Cooler air has begun making its presence felt across the northern portion of the country the last few weeks. Ever wonder how seasonably mild temperatures during the day, suddenly dip into the frosty 30s overnight? Some may be thinking a cold front moved through, but many times this cooling happens with high pressure in control. Give up? The answer is radiational cooling.
(Above) Frost on WeatherWorks Meteorologist Mike DiDomizo's windshield.
As we lose solar heating at night, surfaces cool rapidly as outgoing radiation is released. This radiational cooling is optimized when skies are clear and heat is allowed to escape freely into the atmosphere. A layer of clouds, on the other hand, help to reflect some of the radiation back down to the surface, keeping temperatures a bit warmer.
Winds are another important factor to consider for nightime cooling. As the surface cools, it becomes colder than the air above it. With rather light or calm winds, this results in an inversion: a layer in the atmosphere where temperatures increase with height, as opposed to the typical decrease of temperatures with height. However, windy nights allow for the mixing of layers of air, moderating temperatures and preventing maximum cooling.
Autumn is one of the best times for radiational cooling. The jet stream begins to dip further south, allowing for cooler and relatively drier air masses from Canada to reach into the continental United States (which radiate nicely). Although scraping a thin layer of ice off your car may not be the best start to your morning, be sure to give an appreciable nod to this potent cooling process!