Different Types of Wintry Weather

That title just doesn't sound right in the middle of April, does it?  But we had quite the range of wintry weather Wednesday morning and early afternoon.  From heavy sleet, rain, freezing rain, thunder, small hail and eventually snow, Mother Nature gave it all to us.

But what caused the quickly changing weather over northern Illinois and southern Wisconsin?  Well, it all has to do with what the temperatures were like from the bottom of the atmosphere (near the surface) to the very top.  Let's think of the atmosphere as a column of air.  If the column of air above us is below freezing then the precipitation will fall in the form of snow.  But if a 'warm layer' of air is introduced into that column, different types of precipitation will occur.

Air flowing around low pressure systems flows counter-clockwise.  The southerly wind will bring warmer air northward, but it doesn't necessarily happen at the surface.  Sometimes the warm air can ride, or move overtop, the cold air near the surface.  As precipitation falls from the cloud it will initially fall as snow because temperatures at that level are below freezing.  But if the snow falls into a shallow, warm layer of air above the surface the snow will partially melt within that layer.  However, as it moves out of the warm layer it refreezes and will fall as sleet.


If the warm layer of air is a little more deep throughout the atmosphere the snow will completely melt, becoming all liquid.  During freezing rain situations rain falls through the atmosphere but will refreeze on a surface that is below 32 degrees.  And for all rain to form the entire column of air is above freezing.  We had all four, and then some, scenarios Wednesday morning and early afternoon.  But now with the passage of low pressure the transition over to light snow occurred and will continue into the overnight.