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How can snow accumulate when it isn't freezing?

Trees and ground covered with snow in Louisville's Shelby Park.
Ryan Van Velzer
Snow in Shelby Park in Louisville

Every week Wave 3 meteorologist Tawana Andrew breaks down what we know and what we don't about the climate and weather here in Louisville.

Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I am joined by WAVE 3 meteorologist Tawana Andrew. Good morning, Tawana.

Tawana Andrew: Good morning. And today's a little special when it comes to Science Behind the Forecast. Because we're answering a viewer question this round.

BB: Yes, a question that came in just a few days ago. And we're going to talk about how you can get snow accumulation even if it isn't necessarily freezing. What do we need to know?

TA: So Greg did ask about that accumulation. So shout out to Greg, with how can snow accumulates when air and ground temperatures are above freezing. And yes, air and ground temperatures are so vital to snow forecasting, you have to have air temperatures at or below 32 degrees for snow to actually form. Now snow can reach the ground when temperatures are above freezing. And the way this works is as snowflakes start to fall, they begin to melt in those above freezing temperatures, that actually creates a bubble of colder air because evaporative cooling, which drops the temperature of the air immediately surrounding the snowflake, that happens as the snowflake begins to melt. And the evaporative cooling slows down the melting process, allowing it to continue to snow while temperatures are warmer than the freezing mark. And in a situation with heavy snow evaporative cooling can help to drop temperatures to 32 degrees in less than half an hour. So that's snowfall rate is super important. And by the way, snow falling through dry air will cool the atmosphere faster compared to if that air is already saturated. Now I mentioned snowfall rates, that's super important as well, because that is how you can get snow to accumulate, let's say 36, 38 and even 40 degree temperatures. Once you start to get that snowfall rate of one inch per hour or higher, then you can start to see that accumulation happening on warmer surfaces because the snowfall rate is greater than the melting rate. Now once the snow stops, then the melting will take over once again, everything that's on the ground will start to slowly but surely fade away. And you're more likely to have these higher snowfall rates in systems where there are substantial amounts of moisture. Let's say if we have moisture coming in from the Gulf of Mexico, you're more likely to have higher snowfall rates in that situation than you would per se in let's say a clipper where that moisture isn't really available in that area. So we'll have to watch for that. Without enough moisture in the mid to upper levels of the atmosphere, you just can't have that snow forming, there must also be a significant amount of rising air across our area, like with a cold front or where with an area of low pressure. In addition to that you need cold temperature. So you have to have that layer of cold air in the atmosphere all the way just to above the ground. Otherwise, everything is going to melt before it actually reaches the ground. And it's in this same way that you can have rain with temperatures below freezing because we've all heard a freezing rain, right. So when you have the snow falling through a warm layer, it'll start to melt and it will remain as rain all the way to the ground. And then at that ground level, you'll have sub freezing surfaces, and then you'll have the freezing rain. The last factor in all of this is how long it's actually falling because the most significant melting will start to happen when the heavy still begins to fall. Now bursts of snow not going to be as significant as hours upon hours of heavy snow. So you got to keep that in mind as well. So hopefully I answered Greg's question and anybody else's question who are worried about snowfall writes in snowing in warmer temperatures.

BB: I think you definitely did. And now we all have a better understanding of how snow can accumulate even if it isn't necessarily freezing down on the ground. And we know this now, thanks to the latest edition of Science Behind the Forecast with WAVE 3 meteorologist Tawana Andrew. Thanks for the knowledge, Tawana.

TA: Of course.

This transcript was lightly edited for clarity.

Bill Burton is the Morning Edition host for LPM. Email Bill at bburton@lpm.org.