Published: 2007-11-25 13:03:00
Updated: 2016-11-02 17:59:07
Posted November 25, 2007
Updated November 2
By Mike Moss
Mike Moss: The coldest time of day can shift considerably if we have fronts moving through, varying winds and cloud cover or precipitation starting and stopping during the night or morning. However, assuming we have a more tranquil pattern, as we frequently do, with fair skies and light winds late at night and early in the day, the lowest temperature usually occurs a little after sunrise, sometimes as much as a half hour to an hour later.
The reason for this is that the earth's surface continually emits longwave infrared radiation. During the day, it also receives incoming shortwave radiation from the sun. At night, outgoing energy exceeds incoming, causing temperatures to fall. This process typically continues until the sun rises and climbs high enough for incoming radiation to become stronger than the outgoing, so that temperatures begin to climb again. In addition, there can be an effect on calm, clear mornings where a very shallow layer of especially cold air near the ground is mixed to a few feet higher when the sun first begins to warm the surface, causing temperatures a few feet off the ground (where we are more likely to notice it and where "official" temperature readings are taken) to take an added dip at that time.
Full question from Robert: When does the coldest temperature occur in a 24 hr day?
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