It's Raining Fish and Spiders (19 page)

When hexagonal columns form with hollow conical regions at each end, such forms are called hollow columns. These crystals are small, so you need a good magnifier to see the hollow regions. Columns form near 23°F (-5°C), and near -22°F (-30°C).

Needles

A shift of just a few degrees in temperature changes the pattern of crystal growth from thin, flat plates to long, slender needles. Why this happens is still unknown.

 

Needles are slender, columnar ice crystals that grow when the temperature is around 23ºF (-5ºC). On your sleeve these snowflakes look like small bits of white hair.

Capped Columns

The three small illustrations at the right are three views of a capped column. The top 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 each of the two plates. The other three images are also of capped columns from the side.

 

These crystals first grow as stubby columns which are blown into a region of the clouds where conditions cause crystals to grow into plates. The result is two thin, platelike 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.

Double Plates

The picture at the far right shows a double plate from the side. The center picture shows a double plate with the microscope focused on the smaller plate. In the picture on the left, 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.

 

A double plate is basically a capped column with an especially short central column. One plate grows more quickly than the other, and because the two plates are so close together, the larger one shields the smaller from some water vapor. These crystals are common—many snowflakes that look like ordinary stellar plates are actually double plates if you look closely.

Split Plates and Stars

You may have to stare at these pictures a bit to see how the two distinct pieces fit together. Note how in each case the crystals are connected in the center with short axles.

 

These are forms of double plates, except that part of one plate grows large along with part of the other plate. Split plates and stars, like double plates, are common but often go unnoticed.

Triangular Crystals

Surprisingly, no one knows why snow crystals grow into these threefold 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.)

 

Plates sometimes grow as truncated triangles when the temperature is near 28ºF (2ºC). If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal. These crystals are relatively rare.

12-Sided Snowflakes

These crystals are very rare, but sometimes a snowfall will bring quite a few. The picture in the center shows a 12-sider where the two halves are widely separated.