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How Much Snow Would All This Rain Be?

by Steve Templeton

KMOV.com

Posted on November 18, 2009 at 1:36 AM

Hello to the 7th graders at Carrollton Middle School!  They'll be following the blog this week and I'll be taking a question and answering it later in the week. 

So, with education in mind, I thought I'd blog a bit more in depth on the topic of how much all this rain we've been having would be in snowfall if it were cold enough?

So, the general ratio we use is 1 to 10.  That means 1 inch of rainfall would be equivalent to 10 inches of snow.  So far we've had about 2.70" of rainfall this week.  So that would be 27" of snow!!!  St. Louis averages 22 inches of snowfall a year, so that would be...well that would be ridiculous from one storm.  In fact, as the air is colder it tends to have less moisture.  So, it's not realistic to compare a rainy period to a snowy period. 

But it turns out we use this 1 to 10 ratio in order to make our snowfall forecasts.  That's because the computer projections we use can tell us how much liquid will precipitate out of a column of air, but not how much snow.  Sorry for the big words, let's simplify...the computers I rely on to project what the future weather looks like will give me rainfall but not snowfall.  So, I need to convert rain to snow in order to make my snowfall forecast for the St. Louis area.

There are very few things in meteorology that are clear, cut and dry (no pun intended). So, it turns out you can't apply the 1 to 10 ratio all the time.  This makes converting rainfall to snowfall even more difficult.  When the air is close to freezing (32 degrees) the ratio is closer to 1 to 7.  When the air is cold but not frigid, like a temperature of 22, the ratio is probably close to 1 to 10.  When the air is really cold, like 10 degrees, the ratio 1 to 15 or higher like 1 to 25 in extreme cases.   To make things more difficult in forecasting, you have to look at the air temperature from the surface and aloft, after all the snow is forming above and falling from above, so that's important.

Why is the rain to snow ratio higher for snow formed in dry snow?

Dry snow (Snow formed in dry air) is less dense than wet snow.  Another way to think of this is to imagine those packing "peanuts" that are so annoying when they get out of the box and all over the floor.  Anyway, when they are dry they are fluffy, not very dense and stack up rather high (like dry snow).  If you were to pour water on them, they would be more dense and shrink down so they don't stack very high (like wet snow).

Class dismissed!

I've always wanted to say that, blogging it is the next best thing.

Meteorologist Steve Templeton 

 

 

 

 

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