Outdoor America 2021 Issue 1
Technically, snowflakes are ice crystals. Sometimes these crystals fall from the clouds as tiny, individual flakes. You can see their delicate patterns when they land on your jacket. Other times, the crystals stick together as they fall, forming bigger flakes, the kind that blanket the ground.
Snow happens when water vapor in the air, a gas, freezes to a tiny dust particle, converting directly into ice, a solid, without first becoming liquid raindrops. (When raindrops freeze, sleet forms.)
Due to their molecular make-up, all snowflakes start as six-sided crystal plates high up in the atmosphere. As they fall, more water vapor molecules attach to them, causing “arms” to grow from each side of the hexagon. Their unique crystalline patterns are a result of the various temperatures and humidity levels they tumble through. They never look alike because no two flakes fall precisely the same way. That’s also the reason the six arms of some flakes aren’t always perfectly symmetrical, and why some flakes have complex structures with fern-like side branches and others have simpler patterns.
What’s more, it takes 100,000 water vapor molecules to make the average snowflake. The odds are extremely low that so many molecules would arrange themselves into identical lattices, another reason why each snowflake is unique.
Snow insulates the ground, preventing soil from blowing away and mitigating how deep the ground freezes. When it melts, it provides water for plants, wildlife and humans.
IT'S A FACT
Subtropical Florida and Louisiana are the least snowy states in the country, each rarely receiving more than a dusting.
Many states watch their snowpack levels closely to determine how much water will be available during non-winter months, a measurement called “snow water equivalent” (SWE). According to USDA’s Natural Resources Conservation Service (NRCS), SWE is determined by the density of the snowfall, which can range from five percent when the air temperature is 15 degrees Fahrenheit or colder, to 20 percent when it’s barely below freezing.
For example, if 10 inches of light, fluffy snow melts down to one inch of water, that 10 inches of snow has an SWE of one inch. Likewise, if 10 inches of heavy, wet snow becomes two inches of water, that 10 inches of snow has an SWE of two inches. The higher the SWE, the more water resulting from a snowstorm. NRCS measures SWE to forecast streamflow.
Snow depth, which is not the same as snow water equivalent, is another measurement of interest to not only scientists, but also skiers, snowmobilers, snowshoers and others who rely on snow for their winter activities. After snow lies on the ground, its density increases due to settling, wind packing, melting and recrystallization. Some western states closely monitor snow depth since up to 75 percent of their water supply is directly derived from snowmelt.
The amount of accumulated snow, or snowpack, that mountainous regions receive, plays an important role in the annual water cycle, not only locally, but also downstream where snow might not fall.
As part of the natural water cycle, melting snow is part of the global movement of water.