When does the Overnight Low Temperature Occur on a Windless Day?

As the summer begins to fade we have begun to have the first cool nights of the season. When the sky is clear, the winds are calm, and the ground is dry as the sun sets, the conditions are perfect for an interesting phenomena to occur the next morning.

If you pay careful attention to the temperatures in the hour surrounding sunrise the next morning, you will most likely observe that the lowest temperature will be recorded some minutes after the sun rises. This repeatable observation requires explanation.

All objects both absorb and emit energy in the form of radiation. The surface of the Earth is one such object and it absorbs a lot of solar energy from sunlight during the daylight hours. We all know that this causes the surface temperature to increase. But, while the ground is absorbing energy from the sun, it is also emitting radiant energy (infra-red energy).

As long as the amount emitted is less than the amount absorbed, the temperature of the surface increases. After sunset, absorption ceases but emission continues. Consequently, since the emission is greater than the non-existent solar absorption, the surface temperature drops. This condition persists throughout the night.

When the sun finally rises and sends the first, low-intensity beams of radiation to the surface, it is still true (for a short time) that the meager amount of solar absorption is smaller than the emission from the surface.

Until the absorption is equal to the emission, the temperature continues to fall, even if only slightly.

Hence, the lowest temperature on such a morning occurs after sunrise.

Category: Meteorology
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