What Determines our Precipitation Types?

We are now into December, and the official start to winter is less than three weeks away.  This is the time of year that we start the transition to more wintry precipitation, which is far more varied than what we see in summer.  December is when, on average, most locations in Connecticut see their first accumulating snowfall.  Snow, however is not the only winter weather hazard that we experience here in Southern New England.  In addition to rain and snow, we can also see sleet and treacherous freezing rain.  So, what determines what type of precipitation we get?

Believe it or not, almost all precipitation, even rain in summer, starts as snow.  This is because the atmosphere is not all one temperature.  As you move up in the atmosphere, you encounter layers of air that are different temperatures.  Usually, the temperatures get cooler the higher you go (this is why the tallest mountains are capped with snow, even in summer when it may be in the 80s at the bottom of the mountain).  When precipitation forms up in the sky, it is usually below freezing, and snow forms.  So, let’s take a look at what causes this snow to become what we see on the ground.

When it rains, even in summer, once snow forms high in the sky and begins to fall, the snow falls into a layer of air that is above freezing, and all of the layers of air stay above freezing all the way to the ground.  The snowflakes melt, and we see rain.  In summer, the lower layers of the atmosphere are always above freezing, which is why we almost never see snow in summer in Connecticut.

Once we get into the later part of fall into winter, many layers of the atmosphere over Connecticut can be below freezing, and we can see a much more diverse array of precipitation.  Which layers are above and below freezing determines whether we see rain, snow, sleet, or freezing rain.

If the entire atmosphere above you is below freezing, from way above to the ground, the snow that forms up in the sky never melts, and we get snow.

Sleet happens when the surface and the height where precipitation forms are below freezing, but there is a shallow layer of air that is above freezing.  The snow that forms high in the sky falls through this warmer layer and partially melts.  It then falls into a thick layer of below freezing air all the way to the ground and refreezes into little balls of ice.  Sleet is sometimes hard to see as it falls, so you might think it is raining, but if you look closely at the ground outside, you will see the sleet pellets bounce when they hit a hard surface.

Freezing rain is the most treacherous of winter precipitation.  This is the ice that forms a glaze which can turn roads into skating rinks and snap tree branches and power lines.  Freezing rain falls through an atmosphere that is similar to the atmosphere that forms sleet, but the warm, above freezing layer is much thicker, with a shallow below freezing layer at the ground.  When the snow from high in the sky falls, it melts in the thick warm layer and turns to liquid rain.  However, when it falls through the below freezing layer near the ground, the layer is so shallow that the rain cools to freezing but doesn’t have time to refreeze before it hits the ground.  When the still liquid raindrop hits the ground, which is below freezing, the raindrop freezes instantly, forming a clear ice glaze.  If freezing rain continues, the glaze can become more than ½” thick.  This glaze quickly becomes very heavy and can cause widespread power outages and closed roads because of the dangerous driving conditions.

So, now you know a little more about the variety of precipitation we see here in Connecticut in winter.  There’s a lot more going on above our heads than you may think!

-by John Conway, StormTeam8 Intern

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