Few things brighten a typical Montana winter day like a little snowfall. Right when you feel that you can't take another grey day, a couple of inches of heaven fall to raise your spirits. But just how do these floating flakes come to be? Snow obviously comes from clouds, but not just any cloud.
The conditions must be right. Snow crystals will begin to form in clouds that have temperatures of anywhere between 32 and -39 degrees Fahrenheit. These clouds contain microscopic water droplets so small that several thousand could fit on the head of a pin. This means that what we're seeing when we look at a cloud is a tremendous concentration of these little droplets in one place. Along with the water droplets found in a cloud are tiny particles of dust and salt from the earth's surface and oceans. These particles are transported high into the atmosphere by constant global winds, where they eventually mix with clouds. Once inside a cloud, the particles cool and become a sort of nucleus upon which water molecules can build. Soon the water molecules begin to freeze and form ice crystals, giving birth to fledgling snowflakes. The ice crystals continue to form and grow until they become heavy enough to fall from the cloud, and voila, it's snowing.
Sometimes, if enough ice crystals are falling at once, they'll start to collide with one another and break apart. This creates more bases upon which ice crystals can build, and the result is a chain reaction that soon has you standing in the middle of a major blizzard.
"Field Notes" is produced by the Montana Natural History Center.