Coldest After Dawn
Mon Oct 10, 2005
Listen in RealAudio
An old but erroneous proverb states that: itís always darkest just before the dawn. A related belief contends it is always coldest at the same time of day. Hi, Iím Bryan Yeaton for The Weather Notebook.
It seems logical to think that air temperature bottoms out just before sunrise and then begins to warm with dawnís early light. Observations and physical theory, however, show the coldest time of day generally occurs some time after sunrise.
To simplify why, letís only consider a clear night, with no passing fronts.
All objects gain heat from outside sources and radiate it away at the same time. When more radiant heat is lost than gained, the object cools. When more heat is gained than lost, it warms. If they are balanced, the temperature remains constant. OkayÖ thatís pretty basic.
Between sunset and sunrise, the Earthís surface gathers no solar energy but continues to radiate away its stored heat. During the night, the surface also loses radiant heat faster than it steals heat from other sources, and thus its temperature, and that of the air in contact with it, drops steadily.
At dawn, when the first light beams across the landscape, the incoming solar radiation is very weak. It does not yet have enough strength to counter all the heat escaping from the surface. As a result, the surface continues to lose heat for some time following sunrise, and the air temperature continues to fall.
At some point, the solar rays shine strongly enough to counter the heat loss. The gain-loss balance is shifted, and the air finally begins to warm up. As a rule of thumb: the coldest temperature is about an hour after sunrise.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is funded by Subaru of America. We are a program of the Mount Washington Observatory.
Why is coldest after dawn?