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Wegener–Bergeron–Findeisen process

The Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron and Walter Findeisen), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions where the ambient vapor pressure falls between the saturation vapor pressure over water and the lower saturation vapor pressure over ice. This is a subsaturated environment for liquid water but a supersaturated environment for ice resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition. If the number density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough.

The Bergeron process, if occurring at all, is much more efficient in producing large particles than is the growth of larger droplets at the expense of smaller ones, since the difference in saturation pressure between liquid water and ice is larger than the enhancement of saturation pressure over small droplets (for droplets large enough to considerably contribute to the total mass). For other processes affecting particle size, see rain and cloud physics.

History
The principle of ice growth through vapor deposition on ice crystals at the expense of water was first theorized by the German scientist Alfred Wegener in 1911 while studying hoarfrost formation. Wegener theorized that if this process happened in clouds and the crystals grew large enough to fall out, that it could be a viable precipitation mechanism. While his work with ice crystal growth attracted some attention, it would take another 10 years before its application to precipitation would be recognized.

In the winter of 1922, Tor Bergeron made a curious observation while walking through the woods. He noticed that on days when the temperature was below freezing, the stratus deck that typically covered the hillside stopped at the top of the canopy instead of extending to the ground as it did on days when the temperature was above freezing. Being familiar with Wegener's earlier work, Bergeron theorized that ice crystals on the tree branches were scavenging vapor from the supercooled stratus cloud, preventing it from reaching the ground.

In 1933, Bergeron was selected to attend the International Union of Geodesy and Geophysics meeting in Lisbon, Portugal where he presented his ice crystal theory. In his paper, he stated that if the ice crystal population was significantly small compared to the liquid water droplets, that the ice crystals could grow large enough to fall out (Wegener's original hypothesis). Bergeron theorized that this process could be responsible for all rain, even in tropical climates; a statement that caused quite a bit of disagreement between tropical and mid-latitude scientists. In the late 1930s, German meteorologist Walter Findeisen extended and refined Bergeron's work through both theoretical and experimental work.

Required conditions
The condition that the number of droplets should be much larger than the number of ice crystals depends on the fraction of cloud condensation nuclei that would later (higher in the cloud) act as ice nuclei. Alternatively, an adiabatic updraft has to be sufficiently fast so that high supersaturation causes spontaneous nucleation of many more droplets than cloud condensation nuclei are present. In either case, this should happen not far below the freezing point as this would cause direct nucleation of ice. The growth of the droplets would prevent the temperature from soon reaching the point of fast nucleation of ice crystals.

The larger supersaturation with respect to ice, once present, causes it to grow fast thus scavenging water from the vapor phase. If the vapor pressure $$p$$ drops below the saturation pressure with respect to liquid water $$p_w$$, the droplets will cease to grow. This may not occur if $$p_w$$ itself is dropping rapidly, depending on the slope of the saturation curve, the lapse rate, and the speed of the updraft, or if the drop of $$p$$ is slow, depending on the number and size of the ice crystals. If the updraft is too fast, all the droplets would finally freeze rather than evaporate.

A similar limit is encountered in a downdraft. Liquid water evaporates causing the vapor pressure $$p$$ to rise, but if the saturation pressure with respect to ice $$p_i$$ is rising too fast in the downdraft, all ice would melt before large ice crystals have formed.

Korolev and Mazin derived expressions for the critical updraft and downdraft speed:

 * $$u_{up} = \frac{p_w - p_i}{p_i} \eta N_i \bar{r_i} \,$$
 * $$u_{dn} = \frac{p_i - p_w}{p_w} \chi N_w \bar{r_w} \,$$

where η and χ are coefficients dependent on temperature and pressure, $$N_i$$ and $$N_w$$ are the number densities of ice and liquid particles (respectively), and $$\bar{r_i}$$ and $$\bar{r_w}$$ are the mean radius of ice and liquid particles (respectively).

For values of $$N_i \bar{r_i}$$ typical of clouds, $$u_{up}$$ ranges from a few cm/s to a few m/s. These velocities can be easily produced by convection, waves or turbulence, indicating that it is not uncommon for both liquid water and ice to grow simultaneously. In comparison, for typical values of $$N_w \bar{r_w}$$, downdraft velocities in excess of a few $$m s^{-1}$$ are required for both liquid and ice to shrink simultaneously. These velocities are common in convective downdrafts, but are not typical for stratus clouds.

Formation of ice crystals
The most common way to form an ice crystal starts with an ice nucleus in the cloud. Ice crystals can form from heterogeneous deposition, contact, immersion, or freezing after condensation. In heterogeneous deposition, an ice nucleus is simply coated with water. For contact, ice nuclei will collide with water droplets that freeze upon impact. In immersion freezing, the entire ice nucleus is covered in liquid water.

Water will freeze at different temperatures depending upon the type of ice nuclei present. Ice nuclei cause water to freeze at higher temperatures than it would spontaneously. For pure water to freeze spontaneously, called homogeneous nucleation, cloud temperatures would have to be -35 C. Here are some examples of ice nuclei:

Ice multiplication
As the ice crystals grow, they can bump into each other and splinter and fracture, resulting in many new ice crystals. There are many shapes of ice crystals to bump into each other. These shapes include hexagons, cubes, columns, and dendrites. This process is referred to as "ice enhancement" by atmospheric physicists and chemists.

Aggregation
The process of ice crystals sticking together is called aggregation. This happens when ice crystals are slick or sticky at temperatures of -5 C and above, because of a coating of water surrounding the crystal. The different sizes and shapes of ice crystals fall at different terminal velocities and commonly collide and stick.

Accretion
When an ice crystal collides with supercooled water droplets it is called accretion (or riming). Droplets freeze upon impact and can form graupel. If the graupel formed is reintroduced into the cloud by wind, it may continue to grow larger and more dense, eventually forming hail.

Precipitation
Eventually this ice crystal will grow large enough to fall. It may even collide with other ice crystals and grow larger still through collision coalescence, aggregation, or accretion.

The Bergeron process often results in precipitation. As the crystals grow and fall, they pass through the base of the cloud, which may be above freezing. This causes the crystals to melt and fall as rain. There also may be a layer of air below freezing below the cloud base, causing the precipitation to refreeze in the form of ice pellets. Similarly, the layer of air below freezing may be at the surface, causing the precipitation to fall as freezing rain. The process may also result in no precipitation, evaporating before it reaches the ground, in the case of forming virga.

title: Wegener–Bergeron–Findeisen process


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Naturmuseum Senckenberg

The Naturmuseum Senckenberg (SMF) is a museum of natural history, located in Frankfurt am Main. It is the second-largest of its kind in Germany. In 2010, almost 517,000 people visited the museum, which is owned by the Senckenberg Nature Research Society. Senckenberg's slogan is "world of biodiversity". , the museum exhibits 18 reconstructed dinosaurs.

History
In 1763, Johann Christian Senckenberg donated 95,000 guilders–his entire fortune–to establish a community hospital and promote scientific projects. Senckenberg died in 1772. In 1817, 32 Frankfurt citizens founded the non-profit Senckenberg Nature Research Society, Senckenberg Gesellschaft für Naturforschung (SGN), which is a member of the Leibniz Association. Soon after, Johann Georg Neuburg donated his collection of bird and mammal specimens to the society. The Naturmuseum Senckenberg was founded in 1821, just four years later. Initially located near the Eschenheimer Turm, the museum moved to a new building on Senckenberganlage in 1907. In 1896 a mummified Egyptian child in their collection (inventory number ÄS 18) was the subject of the first mummy X-ray. During World War II, the building was partly destroyed. However, the exhibits had been evacuated before.

Building
The neo-baroque building housing the Senckenberg Museum was erected between 1904 and 1907 by Ludwig Neher outside of the center of Frankfurt in the same area as the Johann Wolfgang Goethe University, which was founded in 1914. The museum is owned and operated by the Senckenberg Nature Research Society. The exhibition area covers 6000 m2.

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Expansion plans
, the museum has been expanded to 10000 m2. New planned sections: Human, Earth, Cosmos, Future.

Directors

 * 2021–present Brigitte Franzen

Collections
The Senckenberg Museum Frankfurt has a large collection of animal, plant and geology exhibits from every epoch of Earth's history.

Diplodocus
Main attraction is a Diplodocus from Bone Cabin Quarry, Wyoming, donated by the American Museum of Natural History on the occasion of the present museum building's inauguration on 13 October 1907,   The 18 m mounted skeleton with additions contains bones of three different sauropod genera (Diplodocus and closely related Apatosaurus and Barosaurus).

Psittacosaurus
, a key holding is a fossilized Psittacosaurus (specimen SMF R 4970) from Liaoning, China, with clear bristles around its tail and visible fossilized stomach contents. The specimen was first reported in 2002. The exact date and locality of the discovery within Liaoning is unknown. A controversial debate about the legal ownership arose. In 2021, researchers described its cloaca in more detail and found similarities with the body outlet of birds. In 2022, for the first time a belly button was found in a dinosaur fossil. A physical life reconstruction of the animal was prepared by paleoartist Robert Nicholls.

Edmontosaurus and Triceratops
Another originals are an Edmontosaurus annectens mummy (specimen SMF R 4036) from Lance Formation, Wyoming. and two Triceratops skulls. The museum bought the three specimen from fossil collector Charles Hazelius Sternberg and his sons in the early 20th century. The museum also exhibits a cast of a complete Triceratops, the museum's mascot.

Casts
Big public attractions also include the casts of Tyrannosaurus rex and Diplodocus longus (in front of the museum), an Iguanodon, the crested Hadrosaur Parasaurolophus and an Oviraptor.

Further casts or single bones:

 * Archaeopteryx lithographica
 * Brachiosaurus brancai
 * Compsognathus longipes
 * Euoplocephalus tutus
 * Plateosaurus engelhardti
 * Protoceratops
 * Sinosauropteryx prima
 * Stegosaurus stenops
 * Amargasaurus cazaui
 * Argentinosaurus huinculensis
 * Austroraptor cabazai
 * Carnotaurus sastrei
 * Eoraptor lunensis
 * Giganotosaurus carolinii
 * Kritosaurus australis
 * Panphagia protos

Birds
A living reconstruction of the extinkt dodo and many other stuffed birds are shown in a permanent exhibition in the upper level. Additionally, the museum owns a large and diverse collection of birds with 90,000 bird skins, 5,050 egg sets, 17,000 skeletons, and 3,375 spirit specimens (a specimen preserved in fluid). This is 75% of the known bird species, only a minor part is exhibited.

Reptiles
Anaconda is one of the oldest and most popular exhibits. Since the remodeling finished in 2003, a new reptile exhibit addresses both the biodiversity of reptiles and amphibians and the topic of nature conservation.

Messel research
The museum houses many originals from the nearby Messel pit, Germany's first UNESCO World Natural Heritage Site, among them a predecessor to the modern horse that lived about 50 million years ago and stood less than 60 cm tall. In 2015, researchers found an foal fetus in the body of the petrified primeval horse mare. Also primates, crocodiles, bats, snakes, turtles and other fossils were found at Messel pit.

Mammals
Display collections full of stuffed animals are arranged in the upper levels; among other things one can see one of twenty existing examples of the quagga, which has been extinct since 1883.

The mammal collection focuses on bats, primates, rodents, and insectivores (not exhibited).

Human evolution
Unique in Europe is a cast of the famous Lucy, an almost complete skeleton of the upright, 1 metre tall, hominid Australopithecus afarensis. The exhibition also includes reconstructions of the heads of human ancestors.

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

title: Naturmuseum Senckenberg