Amazing Materials in Nature – Snow
If you are a winter enthusiast, we are off to a great start this year. Snowshoers, skiers, and snowmobilers have had excellent snow conditions. To add to the great snow conditions, we have enjoyed relatively mild temperatures. So how does snow form and what are the different types of snow?
Atmospheric conditions affect how snow crystals form and what happens to them as they fall to the ground. Recall that a snowflake begins to form when extremely cold water droplets freeze onto pollen or dust particles in the atmosphere. These particles act as the foundation or growing point of the ice crystals. As the ice crystals fall to the ground, water vapor freezes onto the primary crystal, building new crystals – the six arms of the snowflake. Snowflakes are always six sided because of how water molecules bond - each with one oxygen and two hydrogen atoms – hydrogen bonding together in the most efficient way.
There are four types of snow crystals.
The first of snow crystal are snowflakes, which are single ice crystals or clusters of ice crystals that fall from a cloud. We have all seen them as single snow crystals falling from the sky, or larger clumps of these crystals that can accumulate very quickly.
The second type of snow crystal is called hoarfrost. Hoarfrost is the deposition of ice crystals on a surface when the temperature of the surface is lower than the frost point of the surrounding air. I think we are all familiar with the dew point in the warmer months. The frost point is a similar process when temperatures fall below the freezing point. In this process, moisture goes directly from vapor to solid, skipping the liquid phase. Hoarfrost is usually composed of interlocking ice crystals, and tends to form on objects of small diameter that are freely exposed to air, such as wires, tree branches, and plant stems. We can all image the picture postcard image of a tree covered in hoarfrost on a calm winter’s morning.
The third type of snow crystal is called graupel. Graupel consists of snowflakes that become rounded, opaque pellets ranging from 2 to 5 millimeters in diameter. They start as ice crystals that fall through supercooled cloud droplets, which are below freezing but remain a liquid. The cloud droplets then freeze to the crystals, forming a lumpy mass. Graupel is sometimes mistaken for hail, but tends to have a texture that is softer and more crumbly. Graupel is sometimes called snow pellets.
The fourth type of snow crystal are called polycrystals. Polycrystals are a collection of many different ice crystals that have stuck together as opposed to water vapor condensing around one particle.
Snow on the ground, called snow cover or snowpack, is the total of all the snow and ice on the ground. It includes both new snow and previous snow and ice that have not melted. The snowpack may assume different qualities depending on local temperature changes, whether winds blow the snow around, or how long the snow has been on the ground. New snow is a recent snow deposit in which the original form of the ice crystals can be recognized. Old snow indicates deposited snow whose transformation is so far advanced that the original form of the snow crystals can no longer be recognized. Perennial snow is snow that persists on the ground year after year. After last year’s record snowfall in the area, I thought we might have some perennial snow at Kemp Station, but it did manage to melt before winter came again.
One final interesting note is that snow only appears white in aggregate. If you look at an individual snowflake, the crystal is clear, just as you would expect ice to be. However, light is scattered and bounces off the many ice crystals in snow. The reflected light includes all the colors, which, together, look white.
Take the opportunity to get outside and enjoy this year’s snow in our beautiful Northwoods. Snow is another example of our many renewable natural resources.
Photo: Kemp Station’s Boathouse in its winter blanket of snow.