Recently I noticed that the street was wet, but there was no rain. Where did the water come from?
Sam Morteri, Chicago
The wet streets resulted from condensation of moisture in the air directly onto the street surface. This can happen on mild winter days when the air’s saturation temperature is at least several degrees higher than the surface temperature of the concrete in the street. Condensation will occur when mild, moist air sweeps in so quickly — usually just a matter of hours — that the concrete, which can retain a chill from earlier cold weather, has not had a chance to warm up. This phenomenon does not occur during the summer, when relatively intense sunlight keeps the ground quite warm. In the southern Plains, it can be a real problem for motorists. Following a frigid Arctic outbreak, the return of moist air from the Gulf of Mexico can be so swift that street temperatures are still below freezing — and the streets frost over.