CEDAR RAPIDS, Iowa (KCRG) - It's our number one question right now: Why isn't it snowing? It's cold enough to do so, but won't. It all has to do with how the atmosphere is layered and where things are saturated.
Snowflakes will grow when temperatures are between -10C and -20C, which is 14F to -4F. Anything outside of that range is very small plates, needles, or nothing at all. In our case, the snow growth zone (technically called the Dendritic Growth Zone) is void of saturation, meaning there's no moisture to form snowflakes. So in this case, no matter what the surface temperature, we will not have decent snowflakes today. I have circled that dry layer in the weather balloon image from this morning, launched by NWS.
Instead, we are saturated in the lowest kilometer or so of the atmosphere. Lines that are close together depict this. This is enough to allow for drizzle drops to develop. Because there isn't much of anything to condense to (called Cloud Condensation Nuclei), these drops are staying as drops despite it being below freezing. This saturated layer is also circled. This layer is also nowhere near the snow growth layer in this case, keeping things as drizzle drops.
These drops slowly fall to Earth then and freeze on contact to everything. I mean everything. Right down to your lenses on your eyeglasses. Freezing drizzle leaves a very thin coating behind and is difficult to detect on radar due to the shallow nature of it.
Freezing drizzle is the most common when snow is already on the ground as that traps the coldest air near the surface and allows for faster saturation. This will not be the last of the drizzle this year as plenty of snow and cold is on the way this weekend.