Cloudy or Clear? It Makes a Difference!

After a clear to partly cloudy start to the evening, some clouds have drifted back into the Stateline. Meteorologists will often talk about clouds as being an "insulating" factor at night, and this is a prime example. The 10:00pm surface map at left shows our temperature holding at 31 degrees with mostly cloudy skies (the circle near the RFD text is 3/4-full, indicating roughly 3/4 of the sky is cloud-covered). With clear skies, Pontiac, IL is at 23 degrees (the circle near the PNT text is open, indicating little or no cloud cover). The clear skies over Pontiac are allowing radiational cooling, or heat-loss to space, to occur much more efficiently than at Rockford.

Eventually, clouds will again decrease over the Stateline overnight. But a developing south wind will prevent us from falling much below the upper-20s, despite the clearing trend.       --TS