Getting Frost When the Temperature Isn't at Freezing

A Frost Advisory has been issued for Southern Wisconsin and far Northern Illinois for late Tuesday night, but our overnight low temperature isn't expected to drop below freezing.  So how is it possible that we could get frost?  There will be a couple elements at play tonight, but we also have to look at where our temperature sensors are located.

A strong high pressure system for the middle of May will continue to drop south through Wisconsin.  In Wisconsin temperatures will likely drop into the low 30's, but we probably won't get that cold closer to home.  Skies will clear and the wind will turn very light out of the north.  As drier air moves in our dew points will continue to drop, eventually falling into the middle 30's.  As surface temperatures cool overnight they tend to fall towards the dew point temperature.  So with clearing skies, light winds and lower dew points, temperatures area wide will fall into the middle to upper 30's.  But how can we still get frost?

We have to look at where our temperature sensors (thermometers) are located.  Most backyard thermometers are hanging on our porch or deck wall, away from the ground.  And if you have a weather sensor located inside the house, likely the temperature sensor is just outside the window.  Since cold air is very dense, it sinks to the ground and into low spots.  With temperature sensors located at least a couple feet off the ground, it's not a true measurement of what the actual temperature is at ground level where frost forms.  So this is why your thermometer could be reading a temperature of 37 or 38 degrees, but when you look out the window there is frost on the grass.  I'm sure the next question you may be asking is why we don't put our temperature sensors on the ground.  It's important to keep them slightly elevated so we're not recording any heat radiation that may be coming off of the ground or surrounding buildings or roads.  If that were to occur, then we wouldn't get a true reading of the temperature.