These past couple of weeks have been challenging with all the frozen precipitation we’ve gotten. There was a period one morning at my place where it was switching from snow, to sleet, to rain, and back again, all within an hours’ time. I kept looking at the thermometer, which was below freezing but holding steady, and thinking to myself “what is the deal?” Obviously something was going on beyond the temperature of at my patio, so I decided to dig in and figure it out.
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 to the ground. If the atmosphere is above freezing from the clouds to the ground, the snowflakes melt and hit 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 it 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 cold air, which cools it to below freezing, but it remains a liquid through a weird phenomenon called “super cooling”. I numbed my brain trying to understand that, so I won’t go there. When super cooled rain strikes a frozen 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.