Mark Cassino - The Story of Snow: The Science of Winters Wonder

When a speck gets cold enough, water vapor will stick to it. If you had a microscope that could see such small things, here is what you would see

• Water vapor sticks to the cold speck, making the speck wet.
• More water vapor sticks to the wet speck, forming a water droplet.
• The droplet freezes into a ball of ice.
• More water vapor sticks to the ball of ice, and it grows into a hexagon-shaped ice crystal.
• Water vapor continues to stick to the crystal. Faster growth on the corners causes six branches to sprout.
• The branches keep growing, sprouting little arms of their own
• and a beautiful snow crystal is born!

These photographs of real snow crystals are shown much larger than their actual size. The crystals were collected during many different snowfalls.

As the snow crystal gets bigger and heavier, it starts to fall to earth. It keeps growing as it falls through its cloud, taking on its own special shape. The shape depends on how wet the cloud is and how cold it is. A snow crystal can start to grow one way, but then grow another way when it passes through a wetter or colder part of its cloud. The crystal stops growing soon after falling below the clouds.

Parts of a snow crystal can break during the fall to earth, causing the arms to look different.

One common snow crystal shape is the star. Star-shaped snow crystals usually have six arms reaching out from a center point. The center point is the home of the speck that started the crystal. The six arms look alike, but they are almost never exactly alike.

Star-shaped snow crystals are called dendrites (which means tree-like). They form when a cloud is full of moisture, and when the temperature hovers around 5 degrees Fahrenheit (15 degrees Celsius).

This is the simplest kind of plate crystal, a hexagon. Plates form when theres not enough moisture in the cloud for stars to form, and when the temperature conditions are a few degrees warmer or colder than the temperature range that stars require.

Plate crystals are thin like star crystals, but they dont have arms. The simplest kind of plate is a hexagon with six straight sides. More complicated plates have points where arms almost grew.

Simple plate crystals are much smaller than stars. They can be as wide as 1 millimeter, but theyre usually a lot smaller.

The points on this plate crystal are the beginnings of arms that were just starting to develop when the crystal fell out of its cloud and stopped growing.

Column-shaped snow crystals are shaped like pencils. Theyre not flat like stars and plates. Columns can form high in the clouds and at very cold temperatures. They are very tiny, and when they fall, they make for very slippery snow.

A column has six sides. These are the three types:

These are the smallest type of column.	These are longer and more common than solid columns.	The caps on each end of these columns can be plate crystals or star crystals.

Capped columns like this one develop when a column crystal moves into a part of its cloud where the temperature is right for plates or stars to grow at the ends. The two end caps can grow to different sizes, as you can see here.

If you think of a star crystal as a clock, the arms of a star crystal can point to 2, 4, 6, 8, 10, and 12 oclock. Only those times!