pattern-based varieties: Radiatus (arranged in parallel bands) is sometimes seen with the stratformis and castellanus species. Altocumulus stratiformis radiatus of any opacity is coded CM5 if it is increasing in amount.  The duplicatus or undulatus varieties are occasionally seen with the stratiformis and lenticularis species.  Altocumulus stratiformis duplicatus is coded CM7 if it is not overridden by another coding of higher importance.  Lacunosus is very occasionally associated with altocumulus of the species stratiformis, castellanus, or floccus.
 * Precipitation-based supplementary feature: Altocumulus often produces virga but usually not precipitation that reaches the ground.
 * Cloud-based supplementary feature: Mamma caused by localized downdrafts in the cloud layer are occasionally seen with altocumulus. A newly recognized type is the asperitas feature that is characterized by chaotic undulations caused by severe wind shear.

 * Genitus Mother clouds: Altocumulus stratiformis cumulogenitus or cumulonimbogenitus can form when the middle or upper part of a towering free convective cloud begins to spread horizontally due to a loss of convective lift. It is coded CM6.
 * Mutatus mother clouds: Altocumulus can form due to the complete transformation of cirrocumulus, altostratus, nimbostratus, or stratocumulus.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Altocumulus cloud


text:
Altostratus cloud

Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.

Altostratus clouds usually predict the arrival of warm fronts. Once altostratus clouds associated with a warm front arrive, continuous rain or snow will usually follow in the next 12 to 24 hours. Although altostratus clouds predict the arrival of warmer, wetter weather, they themselves do not produce significant precipitation. Thunderstorms can be embedded in altostratus clouds, however, bringing showers.

Because altostratus clouds can contain ice crystals, they can produce some optical phenomena like iridescence and coronas.

Description
Altostratus clouds are generally gray or blue-tinged with a largely-uniform blanket-like appearance. They do not have distinct features, and usually do not produce precipitation. The name "altostratus" comes from the conjugation of the Latin words "altum", meaning "high", and "stratus", meaning "flat" or "spread out". Altostratus clouds can produce virga, causing the cloud base to appear hazy. While they do not produce significant precipitation, altostratus clouds can cause light sprinkles or even small rain showers. Consistent rainfall and lowering of the cloud base causes altostratus to become nimbostratus.

Unlike most other types of clouds, altostratus clouds are not subdivided into cloud species due to their largely-featureless appearance. However, they still appear in five varieties: Altostratus duplicatus, opacus, radiatus, translucidus, and undulatus. Altostratus duplicatus is a rare form of altostratus clouds composed of two or more layers of cloud. Translucidus is a translucent form of altostratus clouds, meaning that the sun or moon can be seen through the cloud, whereas the opacus variety is opaque. Radiatus is another rare variety. It has parallel bands of cloud that stretch toward the horizon. The undulatus variety has an wavy appearance—the underside of the cloud appears to rise and fall.

Altostratus and altocumulus clouds, both of which are mid-level clouds, are commonly measured together in cloud cover studies. Together, they cover around 25% of the Earth's surface on average based on CALIPSO satellite data. This constitutes roughly one third of the Earth's total cloud cover. By itself, separated from altocumulus, altostratus covers ~16% of the Earth's surface. Altostratus cloud cover varies seasonally in temperate regions, with significantly less coverage in the summer months as compared to the other seasons. Additionally, altostratus cloud cover varies by latitude, with tropical regions having vastly fewer altostratus clouds when compared to temperate or polar regions. Altostratus and altocumulus cover roughly 22% of the ocean's surface based on surface measurements, with minimal variation based on season.

Altostratus clouds are warmest at the bottom and coldest at the top, with a fairly consistent lapse rate of 5 to 7 °C per kilometer (14 to 20 °F per mile) inside the cloud. The lapse rate is the rate at which the temperature decreases with altitude. Higher lapse rates (i.e. the faster temperature drops with increasing altitude) were associated with colder clouds. The average temperature of altostratus clouds, based on data collected from roughly 45° to 80° latitude, varied from around -16 to -45 C. Warmer temperatures occurred during summer and colder temperatures during winter.

Inside altostratus clouds, the relative humidity is generally greatest towards the top of the cloud decreasing slowly and roughly linearly towards the bottom. The lowest part of the cloud has the lowest relative humidity. Below the bottom of the cloud, the relative humidity drops rapidly.

Microphysical properties
Altostratus can be composed of water droplets, supercooled water droplets, and ice crystals, but ice crystals make up the vast majority. In some altostratus clouds made of ice crystals, very thin horizontal sheets of water droplets can appear seemingly at random, but they quickly disappear. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again. During the sampling of one cloud, the scientists noted a halo while flying near the top of the cloud, which indicated that the ice crystals were hexagonal near the top. However, farther down, the ice crystals became more conglomerated. Mixed-phase (containing both ice and water) altostratus clouds contain a "melt layer", below which the ice crystals tend to melt into water droplets. These water droplets are spheres and thus fall much faster than ice crystals, collecting at the bottom of the cloud.

Formation
Altostratus clouds form when a large mass of warm air rises, causing water vapor in the atmosphere to condense onto nuclei (small dust particles), forming water droplets and ice crystals. These conditions usually happen at the leading edge of a warm front, where cirrostratus clouds thicken and lower until they transition into altostratus clouds. Alternatively, nimbostratus clouds can thin into altostratus. Altostratus can even form from the spreading of the upper anvil cloud or the middle column of a thunderstorm.

Altostratus clouds are mid-level clouds that form from 2000 to 4000 m above sea level in polar regions. In temperate regions, the ceiling increases drastically, allowing altostratus clouds to form between 2000 to 7000 m. In tropical regions, altostratus can reach even higher, forming from 2000 to 8000 m. They can range from 1000 to 5000 m in thickness and can cover hundreds of kilometers of the Earth's surface.

Use in forecasting
Altostratus clouds tend to form ahead of warm fronts or occluded fronts and herald their arrival. These warm fronts bring warmer air into the region. Occluded fronts form when a faster-moving cold front catches up to a warm front, and the temperature after the frontal system passes may rise or fall. As the frontal system approaches, cirrostratus clouds will thicken into altostratus clouds, which then gradually thicken further into nimbostratus clouds. If the frontal system is occluded, cumulonimbus clouds may also be present. Once the altostratus clouds have arrived, rain or snow will usually follow in the next 12 to 24 hours.

Instability in the atmosphere can embed thunderstorms in an altostratus cloud, although altostratus clouds themselves do not produce storms.

Effects on climate
Globally, clouds reflect around 50 watts per square meter of short-wave solar radiation back into space, cooling the Earth by around 12 C-change, an effect largely caused by stratocumulus clouds. However, at the same time, they reflect around 30 watts per square meter of long-wave (infrared) black body radiation emitted by the Earth back to Earth's surface, heating the Earth by around 7 C-change—a process called the greenhouse effect. Cirrus and altostratus clouds are the top two sources of this heating effect. This combination of heating and cooling sums out to a net loss of 20 watts per square meter globally, cooling the Earth by roughly 5 C-change.

Altostratus clouds are the only cloud genus besides cirrus clouds to exhibit a net global heating effect on Earth and its atmosphere; however, cirrus have a heating effect that is four times as potent as altostratus (2 watts per square meter versus only 0.5 watts per square meter).

Optical phenomena
Altostratus clouds can produce bright halos when viewed from the air, but not when viewed from the ground. Halos can take the appearance of rings, arcs, or spots of white or multicolored light and are formed by the reflection and refraction of sunlight or moonlight shining through ice crystals in the cloud. Light diffraction through altostratus clouds can also produce coronas, which are small, concentric pastel-colored rings of light around the sun or moon. They can also be iridescent, with often-parallel bands of bright color projected on a cloud. Unlike the halos, the coronas and iridescence can be seen from Earth's surface.

Relation to other clouds
Altostratus and altocumulus clouds are the two genera of mid-level clouds that usually form between 6500 and 20000 ft. These are given the prefix "alto-". These clouds are formed from ice crystals, supercooled water droplets, or liquid water droplets.

Above the mid-level clouds are three different genera of high-level clouds, cirrus, cirrocumulus, and cirrostratus, all of which are given the prefix "cirro-". High-level clouds usually form above 20000 ft. Cirrocumulus and cirrostratus are sometimes informally referred to as cirriform clouds because of their frequent association with cirrus.

Below the mid-level clouds are the low-level clouds, which usually form below 6500 ft and do not have a prefix. The two genera that are strictly low-level are stratus, and stratocumulus. These clouds are composed of water droplets, except during winter when they are formed of supercooled water droplets or ice crystals if the temperature at cloud level is below freezing. Three additional genera usually form in the low altitude range, but may be based at higher levels under conditions of very low humidity. They are the genera cumulus, and cumulonimbus, and nimbostratus. These are sometimes classified separately as clouds of vertical development, especially when their tops are high enough to be composed of supercooled water droplets or ice crystals.

Cirrostratus

Cirrostratus clouds can appear as a smooth veil in the sky or as a striated sheet. They are sometimes similar to altostratus and are distinguishable from the latter because the sun or moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent. Cirrostratus come in two species, fibratus and nebulosus. The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around -35 to -45 C, the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around -47 to -52 C, the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns. These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present. They are formed by warm, moist air being lifted slowly to a very high altitude. When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds, and rain usually begins 12 to 24 hours later.

Altocumulus
Altocumulus clouds are small patches or heaps of white or light gray cloud. Like altostratus, altocumulus are composed of a mixture of water droplets, supercooled water droplets, and ice crystals. Although altocumulus clouds are mid-level clouds that form at roughly the same altitude as altostratus clouds, their formation methods are completely different. Altocumulus forms from convective (rising) processes, whereas altostratus is usually formed by descending and thickening cirrostratus.

Stratus
Stratus are low-level clouds that are usually visually similar to altostratus. Stratus comes in two species: nebulosus, a largely-featureless flat gray cloud sheet, and fractus, shattered fragments of cloud often called "scud". Opaque varieties of altostratus and stratus nebulosus clouds can be virtually indistinguishable from each other to the naked eye, to the point that the World Meteorological Organization suggests that one of the few ways to distinguish between these clouds is to check what types of clouds came before them. Altostratus clouds, because they tend to form from warm fronts, are usually preceded by high-level cirriform clouds. Stratus clouds tend to form by cooling air masses, often at night, and thus are not usually preceded by other types of clouds.

Nimbostratus
Nimbostratus are low-level (sometimes classified as vertical) rain-bearing stratus clouds. Unlike the sprinkles or light drizzles that altostratus or stratus can produce, nimbostratus produces heavy, continuous rain or snow. These clouds are thick and dark enough to entirely blot out the sun. Nimbostratus has no species or varieties. Like altostratus, nimbostratus clouds can be made of ice crystals, supercooled water droplets, or water droplets.

title: Altostratus cloud


text:
Stratocumulus cloud

A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and the whole being at a lower height, usually below 2000 m. Weak convective currents create shallow cloud layers (see also: sea of clouds) because of drier, stable air above preventing continued vertical development. Historically, in English, this type of cloud has been referred to as a twain cloud for being a combination of two types of clouds.

Description
Stratocumulus clouds are rounded clumps or patches of white to dark gray clouds that normally form in groups. The individual cloud elements, which cover more than 5 degrees of arc each, can connect with each other and are sometimes arranged in a regular pattern.

Occurrence
Vast areas of subtropical and polar oceans are covered with massive sheets of stratocumulus. These may organize into distinctive patterns which are currently under active study. In subtropics, they cover the edges of the horse latitude climatological highs, and reduce the amount of solar energy absorbed in the ocean. When these drift over land the summer heat or winter cold is reduced. 'Dull weather' is a common expression incorporated with overcast stratocumulus days, which usually occur either in a warm sector between a warm and cold front in a depression, or in an area of high pressure, in the latter case, sometimes persisting over a specific area for several days. If the air over land is moist and hot enough, stratocumulus may develop to various cumulus clouds, or, more commonly, the sheet of stratocumulus may become thick enough to produce some light rain. On drier areas they quickly dissipate over land, resembling cumulus humilis. This often occurs in late morning in areas under anticyclonic weather, the stratocumulus breaking up under the sun's heat and often reforming again by evening as the heat of the sun decreases again.

Precipitation
Most often, stratocumulus produce no precipitation, and when they do, it is generally only light rain or snow. However, these clouds are often seen at either the front or tail end of worse weather, so they may indicate storms to come, in the form of thunderheads or gusty winds. They are also often seen underneath the cirrostratus and altostratus sheets that often precede a warm front, as these higher clouds decrease the sun's heat and therefore convection, causing any cumulus clouds to spread out into stratocumulus clouds.

Comparison with altocumulus
Stratocumulus clouds are similar in appearance to altocumulus and can be mistaken for such. A simple test to distinguish these is to compare the size of individual masses or rolls: when pointing one's hand in the direction of the cloud, if the cloud is about the size of the thumb, it is altocumulus; if it is the size of one's fist, it is stratocumulus. This often does not apply when stratocumulus is of a broken, fractus form, when it may appear as small as altocumulus.

Optical effects
Stratocumulus clouds are the main type of cloud that can produce crepuscular rays. Thin stratocumulus clouds are also often the cause of corona effects around the Moon at night. All stratocumulus subtypes are coded CL5 except when formed from free convective mother clouds (CL4) or when formed separately from co-existing (CL8).

Formation
Stratocumulus clouds usually form from the rising and breakup of a stratus cloud. They can also form from altostratus and nimbostratus clouds, either as evaporating precipitation condenses into a cloud or as the nimbostratus cloud itself thins and breaks up. If a cumulus cloud becomes flattened (for example, by wind shear or temperature inversion), it too can become a stratocumulus cloud.

Species
Stratocumulus Stratiformis are extensive flat but slightly lumpy sheets that show only minimal convective activity.

Stratocumulus Lenticularis are separate flat elongated seed-shaped clouds. They are typical for polar countries or warmer climate during winter seasons. They also can be formed by winds passing hills or mountains, such as Foehn winds, and in this case they can be very regularly shaped.

Stratocumulus Castellanus have stronger convective activity due to the presence of increasingly unstable air. They are distinct from other stratocumulus by puffy tower-like formations atop the cloud layer. They look like cumulus congestus, but can be easily confused: "towers" of cumulus congestus grow above separate clouds, whereas in the case of stratocumulus castellanus, there is always a more or less defined layer of clouds. Stratocumulus castellanus may develop into cumulus congestus (and even further into cumulonimbus) under auspicious conditions. Any showers from stratocumulus castellanus are not usually as heavy as those from cumulus congestus.

Opacity-based varieties
Stratocumulus Opacus is a dark layer of clouds covering entire sky without any break. However, the cloud sheet is not completely uniform, so that separate cloud bases still can be seen. This is the main precipitating type, however any rain is usually light. If the cloud layer becomes grayer to the point when individual clouds cannot be distinguished, stratocumulus turn into stratus clouds.

Stratocumulus Perlucidus is a layer of stratocumulus clouds with small spaces, appearing in irregular pattern, through which clear sky or higher clouds can be seen.

Stratocumulus Translucidus consist of separate groups of stratocumulus clouds, with a clear sky (or higher clouds) visible between them. No precipitation in most cases.

Pattern-based varieties
Stratocumulus Undulatus clouds appear as nearly parallel waves, rolls or separate elongated clouds, without significant vertical development.

Stratocumulus Radiatus clouds appear as the same as stratocumulus undulatus, but stratocumulus undulatus move perpendicular to the wind shear, while stratocumulus radiatus move parallel to the wind shear.

Stratocumulus Duplicatus clouds appear as stratocumulus clouds with two or more layers or sheets. Stratocumulus duplicatus is common on species lenticularis or lenticular cloud.

Stratocumulus Lacunosus clouds are very uncommon. They only occur when there are localized downdrafts striking through the stratocumuliform cloud.

Supplementary feature
Stratocumulus Mamma is a type of mammatus cloud.

Stratocumulus Asperitas is a rare, newly recognized supplementary feature that presents itself as chaotic, wavy undulations appearing in the base of a stratocumulus cloud cover. It is thought these clouds are formed by severe wind shear.

Stratocumulus Fluctus is also a rare, newly recognized supplementary feature in which short-lived "sea waves" form on top of a stratocumulus cloud, they are caused by wind speed and direction differences directly under and over the cloud.

Precipitation-based supplementary features
Stratocumulus Virga is a form of precipitation that evaporates in mid-air and doesn't reach the ground.

Stratocumulus Praecipitatio is a form of precipitation that reaches the ground as light rain or snow.

Mother clouds
Stratocumulus Cumulomutatus the specific type of stratocumulus clouds, are flat and elongated. They form in the evening, when updrafts caused by convection decrease making cumulus clouds lose vertical development and spread horizontally. They also can occur under altostratus cloud preceding a warm or occluded front, when cumulus usually lose vertical development as the sun's heat decreases. Like all other forms of stratocumulus apart from castellanus, they are also often found in anticyclones.

Stratocumulus Cumulogenitus out of cumulus or cumulonimbus clouds, disrupted by decreasing convection. During formation period, puffy tops of cumulus clouds can protrude from stratocumulus cumulogenitus for a relatively long time until they completely spread in horizontal direction. Stratocumulus cumulogenitus appear as lengthy sheet or as group of separate elongated cloud rolls or waves.

title: Stratocumulus cloud


text:
Nimbostratus cloud

A nimbostratus cloud is a multilevel, amorphous, nearly uniform, and often dark-grey cloud that usually produces continuous rain, snow, or sleet, but no lightning or thunder.

Although it is usually a low-based cloud, it actually forms most commonly in the middle level of the troposphere and then spreads vertically into the low and high levels. Nimbostratus usually produces precipitation over a wide area.

The prefix nimbo- comes from the Latin word , which denotes "cloud" or "halo".

Downward-growing nimbostratus can have the same vertical extent as most large upward-growing cumulus, but its horizontal expanse tends to be even greater.

Appearance
Nimbostratus has a diffuse cloud base generally found anywhere from near surface in the low levels to about 3000 m in the middle level of the troposphere. Although usually dark at its base, it often appears illuminated from within to a surface observer. Nimbostratus usually has a thickness of about 2000 to 4000 m. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes. It is coded CM2 on the SYNOP report.

Formation
Nimbostratus occurs along a warm front or occluded front where the slowly rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher-level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus.

Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable warm front caused as a result of the advancing warm air being hot, humid and unstable, cumulonimbus clouds may be embedded within the usual nimbostratus. Lightning from an embedded cumulonimbus cloud may interact with the nimbostratus but only in the immediate area around it. In this situation with lightning and rain occurring it would be hard to tell which type of cloud was producing the rain from the ground, however cumulonimbus tend to produce larger droplets and more intense downpours. The occurrence of cumulonimbus and nimbostratus together is uncommon, and usually only nimbostratus is found at a warm front and sometimes in cold front.

Forecast

Nimbostratus is generally a sign of an approaching warm or occluded front producing steady moderate precipitation, as opposed to the shorter period of typically heavier precipitation released by a cold-frontal cumulonimbus cloud. Precipitation may last for several days, depending on the speed of the frontal system. A nimbostratus virga cloud is the same as normal nimbostratus, but the rain or snow falls as virga which doesn't reach the ground. Stratus or stratocumulus usually replace the nimbostratus after the passage of the warm or occluded front.

Origin of name
Under Luke Howard's first systematized study of clouds, carried out in France in 1802, three general cloud forms were established based on appearance and characteristics of formation: cirriform, cumuliform and stratiform. These were further divided into upper and lower types depending on altitude. In addition to these three main types, Howard added two names to designate multiple cloud types joined together: cumulostratus, a blending of cumulus clouds and stratus layers, and nimbus, a complex blending of cirriform, cumuliform, and stratiform clouds with sufficient vertical development to produce significant precipitation.

Later, in the 20th century, an IMC commission for the study of clouds put forward a refined and more restricted definition of the genus nimbus, effectively reclassifying it as a stratiform cloud type. It was then renamed nimbostratus, and published with the new name in the 1932 edition of the International Atlas of Clouds and of States of the Sky. This left cumulonimbus as the only nimboform type as indicated by its root name.

Subtypes and derivative types

 * Species and varieties: Nimbostratus is very thick, opaque, and featureless, so this genus type is not subdivided into species or varieties.
 * Precipitation-based supplementary features: Nimbostratus is a major precipitation cloud and produces the virga or precipitation features. The latter can achieve heavy intensity due to the cloud's vertical depth.
 * Accessory cloud: Nimbostratus pannus is an accessory cloud of nimbostratus that forms as a ragged layer in precipitation below the main cloud deck. Pannus is coded CL7.
 * Genitus mother clouds: This genus type can form from cumulus and cumulonimbus.
 * Mutatus mother clouds: Nimbostratus can form due to the complete transformation of altocumulus, altostratus and stratocumulus.

Relation to other clouds
Multi-level nimbostratus is physically related to other stratiform genus-types by way of being non-convective in nature. However, the other sheet-like clouds usually each occupy only one or two levels at the same time. Stratus clouds are low-level and form from near ground level to 2000 m at all latitudes. In the middle level are the altostratus clouds that form from 2000 m to 7000 m in polar areas, 7000 m in temperate areas, and 7600 m in tropical areas. Although altostratus forms mostly in the middle level of the troposphere, strong frontal lift can push it into the lower part of the high-level. The main high-level stratiform cloud is cirrostratus which is composed of ice crystals that often produce halo effects around the sun. Cirrostratus forms at altitudes of 3000 to 7600 m in high latitudes, 5000 to 12000 m in temperate latitudes, and 6100 to 18000 m in low, tropical latitudes. Of the non-stratiform clouds, cumulonimbus and cumulus congestus are the most closely related to nimbostratus because of their vertical extent and ability to produce moderate to heavy precipitation. The remaining cumuliform (cumulus) and stratocumuliform (stratocumulus, altocumulus, and cirrocumulus) clouds have the least in common with nimbostratus.

title: Nimbostratus cloud


text:
Omicron

Omicron (, ; uppercase Ο, lowercase ο, ) is the fifteenth letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin:. In classical Greek, omicron represented the close-mid back rounded vowel in contrast to omega which represented  the open-mid back rounded vowel  and the digraph ου which represented the long close-mid back rounded vowel. In modern Greek, both omicron and omega represent the mid back rounded vowel or. Letters that arose from omicron include Roman O and Cyrillic O. The word literally means "little O" (o mikron) as opposed to "great O" (ō mega). In the system of Greek numerals, omicron has a value of 70.

Use
In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation, but its use is limited since both upper case and lower case (Ο ο) are indistinguishable from the Latin letter "o" (O o) and difficult to distinguish from the Arabic numeral "zero" (0).

Mathematics
The big-O symbol introduced by Paul Bachmann in 1894 and popularized by Edmund Landau in 1909, originally standing for "order of" ("Ordnung") and being thus a Latin letter, was apparently viewed by Donald Knuth in 1976 as a capital Omicron, probably in reference to his definition of the symbol (capital) Omega. Neither Bachmann nor Landau ever call it "Omicron", and the word "Omicron" appears just once in Knuth's paper: in the title.

Greek numerals
There were several systems for writing numbers in Greek; the most common form used in late classical era used omicron (either upper or lower case) to represent the value 70.

More generally, the letter omicron is used to mark the fifteenth ordinal position in any Greek-alphabet marked list. So, for example, in Euclid's Elements, when various points in a geometric diagram are marked with letters, it is effectively the same as marking them with numbers, each letter representing the number of its place in the standard alphabet.

Astronomy
Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position. Such stars include Omicron Andromedae, Omicron Ceti, and Omicron Persei.

In Claudius Ptolemy's (c. 100–170) Almagest, tables of sexagesimal numbers $ε$  are represented in the conventional manner for Greek numbers: $ε$ . Since the letter omicron [which represents $ζ$ ($ζ$) in the standard system] is not used in sexagesimal, it is re-purposed to represent an empty number cell. In some copies, zero cells were just left blank (nothing there, value is zero), but to avoid copying errors, positively marking a zero cell with omicron was preferred, for the same reason that blank cells in modern tables are sometimes filled-in with a long dash (—). Both an omicron and a dash imply that "this isn't a mistake, the cell is actually supposed to be empty". By coincidence, the ancient zero-value omicron ($ζ$) resembles a modern Hindu-Arabic zero ($ϙ$).

Medicine
The World Health Organization (WHO) uses the Greek alphabet to describe variants of concern of SARS‑CoV‑2, the virus which causes COVID-19. On November 26, 2021, Omicron was assigned to the B.1.1.529 variant of concern.

History

In the earliest Greek inscriptions, only five vowel letters A E I O Y were used. Vowel length was undifferentiated, with O representing both the short vowel /o/ and the long vowels /o:/ and /ɔː/. Later, in classical Attic Greek orthography, the three vowels were represented differently, with O representing short /o/, the new letter Ω representing long /ɔː/, and the so-called "spurious diphthong" OY representing long /o:/.

Although the Greeks took the character O from the Phoenician letter `ayin, they did not borrow its Phoenician name. Instead, the name of the letter O in classical Attic times was simply the long version of its characteric sound: $π$ (pronounced /o:/) (that of Ω was likewise $ρ$). By the second and third centuries CE, distinctions between long and short vowels began to disappear in pronunciation, leading to confusion between O and Ω in spelling. It was at this time that the new names of $ω$ ("small O") for O $ϡ$ ("great O") for Ω were introduced.

Mispronunciation
During the early outbreak of the Omicron variant of COVID-19, many people unfamiliar with the entire Greek alphabet (or simply lacking the ability to pronounce or sound out words using phonetics) mispronounced Omicron as "Omnicron" due to the unfamiliarity of the letter, and the use of the prefix "Omni-" in many words.

Character encodings

 * Greek omicron / Coptic O

 * Mathematical omicron

These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

title: Omicron


text:
Cirrocumulus cloud

Cirrocumulus is one of the three main genus types of high-altitude tropospheric clouds, the other two being cirrus and cirrostratus. They usually occur at an altitude of 5 to 12 km. Like lower-altitude cumuliform and stratocumuliform clouds, cirrocumulus signifies convection. Unlike other high-altitude tropospheric clouds like  cirrus and cirrostratus, cirrocumulus includes a small amount of liquid water droplets, although these are in a supercooled state. Ice crystals are the predominant component, and typically, the ice crystals cause the supercooled water drops in the cloud to rapidly freeze, transforming the cirrocumulus into cirrostratus. This process can also produce precipitation in the form of a virga consisting of ice or snow. Thus, cirrocumulus clouds are usually short-lived. They usually only form as part of a short-lived transitional phase within an area of cirrus clouds and can also form briefly as a result of the breaking up of part of a cumulonimbus anvil.

Properly, the term cirrocumulus refers to each cloud, but is typically also used to refer to an entire patch of cirrocumulus. When used in this way, each cirrocumulus element is referred to as a "cloudlet".

Appearance
Cirrocumulus is a cloud of the stratocumuliform physical category that shows both stratiform and cumuliform characteristics and typically appears as white, patchy sheets with ripples or tufts without gray shading. Each cloudlet appears no larger than a finger held at arm's length. These often are organized in rows like other cumuliform and stratocumuliform clouds, but since they are so small, cirrocumulus patches take on a finer appearance, sometimes also referred to colloquially as "herringbone" or as a "mackerel sky". Cirrocumulus is coded CH9 for the main genus-type and all subforms.

Cirrocumulus is distinguished from altocumulus in several ways, although the two stratocumuliform genus types can occasionally occur together with no clear demarcation between them. Cirrocumulus generally occur at higher altitudes than altocumulus, thus the "cloudlets" appear smaller, as they are more distant from observation at ground level. They are also colder. Cirrocumulus clouds never cast self-shadows and are translucent to a certain degree. They are also typically found amongst other cirrus clouds in the sky and are usually themselves seen to be transforming into these other types of cirrus. This often occurs at the leading edge of a warm front, where many types of cirriform clouds can be present.

Cirrocumulus clouds tend to reflect the red and yellow colours during a sunset and sunrise, so they have been referred to as "one of the most beautiful clouds". This occurs because they reflect the unscattered rays of light from the early morning or evening sun, and those rays are yellow, orange, red, and sometimes purple.

Forecasting
Cirrocumulus usually only forms in patches. If it forms in patches with cirrus or cirrostratus and the clouds spread across the sky, it usually means rain in 8–10 hours (can be more if the front is slow-moving). Only small patches of cirrocumulus and perhaps some wisps of cirrus usually mean a continuation of good weather (although this may also be seen in conjunction with showers and thunderstorms). If it is seen after rain, it usually means improving the weather.

Subtypes

 * Species: Cirrocumulus stratiformis (Cc str) is one of four species and appears in the form of relatively flat stratocumuliform sheets or patches. The species Cirrocumulus lenticularis (Cc len) takes its name from the lens-shaped structure of this cloud which is tapered at each end. Cirrocumulus castellanus (Cc cas) has cumuliform buildups that give the cloud a partly or mainly turreted appearance.  When the cumuliform parts have more of a tufted appearance, it is given the species name Cirrocumulus floccus (Cc flo).  Cirrocumulus with castellanus buildups can show some vertical extent, but are not usually classified as vertical or multi-étage clouds.
 * Varieties: This genus type is always translucent and so has no opacity-based varieties.  However, like cirrus, certain cirrocumulus species can sometimes be divided into pattern-based varieties.  The undulatus variety has a wavy undulating base and is seen mostly with the stratiformis and lenticularis species types.  The lacunosus variety contains circular holes caused by downdrafts in the cloud and is associated mainly with the species stratiformis, castellanus, and floccus.
 * Precipitation-based supplementary feature: Cirrocumulus occasionally produces virga, precipitation that evaporates before reaching the ground.
 * Cloud-based supplementary feature: Mamma in the form of downward forming bubbles is infrequently seen as a cloud-based supplementary feature.
 * Mother clouds: This genus type has no recognized genitus mother clouds. However cirrocumulus stratiformis cirromutatus or cirrostratomutatus can result from sheets or filaments of high cloud taking on a stratocumuli form structure as a result of high altitude convection.  A high layer of white or light grey altocumulus of a particular species can thin out into pure white cirrocumulus altocumulomutatus of the same species.

title: Cirrocumulus cloud


text:
Cirrostratus cloud

Cirrostratus is a high-altitude, very thin, generally uniform stratiform genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make halos. These are made when the cloud takes the form of thin cirrostratus nebulosus. The cloud has a fibrous texture with no halos if it is thicker cirrostratus fibratus. On the approach of a frontal system, the cirrostratus often begins as nebulous and turns to fibratus. If the cirrostratus begins as fragmented of clouds in the sky it often means the front is weak. Cirrostratus is usually located above 5.5 km (18,000 ft). Its presence indicates a large amount of moisture in the upper troposphere. Clouds resembling cirrostratus occasionally form in polar regions of the lower stratosphere. Polar stratospheric clouds can take on this appearance when composed of tiny supercooled droplets of water or nitric acid.

Cirrostratus clouds sometimes signal the approach of a warm front if they form after cirrus and spread from one area across the sky, and thus may be signs that precipitation might follow in the next 12 to 24 hours or as soon as 6–8 hours if the front is fast moving. If the cirrostratus is broken fibratus, it can mean that the front is weak and that stratus rather than nimbostratus will be the precipitating cloud (meaning drizzle or snow grains instead of moderate rain or snow). Cumulus humilis or stratocumulus clouds are often found below cirrostratus formations, due to the stable air associated with cirrostratus creating an inversion and restricting convection, causing cumuliform clouds to become flattened. Contrails also tend to spread out and can be visible for up to an hour in cirrostratus.

The phrase "milky sunshine" is often, as well as referring to haze or light mist, used to refer to the milky look of the sky when cirrostratus is present.

 * Species: Cirrostratus fibratus (Cs fib) is a high fibrous sheet similar to cirrus but with less detached semi-merged filaments. It is reported in the SYNOP code as CH8 or as CH5 or 6 (depending on the amount of sky covered) if increasing in amount.  If the high cloud covers the entire sky and takes on the form of a featureless veil, it is classified as cirrostratus of the species nebulosus (Cs neb) and is coded CH7.
 * Varieties: Cirrostratus species have no opacity-based varieties as they are always translucent.  Two pattern-based varieties are sometimes seen with the species fibratus.  These are the closely spaced duplicatus and wavy undulatus types similar to those seen with cirrus fibratus.  Pattern-based varieties are not commonly associated with the species nebulosus due to its lack of features.
 * Supplementary features: Cirrostratus produces no precipitation or virga, and is not accompanied by any accessory clouds.
 * Genitus mother clouds: Cirrostratus fibratus cirrocumulogenitus sometimes appears as the latter cloud flattens and loses some of its stratocumuliform structure. Cirrostratus fibratus cumulonimbogenitus may form if the cirriform top of a mature thundercloud spreads and flattens sufficiently to become a high stratiform cloud.
 * Mutatus mother clouds: Cirrostratus fibratus cirromutatus or cirrocumulomutatus are the result of a complete transformation from cirrus and cirrocumulus genus types. Cirrostratus nebulosis altostratomutatus results when a high grey nebulous altostratus layer thins out into a whitish layer of featureless high cloud.

title: Cirrostratus cloud


text:
Xi (letter)

Xi (, ; uppercase Ξ, lowercase ξ; ξι) is the fourteenth letter of the Greek alphabet, representing the voiceless consonant cluster. Its name is pronounced in Modern Greek. In the system of Greek numerals, it has a value of 60. Xi was derived from the Phoenician letter samekh.

Xi is distinct from the letter chi, which gave its form to the Latin letter X.

Greek
Both in classical Ancient Greek and in Modern Greek, the letter Ξ represents the consonant cluster /ks/. In some archaic local variants of the Greek alphabet, this letter was missing. Instead, especially in the dialects of most of the Greek mainland and Euboea, the cluster /ks/ was represented by Χ (which in classical Greek is chi, used for ).

Because this variant of the Greek alphabet was used in Magna Graecia (the Greek colonies in Sicily and the southern part of the Italian peninsula), the Latin alphabet borrowed Χ rather than Ξ as the Latin letter that represented the /ks/ cluster that was also present in Latin.

Cyrillic
The Xi was adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).

Uppercase
The uppercase letter Ξ is used as a symbol in various contexts.

Pure mathematics

 * Harish-Chandra's Ξ function in harmonic analysis and representation theory
 * The Riemann Xi function in analytic number theory and complex analysis

Physics

 * The "cascade particles" in particle physics
 * The partition function under the grand canonical ensemble in statistical mechanics

Other uses

 * Indicating "no change of state" in Z notation in computing
 * Used as the Latin letter E in some logos.
 * Monetary units of the cryptocurrencies Ether (and less commonly ETC), equal to 1018 Wei

Lowercase
The lowercase letter ξ is used as a symbol for:

Pure mathematics

 * Random variables
 * A parameter in a generalized Pareto distribution
 * The symmetric function equation of the Riemann zeta function in mathematics, also known as the Riemann Xi function
 * A universal set in set theory
 * A number used in the remainder term of Taylor's theorem that falls between the limits a and b
 * A number used in error approximations for formulas that are applications of Taylor's theorem, such as Newton–Cotes formulas

Physics and astronomy

 * In fluid dynamics, the Iribarren parameter.
 * The initial mass function in astronomy.
 * The correlation function in astronomy.
 * Spatial frequency; also sometimes temporal frequency.
 * A small displacement in MHD plasma stability theory
 * The x-coordinate of computational space as used in computational fluid dynamics
 * Potential difference in physics (in volts)
 * The radial integral in the spin-orbit matrix operator in atomic physics.
 * The Killing vector in general relativity.
 * Average logarithmic energy decrement per collision (neutron calculations in nuclear physics)
 * Pippard's cohesion length in superconductors
 * The diameter of a crystal nucleus in nucleation theory
 * Microturbulence velocity in a stellar atmosphere
 * The dimensionless longitudinal momentum loss of a beam particle after a two-body interaction in accelerator physics.
 * Dimensionless distance variable used in the Lane–Emden equation

Other uses

 * Propositional variables in some philosophical works, first found in Wittgenstein's Tractatus
 * Extent of reaction, a concept in physical chemistry used often in chemical engineering kinetics and thermochemistry
 * Unknown stereochemistry or stereocentre configuration in a planar ring system in organic chemistry, as well as uppercase Xi for unknown R/S/E/Z configuration in general
 * One of the two different polypeptide chains of the human embryonic hemoglobin types Hb-Portland (ξ2γ2) and Hb-Gower I (ξ2ε2)
 * A parameter denoted as warped time used to derive the equations for homogeneous azeotropic distillation
 * State Price Density in mathematical finance
 * The information vector in the Information Filter, GraphSLAM, and a number of other algorithms used for robot localization and robotic mapping.
 * Used in Support Vector machines in cases where the data is not linearly separable.
 * Used in Microelectronics to represent the distance from a p-n junction to a point in the depletion region where the electric field is strongest.

Greek Xi / Coptic Ksi
Unicode Code Charts: Greek and Coptic (Range: 0370-03FF)

Mathematical Xi
The following characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

Other uses
Uppercase Ξ is used as an 'E' to stylise company names/logos like Razer (styled as RΛZΞR), Tesla (styled as TΞSLA), musician Banners (styled as BANNΞRS), and in South Korean boy group ZE:A's newest logo (styled as "ZΞA") (Compare: Heavy Metal umlaut; Faux Cyrillic)

title: Xi (letter)


text:
Stratus cloud

Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds formed by rising thermals. The term stratus describes flat, hazy, featureless clouds at low altitudes varying in color from dark gray to nearly white. The word stratus comes from the Latin prefix strato-, meaning "layer". Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes. Some call these clouds "high fog" for their fog-like form.

Formation
Stratus clouds form when weak vertical currents lift a layer of air off the ground and it depressurizes, following the lapse rate. This causes the relative humidity to increase due to the adiabatic cooling. This occurs in environments where atmospheric stability is abundant.

Description
Stratus clouds look like featureless gray to white sheets of cloud. They can be composed of water droplets, supercooled water droplets, or ice crystals depending upon the ambient temperature.

Species
Stratus nebulosus clouds appear as a featureless or nebulous veil or layer of stratus clouds with no distinctive features or structure. They are found at low altitudes, and are a good sign of atmospheric stability, which indicates continuous stable weather. Stratus nebulosus may produce light rain and drizzle or flakes of snow. Stratus fractus clouds on the other hand, appear with an irregular shape, and forms with a clearly fragmented or ragged appearance. They mostly appear under the precipitation of major rain-bearing clouds; these are nimbostratus and cumulonimbus clouds, and are classified as types of pannus clouds. Stratus fractus can also form beside mountain slopes, without the presence of nimbus clouds (clouds that precipitate), and their color can be from dark grey to almost white.

Opacity-based varieties
Stratus fractus are not divided into varieties, but stratus nebulosus on the other hand, are divided into two. The Stratus opacus variety appears as a nebulous or milky sheet of the nebulosus species, but are opaque enough to block the sun from view. Stratus Translucidus is another variety of the nebulosus species. These clouds are considered more thin than the opacus variety because this cloud is rather translucent, allowing the position of the Sun or Moon to be observed from Earth's surface.

Pattern-based variety
Stratus clouds only have one pattern-based variety. This is the stratus undulatus variety. Mild undulations can be observed from this cloud, only associated by the nebulosus species. Though rare, this cloud formation is caused by disturbances on the gentle wind shear. Stratus undulatus clouds are more common on stratus stratocumulomutatus clouds where the wind is stronger as height increases.

Genitus mother clouds
Stratus cumulogenitus clouds occur when the base of cumulus clouds spreads, creating a nebulous sheet of stratiform clouds. This can also occur on nimbostratus clouds (stratus nimbostratogenitus) and on cumulonimbus clouds (stratus cumulonimbogenitus). Stratus fractus clouds can also form under the base of precipitation-bearing clouds and are classified as pannus clouds. Stratus clouds may also form from formation mechanisms that are not typical for the cloud type, for example, Stratus homogenitus, which are stratus formed by human activity, Stratus cataractagenitus, which are formed from the spray of waterfalls, and Stratus silvagenitus, which are formed by evaporation or evapotranspiration occurring in a forest.

Mutatus mother cloud
Stratus only has one mutatus mother cloud. Stratus stratocumulomutatus clouds occur when stratocumulus opacus patches fuse to create a stratiform layer.

Accessory clouds and supplementary feature
Stratus clouds do not produce accessory clouds, but a supplementary feature praecipitatio is derived from Latin, which means "precipitation". Stratus clouds are generally too low to produce virga, or rain shears that evaporate before reaching the ground, although higher stratus clouds can produce it.

Forecast
A stratus cloud can form from stratocumulus spreading out under an inversion, indicating a continuation of prolonged cloudy weather with drizzle for several hours and then an improvement as it breaks into stratocumulus. Stratus clouds can persist for days in anticyclone conditions. It is common for a stratus to form on a weak warm front, rather than the usual nimbostratus.

Effects on climate
According to Sednev, Menon, and McFarquhar, Arctic stratus and other low-level clouds form roughly 50% of the annual cloud cover in Arctic regions, causing a large effect on the energy emissions and absorptions through radiation.

Cirrostratus clouds

Cirrostratus clouds, a very high ice-crystal form of stratiform clouds, can appear as a milky sheen in the sky or as a striated sheet. They are sometimes similar to altostratus and are distinguishable from the latter because the Sun or Moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent. Cirrostratus come in two species, fibratus and nebulosus. The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around -35 C to -45 C, the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around -47 C to -52 C, the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns. These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present. They are formed by warm, moist air being lifted slowly to a very high altitude. When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds, and rain usually begins 12 to 24 hours later.

Stratocumulus clouds
A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels; but like cumulus clouds (and unlike stratus clouds), they form via convection. Unlike cumulus clouds, their growth is almost completely retarded by a strong inversion, causing them to flatten out like stratus clouds and giving them a layered appearance. These clouds are extremely common, covering on average around twenty-three percent of the Earth's oceans and twelve percent of the Earth's continents. They are less common in tropical areas and commonly form after cold fronts. Additionally, stratocumulus clouds reflect a large amount of the incoming sunlight, producing a net cooling effect. Stratocumulus clouds can produce drizzle, which stabilizes the cloud by warming it and reducing turbulent mixing.

title: Stratus cloud


text:
Nu (letter)

Nu (uppercase Ν, lowercase ν; ni ) is the thirteenth letter of the Greek alphabet, representing the voiced alveolar nasal. In the system of Greek numerals it has a value of 50. It is derived from the Phoenician nun. Its Latin equivalent is N, though the lowercase ($$\nu$$) resembles the Roman lowercase v.

The name of the letter is written νῦ in Ancient Greek and traditional Modern Greek polytonic orthography, while in Modern Greek it is written νι.

Letters that arose from nu include Roman N and Cyrillic script En.

Symbology
The lower-case letter $ν$ is used as a symbol in many academic fields. Uppercase nu is not used, because it appears identical to Latin N.

 * Mathematics:
 * Degrees of freedom in statistics.
 * The greatest fixed point of a function, as commonly used in the μ-calculus.
 * Free names of a process, as used in the π-calculus.
 * One of the Greeks in mathematical finance, known as "vega".
 * The reciprocal of 1 plus the interest rate in finance.
 * The $p$-adic valuation or $p$-adic order of a number.
 * Physics:
 * Kinematic viscosity in fluid mechanics.
 * The frequency of a wave in physics and other fields; sometimes also spatial frequency; wavenumber
 * The specific volume in thermodynamics.
 * Poisson's ratio, the ratio of strains perpendicular with and parallel with an applied force.
 * Any of three kinds of neutrino in particle physics.
 * The number of neutrons released per fission of an atom in nuclear physics.
 * Molecular vibrational mode, $ν_{x}$ where $x$ is the number of the vibration (a label).
 * The true anomaly, an angular parameter that defines the position of a body moving along an orbit (see orbital elements).
 * Biology:
 * A DNA polymerase found in higher eukaryotes and implicated in translesion synthesis.
 * Chemistry:
 * The stoichiometric coefficient.
 * Psychology:
 * The maximum conditioning possible for an unconditioned stimulus in the Rescorla-Wagner model.

Greek Nu/Coptic Ni
Encodings of Greek Nu and Coptic Ni.

BBX Mathematical Nu
These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

title: Nu (letter)