It wasn't all rain falling from the skies this morning

Many folks around north-central Illinois reported sleet, or ice pellets, with some of the heavier showers shortly after 11am Wednesday morning.  It wasn't freezing rain, because in order to get freezing rain you need temperatures at the surface to be either at, or slightly below or above 32 degrees.  Our surface temperature was in the middle to upper 40's.

So, what happened in the atmosphere to cause this 'winter type' precipitation? 

Have you ever heard the term 'Evaporative Cooling'?  How about this, when you get out of the shower or swimming pool, do you notice how when you first step out of the water you feel a little chilly?  That's because the water is evaporating off of your skin.  Evaporation is a cooling process and causes that chill you feel when the air is relatively dry and you get out the water.

The same process can kind of be applied to our atmosphere.  When looking at the amount of moisture we had in our atmosphere, or looking at our dew point numbers, we noticed that they were very dry.  Just take for example the image on the left.  This is what we call an atmospheric sounding from Davenport's National Weather Service upper air balloon launch at 7am.  The green line represents the dew point, and the red line represents the temperature.  Do you see how at roughly about 800mb the green line quickly shifts to the left?  Well, all throughout the atmosphere the air is dry.  And the further away the temperature and dew point lines are to each other, the drier the air.

So, let's apply this to what happened today.  When the rain started to move into Northern Illinois, some of it may have initially evaporated due to the drier air.  Over time, the temperature above started to cool, causing it to fall closer to 32 degrees.  That's evaporative cooling.  As the air above cooled, it caused freezing levels to drop low enough to allow for those ice pellets to mix in with the heavier rain showers.

Pretty cool, right?  Well, maybe not so cool with the weather today, but it's neat to see how our atmosphere works!