I’ve always been curious as to why calm winds make for colder nights. It seemed so backwards to me. Wind aids evaporative cooling, right? Why wouldn’t wind make things cooler?

Well, after a little Googling I found the answer. Its due to temperature inversion! During the day, the sun heats the ground, which is denser than air and thus warms more quickly. At night, the situation is reversed. The ground radiates its heat back into the air, which soon becomes warmer than the ground. At night the air temperature actually rises with altitude.

A calm night lets the colder, denser air settle to the ground, without mixing with the warmer air above. Thus, surface temperatures are lower when the wind is calm.

Thanks to N.C. State’s Cooperative Extension site and the University of Wisconsin’s Frost Forecast page, The Weather Doctor, and the good ‘ol Wikipedia for clearing that up.