Tuesday, January 27, 2009

Precipitation types 101

With the impending winter storm that includes a forecast of several different precipitation types, I thought it would be an appropriate time to discuss the various precipitation types and how each occurs, with the help of some graphics from USA Today. We'll start with the easy ones.

In the wintertime (and any other time for that matter), rain occurs when precipitation falls from clouds through air that is above freezing all the way to the surface, or at least any sub-freezing air is very limited in depth and the temperature of that air is not too cold.


Snow is exactly the opposite - precipitation falls through air beneath the cloud layer that is below freezing, or the depth of that freezing layer is sufficient to maintain ice and snow crystals all the way to the ground. As happens many times in the Mid-South, we get some snow when the surface temperature is as high as 36-37 degrees. For this to happen, the depth of that above-freezing layer has to be very shallow, otherwise the snow would melt before reaching the ground and rain would result. See the graphic below.


Sleet (or ice pellets) and freezing rain are terms that are typically interchanged, incorrectly I might add, but both can contribute to an "ice storm." They form through different processes, however. In the graphic below, freezing rain results when precipitation falls through a warm layer that melts it, creating rain. Once the rain nears the ground, it encounters a very shallow (sometimes a few hundred feet thick is all) freezing layer and the liquid precipitation freezes on contact with a surface. This produces a glaze of ice that is very dangerous as accumulations can cause downed trees and power lines and very slick road conditions.

Sleet, on the other hand, is produced by liquid precipitation that falls through a deeper sub-freezing layer than in the freezing rain scenario, causing re-freezing and producing ice pellets (sleet). Sleet is already frozen before reaching the ground and typically does not accumulate on trees and power lines as readily as freezing rain, unless it is coming down hard enough and in cold enough temperatures at the surface.

Hopefully this helps explain the differences between the various precip types. Obviously, an icy condition is more dangerous than snow and snow is definitely more fun to play in during and after the winter storm!

4 comments:

sudo said...

Well... interesting weather is a break for the humdrum but... the forecast looks just a little too pact and exact. I'm betting there's gonna'be a likely different scenario than what is now in the forecast. And the reason I'm saying that with no computer models to back me up???? Because that's what ALWAYS seems to happen in Memphis!!!

Daniel

memwxnet said...

Could be Daniel! My philosophy, as you know, is to provide the best and most accurate info I can based on what I know at the time. I'd rather go out there on that proverbial limb and bust than be wishy-washy and "right" every time. Here's to a big bust and a bigger snow! ;-) Of course, if I'm wrong, this IS Memphis and it'll end up being all rain with a few flakes tomorrow morning.

--Erik

jacksonweather said...

Erik - nice touch of explanation and graphics. With you on that philosophy, too. Been out there on the limb alone before. Have scars from the past from falling off! ;-)
Don't like the wishy-washiness, even though at times you have to be! Eddie

sudo said...

Erik,

I'm just playing devil's advocate.You're the professional.. I'm the gadfly. But how many years have we been talking snow in Memphis? And never getting any of it to speak of? I'm about to become a believer in "global warming"

Daniel