Clouds are a combination of microscopic water droplets evaporated by energy from the sun and condensed when they meet cold air and billions of minute particles of dust and salt lifted by wind from the surface of the earth and sea. When the temperature inside a cloud is between thirty-two and minus forty degrees, molecules of water gather on the particles, freezing and building snow crystals. Because water molecules are triangular in shape, snow crystals are always six-sided. When the crystals become too heavy to float in the cloud, they fall as snowflakes.
There are seven common shapes a snow crystal can take, depending on the temperature and humidity of the air in which it forms: hexagonal plate
crystals, the most common, are armless; stellar
crystals are classic snowflakes with arms that radiate from the center; columns
are hexagonal tubes with flat or pointed ends; capped columns
are hollow tubes with plate crystals on the ends; needles
are long, slender crystals with pointed ends that readily combine, spatial dendrites
are three-dimensional, six-sided stars; and irregular
, the catch-all category that includes graupel or tapioca snow. A crystal commonly passes through a number of atmospheric conditions before landing, often forming combination crystals.
Carol McFeeters Thompson is a regular contributor to Outdoor
Illinois and the Site Interpreter at Weldon Springs State Park