By Steve Roark
Volunteer, Cumberland Gap National Historical Park

With winter coming on, folks start thinking about precipitation of the frozen kind. In what form frozen water come to us meteorologically depends on a whole lot more than what the temperature our thermometer says, and I’m sure you’ve see days when the weather goes from rain to sleet to snow and back again. So let us delve into the frozen world a little while.

What falls out of the sky depends on the temperature of various layers of the air column between the ground and the clouds. In winter all forms of precipitation start out as snow crystals floating around in the clouds. As the crystals bump around they cling to each other to form flakes, which eventually grow heavy enough to fall towards us here on the ground. If the atmosphere is below freezing the whole way, it will remain snow and perhaps accumulate. If the atmosphere is above freezing from the clouds to the ground, the snowflakes melt and hits the ground as liquid rain drops.

Sleet is a hybrid between snow and rain. It occurs when snowflakes high up in the air column fall through a narrow band of warm air that is above a deeper layer of cold air. The snowflakes melt slightly in the warm air, then re-freeze in the lower cold layer and make it to the ground as some form of frozen pellets that bounce off the ground when they hit. Sleet can take on all kinds of shapes.

Freezing rain is the most complex form of precipitation, and the most dangerous as far as hazardous roads and downed utility lines. Snow falls through a cold air layer below the cloud. It then falls through a thick layer of warm air that melts it completely into rain. Near the ground the rain then falls through a thin layer of really cold air, which cools it to below freezing, but it remains a liquid through a weird phenomenon called “super cooling”. I hurt my brain trying to understand that, so I won’t go there. When super cooled rain strikes a cold surface, it instantly freezes and creates a dangerous world of ice

So there you have it. It’s not just the temperature on your thermometer that counts. If the air above us is busy, things get interesting.