Dear Tom,
Why doesn’t frost form under trees? Craig Ward, Geneva IL
Dear Craig,
Frost forms when air near the ground has cooled below its saturation temperature (which causes water vapor in the air to begin condensing) and has also cooled to below freezing (which means frost, rather than dew, will form).
At night, most objects get cold because they are radiating heat energy. The temperature of objects that do not contain much stored heat (like blades of grass) will fall rapidly and, in turn, chill the air immediately in contact with them. When saturation occurs at below-freezing temperatures in the chilled air, frost forms.
However, under a tree the dense canopy of leaves interferes with the radiational-cooling process. The tree leaves actually radiate heat downward, and grass underneath will accumulate little if any frost.
———-
Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
Send your questions to:
ASK TOM WHY, 2501 Bradley Place, Chicago, Il. 60618
e-mail:asktomwhy@tribune.com
