Dear Tom,
Why does the temperature always seem to drop a degree or two at dawn?
—Marilyn Wenzel, Oak Park
Dear Marilyn,
Atmospheric heat is constantly radiating from the Earth’s surface. During daylight hours, more heat arrives from sunlight than the amount being lost, so air temperatures rise; the opposite occurs at night. Transition periods occur at sunrise and sunset.
At dawn, more heat is still being lost than is arriving and air temperatures continue to fall, as they have been doing through the proceeding night. The process reverses at dawn, but the reversal takes a little while. Heat input from sunlight begins to increase, a little at first when the sun first rises, but increasing as the sun rises higher into the sky. Heating usually overcomes heat loss within a couple hours after sunrise.