Posted by Snow Addicted Wednesday, August 7, 2013
Take a minute to have a good look and try to understand what snow is.
A snowflake is either a single ice crystal or an aggregation of ice crystals which falls through the Earth's atmosphere. They begin as snow crystals which develop when microscopic supercooled cloud droplets freeze. Snowflakes come in a variety of sizes and shapes. Complex shapes emerge as the flake moves through differing temperature and humidity regimes, such that individual snowflakes are nearly unique in structure. Snowflakes appear white in color despite being made of clear ice. This is due to diffuse reflection of the whole spectrum of light by the small crystal facets.
The video tracks formation of snowflakes from their origins in bits of dust in clouds that become droplets of water falling to Earth. When the droplets cool, six crystal faces form because water molecules bond in hexagonal networks when they freeze. It explains that ice crystals grow fastest at the corners between the faces, fostering development of the six branches that exist in most snowflakes. As snowflakes continue to develop, the branches can spread, grow long and pointy, or branch off into new arms.
If you look closely at falling snow, you can see a great many different crystal shapes. There's a lot more to see than you might think!
Types of Snowflakes!
"Simple Prisms"!A hexagonal prism is the most basic snow crystal geometry. Depending on how fast the different facets grow, snow crystal prisms can appear as thin hexagonal plates, slender hexagonal columns, or anything in between. Simple prisms are usually so small they can barely be seen with the naked eye.
"Stellar Plates"!These common snowflakes are thin, plate-like crystals with six broad arms that form a star-like shape. Their faces are often decorated with amazingly elaborate and symmetrical markings. Plate-like snowflakes form when the temperature is near -2 C (28 F) or near -15 C (5 F)
"Sectored Plates"!Stellar plates often show distinctive ridges that point to the corners between adjacent prism facets. When these ridges are especially prominent, the crystals are called sectored plates. The simplest sectored plates are hexagonal crystals that are divided into six equal pieces, like the slices of a hexagonal pie. More complex specimens show prominent ridges on broad, flat branches.
"Stellar Dendrites"!Dendritic means "tree-like", so stellar dendrites are plate-like snow crystals that have branches and sidebranches. These are fairly large crystals, typically 2-4 mm in diameter, that are easily seen with the naked eye. Stellar dendrites are clearly the most popular snow crystal type, seen in holiday decorations everywhere. You can see these crystals for yourself quite well with just a simple magnifier
"Fernlike Stellar Dendrites"!Sometimes the branches of stellar crystals have so many sidebranches they look a bit like ferns, so we call them fernlike stellar dendrites. These are the largest snow crystals, often falling to earth with diameters of 5 mm or more. In spite of their large size, these are single crystals of ice. Some snowfalls contain almost nothing but stellar dendrites and fernlike stellar dendrites. It can make quite a sight when they collect in vast numbers, covering everything in sight. The best powder snow is made of stellar dendrites. These crystals can be extremely thin and light, so they make a low density snowpack.
"Double Plates"!A double plate is basically a capped column with an especially short central column. The plates are so close together that inevitably one grows out faster and shields the other from its source of water vapor. The result is one large plate connected to a much smaller one. The first picture at right shows a double plate from the side. The second picture shows a double plate with the microscope focused on the smaller plate. In the third picture, note the slightly out-of-focus hexagon that is about one-sixth as large as the main crystal.
"Triangular Crystals"!Plates sometimes grow as truncated triangles when the temperature is near -2 C (28 F). If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal. These crystals are relatively rare. Surprisingly, no one knows why snow crystals grow into these three-fold symmetrical shapes.
"12-Sided Snowflakes"!Sometimes capped columns form with a twist, a 30-degree twist to be specific. The two end-plates are both six-branched crystals, but one is rotated 30 degrees relative to the other. This is a form of crystal twinning, in which two crystals grow joined in a specific orientation. These crystals are quite rare, but sometimes a snowfall will bring quite a few. The picture at the far right shows a 12-sider where the two halves are widely separated.
"Radiating Dendrites"!When the pieces of a polycrystal grow out into dendrites, the result is called a radiating dendrite (also called a spatial dendrite). The image shows a fernlike stellar dendrite with two errant branches growing up out of the main plane of the crystal.
"Rimed Crystals"!Clouds are made of countless water droplets, and sometimes these droplets collide with and stick to snow crystals. The frozen droplets are called rime. When the coverage is especially heavy, so that the assembly looks like a tiny snowball. The first two pictures at right have relatively light rime coverage. The final example is completely covered with rime, but you can still see the six-fold symmetry of the underlying stellar crystal.
"A general look at the different types of falling snow "!
Source: Snow Crystals