(c) Anthony K. Grafton 2003

Wet or Dry Snow?

Snow is, of course, just frozen water. But how much water is in a given amount of snow? Well, the answer is, “it depends”.

Snowflakes are a beautiful example of the complexity that can occur in nature during a very simple process: the freezing of water vapor in the air. Because of the underlying shape of the water molecule, every snowflake has hexagonal (six-sided) symmetry. But because of the slightly different, and constantly changing, conditions in which each snowflake is formed, no two snowflakes will ever be exactly the same.

Two primary factors that influence the shape of each snowflake are temperature and humidity. Other things such as collisions between flakes, slight melting, and air-flow can influence the shape, too. All these aspects taken together can result in several basic snowflake shapes, including plates, needles, hollow cylinders, and dendrites. The dendrite is the classic snowflake shape we often imagine.

Depending on what kind (shape) of snow is falling, we usually classify it as “wet” or “dry” in an attempt to describe how much moisture is in the snow. Normally, people use the ten-to-one rule of thumb and estimate that ten inches of snow is equivalent to one inch of rain. But the truth is that the ten-to-one rule only works under certain conditions, for certain snowflake types. Other conditions can give dramatically different results.

The next time is snows, borrow an old cup from Mom or Dad and put it outside. Measure the depth of the snow (in centimeters) in the cup. Bring the cup inside and after the snow melts, measure the depth of water in the cup. Send your results in to Science Corner, and a future column will present everyone’s findings.