Fresh blanket of snow at McKeon Reserve’s Branch Stream Fen.
The Science of Snowflakes
Photo: Kevin McKeon
By Kevin McKeon, Maine Master Naturalist
We haven’t had much of it yet this winter, but snow is a fact of life in Maine. It’s also a more interesting phenomenon than a glance out the window at a snowy landscape might suggest.
When water molecules freeze around a tiny particle — like a speck of dust or piece of pollen — the chemical properties of hydrogen dictate the formation of a six-sided water crystal. As more water droplets attach to it and freeze into more crystals, a six-sided cluster of crystals grows into a snowflake. That’s why all snowflakes have a six-armed, branched structure.
Almost all precipitation begins as snow. As it falls to the ground from the icy-cold upper atmosphere, cloud structure determines what happens to that snowflake. When lower cloud temperatures are 32ºF or lower, it remains snow. Rain happens as the snowflake passes through atmospheric layers above 32ºF and remains above that temperature all the way to the ground, melting into a drop of water. Freezing rain occurs when that melted drop of water passes through a very thin layer of 32ºF or lower air at ground level, doesn’t have time to freeze, but freezes to whatever it lands upon. When this freezing-at-ground-level happens for several hours, it’s called an ice storm.
Sleet is caused by the snowflake partially melting into slush, then passing through a cold layer, re-freezing into an ice pellet before landfall. Hail forms within certain powerful thunderclouds — cumulonimbus (“rain cloud heap”) — with multiple levels of temperature. These “Kings of Clouds,” up to 75,000 feet tall, have powerful updrafts that carry the melted snowflake water droplet up and down through itself, freezing, colliding with other water droplets that freeze, forming an ice ball — a process called accretion — until ultimately becoming too heavy for the thundercloud to hold, falling to the ground as a hailstone.
Sunlight strongly diffuses from snow, which is 90% air, so by the time the light reaches our eyes, we see it as the combination of all colors, which is white. But as the snowpack deepens, the added weight squeezes some air out, increasing density. This causes the longer red wavelengths to get absorbed, giving snow a slight bluish tint. (A stick pushed into deep, weeks-old snow will create this blue tint at the hole’s bottom.) In glaciers, this increased compression eventually forms thick ice holding very little air, creating the blue ice sometimes seen in icebergs. This light behavior is like sunlight causing the sky to appear blue. Various sources of dust, algae, and pollutants can cause snow to appear in shades of orange, black and red.
Snow makes a great insulator. Some of nature’s critters have learned to burrow into it to stay warm at night, and below it for their winter dormancy sleep. Some humans do it too: Igloo interiors can be 100 degrees warmer than the outside air from trapped body heat! And all that air trapped within a fresh powdery snowfall makes for a very quiet, sound-dampened winter walk.
Snowflakes fall from 1 to 9 mph, taking about an hour for a flake to leave its cloud and reach the ground. One snowflake was reported to be 15” across, but they almost always break into smaller pieces as they fall, with the average size less than half an inch. Other planets have snow made from C02 (Mars), ammonia (Jupiter), sulfur (Io, a moon of Jupiter), and methane (Neptune, Uranus, and Pluto). The “snow monkeys” of Japan, Japanese macaques, have been observed making and playing with snowballs, stealing them from each other!
Too cold to snow? Well … as water molecules in warm air gain heat energy, they move around faster, tending to remain in a vapor state. As air cools, molecule movement slows, heat energy is released, and water vapor condenses into water droplets. The TV weather reporters describe this as water being “squeezed out” of the atmosphere. Snow can occur even at incredibly low temperatures, if there is some source of moisture and some way to lift the air to a cooler atmospheric level. So, while it can be too warm to snow, it cannot be too cold to snow. But cold air holds less water vapor, resulting in fewer water droplets being squeezed out as the air rises, making the formation of snow less likely.
Blue sky: https://sanfordspringvalenews.com/out-in-the-woods-52/
Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net
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