Snowflakes why

As a result, the water molecules arrange themselves in predetermined spaces and in a specific arrangement. This process is much like tiling a floor in accordance with a specific pattern: once the pattern is chosen and the first tiles are placed, then all the other tiles must go in predetermined spaces in order to maintain the pattern of symmetry. Water molecules simply arrange themselves to fit the spaces and maintain symmetry; in this way, the different arms of the snowflake are formed.

Howard T. Evans, Jr. Geological Survey, adds a few details: Snowflakes are mysterious things. Their fundamental form derives from the arrangement of the water molecules in the ice crystal. When a liquid freezes, the molecules tend to settle in the lowest-energy state, and that almost always involves some form of symmetry. The higher the symmetry, the more stable the crystal is. Water molecules floating freely in a vapor begin to arrange themselves into a crystalline solid when the temperature drops below freezing.

The two hydrogen atoms of the molecules tend to attract neighboring water molecules. When the temperature thermal motion is low enough, the molecules link together to form a solid, open framework that has a strict hexagonal symmetry. But why are snowflake shapes so elaborate? Nobody has a good answer for that.

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 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. See Snowflake Watching for more about observing snowflakes. 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. 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. Note how the two hollow regions are symmetrical in each column. Sometimes the ends grow over and enclose a pair of bubbles in the ice, as seen in the last picture on the right.

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. Capped Columns These 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.

The first example at right shows three views of a capped column. The first view is from the side, showing the central column and the two plates edge-on. The other two views show the same crystal from one end, with the microscope focused separately on the two plates.

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. These crystals are common -- many snowflakes that look like ordinary stellar plates are actually double plates if you look closely. 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. This hexagon is the second side of a double plate, connected to the main plate by a small axle. 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.

Ultimately, it is the temperature at which a crystal forms — and to a lesser extent the humidity of the air — that determines the basic shape of the ice crystal. Thus, we see long needle-like crystals at 23 degrees F and very flat plate-like crystals at 5 degrees F. The intricate shape of a single arm of the snowflake is determined by the atmospheric conditions experienced by entire ice crystal as it falls. A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way.