WRAL WeatherCenter Blog

I have always heard that the coldest time of the day is when the sun just does break over the horizon. When I go hunting in the morning, it really does feel like it gets colder at that time. Is this true? If so, why?

Posted February 26, 2007

MIKE MOSS SAYS:      Thomas,      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 morning where a very shallow layer of especially cold air very 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) causing an added dip in the temperature at that time.
Comments

Please with your WRAL.com account to comment on this story. You also will need a Facebook account to comment.

Oldest First
View all