Snow Can Fall, Even When Temperatures Are Above 32.

4:44 PM, Mar 20, 2013   |    comments
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In order for snow to fall, we know that the temperature has to be 32 degrees or colder... cold enough for water to freeze. But if you take a look at Topper's forecast for tonight, you'll see there's a chance for snow showers or flurries, with an overnight low of 33 degrees in the District. So why, you may wonder, is it possible to have snow falling with an air temperature above the freezing point? The answer is in the clouds!

Snowflakes form from crystallized water droplets in the cloud layer. The temperature in the cloud is well below freezing. So, as the snowflake descends toward the Earth's surface from the cloud, it will remain frozen initially.

Whether or not the snowflake melts is determined by the airmass underneath the cloud layer. If the temperature is below freezing everywhere except right at the surface (or the lowest 100 feet or so), then the snowflake will remain frozen until it touches a warmer surface. You've probably seen this many times, on parking lots and on your car, when the snowflake begins melting immediately after hitting the surface. This is because the snowflake literally didn't have time to melt before touching down! Also, different surfaces can have different temperatures, which is why you'll often see snow sticking to grass and tree branches, but melting on black pavement.

If there is a pocket of air underneath the cloud (but above the ground) which is warmer than freezing, then the snowflake will at least partially melt before it hits the ground. In these situations, the precipitation could change into any of a number of things, depending on the depth of the cold and warm layers. You could end up with rain, sleet, graupel, rimed ice pellets, or freezing rain! Pretty complicated, huh?

It's common to see large chunks of snow fall when temperatures are above freezing. One of the reasons is because the larger snowflakes are heavier, and therefore have a stronger gravitational pull. This allows the flakes to move toward Earth faster. So, the snowflakes less time to melt on the way down!

Of course, when the temperature at the surface is above the freezing point, the snow will melt quickly upon landing. The only exception to this is when the snow is falling faster than it can melt.



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