| Types of Snowflakes |
 Simple PrismsA hexagonal prism is the most basic snow crystal geometry (see the Snowflake Primer). Depending on how fast the different facets grow, snow crystal prisms can appear as thin hexagonal plates, slender hexagonal columns (shaped a lot like wooden pencils), or anything in between. Simple prisms are usually so small they can barely be seen with the naked eye. |
   The examples at right show two stubby prisms and one thin plate. Snow crystal facets are rarely perfectly flat, being more typically decorated with various indents, ridges, or other features |
 Stellar PlatesThese 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), |
 Fernlike Stellar DendritesSometimes 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 -- the water molecules are lined up from one end to the other. |
   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, where you sink to your knees while skiing, is made of stellar dendrites. These crystals can be extremely thin and light, so they make a low density snowpack |
 12-Sided SnowflakesSometimes 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 |
 Rimed CrystalsClouds are made of countless water droplets, and sometimes these droplets collide with and stick to snow crystals. The frozen droplets are called rime. All the different types of snow crystals can be found decorated with rime. When the coverage is especially heavy, so that the assembly looks like a tiny snowball, the result is called graupel. |
   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 |
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  Irregular CrystalsThe most common snow crystals by far are the irregular crystals. These are small, usually clumped together, and show little of the symmetry seen in stellar or columnar crystals |
نوشته شده توسط سيده نسيم نعمت اللهي در سهشنبه 28/6/1385 و ساعت 1:12 عصر | نظرات ديگران()
