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How much snow would this rain make?

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It’s a common question meteorologists hear when forecasting rain in winter. “How much snow would this be if it wasn’t rain?”. The problem with that question is that it really doesn’t have an answer, mainly for two reasons.

Reason number 1: The snow to water ratio isn’t set in stone. The average is around 10 to 1, meaning if you melt 10” of snow you’ll end up with 1” of water. But as we know snow comes in a variety of shapes and textures. There is light-fluffy snow, heavy-wet snow, and the hard little pellets of snow to name a few. Heavy-wet snow, often falling with surface temps in the 20s and low 30s can have a ratio as low as 5 to 1. Light and fluffy snow made where many of the flakes contain dendrites (the snow crystals that stick together well) can have a snow to water ratio up to 40 to 1! That means 1” of water would make 40” of snow! Of course this never happens because light-fluffy snow is made at colder temperatures.

This brings us to reason number 2: Cold air cannot hold as much water as warm air. Think back to July 22, 2010 when 5.61” of rain fell on Milwaukee. Now imagine if that was snow with an average 10 to 1 ratio. We would have been bombarded with 56” of snow, right? No way. The temperature that day was 86 degrees. If you took that airmass and cooled it to 32 degrees where snow could live, you would have to get rid of most of the moisture in the air. If my math is right 86 degree air can hold over 7 times as much water as 32 degree air (for weather buffs, a saturation mixing ratio of 27.77 vs. 3.86 at MKE 990mb pressure).

(Below: Brittney and I sharing some laughs and coffee with Brrrt the Snowstick. Brrrt hasn’t had a lot of snow to measure this season).

But what about an air temperature that’s a little above freezing? Well let’s use Wednesday’s example where we may see (at most) 1” of rain on a 48 degree day. How much easier is it for 48 degrees to hold moisture vs. 32 degrees? Well for our elevation 48 degree air can hold almost twice as much as 32 degree air. So if you REALLY want an answer about Wednesday’s rain, let’s just say the same storm system at 16 degrees cooler would bring ½” of water instead of an 1”. And IF it fell as all snow, and IF that snow had an average 10 to 1 ratio, we would get 5” of snow. So there’s your answer… with a few qualifiers.