What is “Radiational Cooling?”

The coldest part of the day is typically just after sunrise. That’s because the atmosphere will have allowed all the heat from the previous day to radiate upward from ground level. This process, known as “radiational cooling” occurs every night. However, there are certain variables that can help or hinder this process.

Cloud cover, for example, severely reduces the amount of heat that is able to radiate upward into the atmosphere keeping the nighttime temperature warmer.  Being in an urban area also hinders the process as the heat from buildings and automobile traffic keeps the city temperature warmer at night than nearby suburban areas.

Ideal conditions for radiational cooling include clear skies and calm winds coupled with low relative humidity.  Dry air heats up and cools off more quickly than more humid air, especially in rural areas. 

Such was the case yesterday at Dulles Airport in Sterling, Virginia.  Conditions were favorable for radiational cooling Monday night with clear skies, calm winds and low relative humidity.  Yesterday’s low temperature at Dulles Airport was a chilly 42 degrees compared to the average low of 53 degrees on September 23.  Given yesterday’s plentiful sunshine, the high temperature rebounded to a delightful 70 degrees. 

Such cool morning lows, coupled with mild afternoons, are what help the leaves begin to change color in the early autumn.  There will certainly be more such days this fall and you will now have a better understanding of what “radiational cooling” is too the next time you hear the term in a weather forecast.