When does the overnight low temperature occur on a windless day?

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When does the overnight low temperature occur on a windless day?
sunrise through window frost
Frosty sunrise by Marilylle Soveran

As summer begins to fade, the first cool nights of the season start. When the sky is clear, the winds are calm, and the ground is dry at sunset, the conditions are perfect for a counter-intuitive phenomena. If you pay careful attention to temperature in the hour surrounding sunrise, you will likely observe that the lowest temperature next morning occurs some minutes after the Sun rises!

This repeatable observation requires explanation.

All objects both absorb and emit energy as radiation. The surface of the Earth absorbs a lot of solar energy during daylight hours. We all know this raises the surface temperature, but while the ground is absorbing this energy, it is also emitting radiant energy as infra-red radiation. As long as less is emitted than absorbed, the surface temperature increases.

After sunset, absorption ceases but emission continues, so surface temperature drops; this cooling continues through the night. When the sun finally rises and sends its first, low intensity beams of radiation to the surface, that meager amount of solar absorption is, briefly, less than the emission from the surface.

Therefore, temperature continues to fall, even if only slightly, until the absorption equals the emission. Hence, the lowest temperature on such a morning follows sunrise.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.