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! The above table shows the more common and/or distinctive types of snowflakes. Click on the table for a more detailed look, then scroll down this page for examples of the different types. Types of Snowflakes1. 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 (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.
2. 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), as dictated by the snow crystal morphology diagram.
3. Sectored PlatesStellar 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.
4. 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.
5. 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 -- 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.
6. Hollow Columns Hexagonal columns often form with conical hollow regions in their ends, and such forms are called hollow columns. These crystals are small, so you need a good magnifier to see the hollow regions.
7. Needles Needles are slender, columnar ice crystals that grow when the temperature is around -5 C (23 F). On your sleeve these snowflakes look like small bits of white hair.
One of the amazing things about snow crystals is that their growth changes from thin, flat plates to long, slender needles when the temperature changes by just a few degrees. Why this happens remains something of a scientific mystery.
8. Capped ColumnsThese crystals first grow into stubby columns, and then they blow into a region of the clouds where the growth becomes plate-like. The result is two thin, plate-like crystals growing on the ends of an ice column. Capped columns don't appear in every snowfall, but you can find them if you look for them.
9. Double PlatesA 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. These crystals are common -- many snowflakes that look like ordinary stellar plates are actually double plates if you look closely.
10. Split Plates and Stars These are forms of double plates, except that part of one plate grows large along with part of the other plate. The picture at right shows all eight ways to make a split star. Split plates and stars, like double plates, are common but often unnoticed.
11. 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. (Note however that the molecular structure of triangular crystals is no different from ordinary six-sided crystals. The facet angles are all the same.)
12. 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.
13. Bullet Rosettes The nucleation of an ice grain sometimes yields multiple crystals all growing together at random orientations. When the different pieces grow into columns, the result is called a bullet rosette. These polycrystals often break up to leave isolated bullet-shaped crystals.
14. 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. 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.
15. Irregular Crystals The 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.
16. Artificial Snow Snow machines shoot a mixture of water and compressed air out of nozzles. The water comes out as fine droplets, and the air cools as it decompresses, causing the droplets to freeze. A fan blows the ice particles onto the slopes.
Source: http://www.SnowCrystals.comخوب حالا میدونید اسم دونه های برف پست قبلی که به این اسم بود چیه؟ لطفاً پاسخ های خود را تا 12 این ماه به آدرس ما صندوق پستی بفرستید تا به قید قرعه به یکی از آشناهای خودمون بلیط رفت و برگشت تهران، میامی کادو بدیم.
