The shape of snow

Snow is a weather event that is either loved or loathed, with not much middle ground. There is no denying however that a gently falling snow is a beautiful, serene sight. As with most things in nature, snow is a more complex phenomenon than it appears.

Snow starts high in the frozen tops of large thick clouds, where minute ice crystals form from water vapor in temperatures below minus 40 degrees. These crystals gradually grow and cling together until they become heavy enough to fall as snowflakes. If the temperature stays below freezing all the way, the snowflakes reach the ground intact, growing larger on their way down.

Snow crystals come in a variety of forms, including needles, hexagons, columns, prisms, and six-pointed stars. The shape depends on the air temperature in which they fall. The star-shaped crystal what everyone mentally pictures as a “snowflake”, and is technically called a “dendritic crystal.” They form in fairly moist air at temperatures around 5°F. The old saying that no two snowflakes are alike is technically true. The growth of crystals involves a complicated mix of evaporation, condensation, sublimation (the direct conversion of water vapor to ice), and deposition around a tiny hexagonal ice nucleus.

The shape of snow readily demonstrates its origin as water. All snow crystals are six-sided, a shape derived directly from the triangular template of the water molecule. The central oxygen and its two bound hydrogen atoms form a bulbous triangle, the basic unit of all shapes of frozen water.

Dry powdery snow falls when temperatures are so low that the falling crystals do not melt and refreeze when they touch each other and so do not form large soft flakes. Dry snow is very light and ideal for skiing. Wet snow is composed of crystals that have melted and refrozen together to form fluffy, soft flakes. It sticks together and makes good snowballs and snowmen. Around 36 inches of dry snow and 7 inches of wet snow have a water content equal to 1 inch of rain.

Steve Roark is the area forester in Tazewell, Tennessee, for the Tennessee Department of Agriculture, Forestry Division.