Why does the lowest temperature of the day occur just after sunrise?

It is coldest just about the time of sunrise because this is the hour at which the atmosphere has been without the heat of the sun for the longest time. The lowest temperature of the day usually occurs then. The atmosphere holds in the heat of the day for some time after the sun sets. The heat gradually dissipates over the hours of the evening. By just before sunrise, the heat is gone, and the atmosphere is at its coolest. This is also the time at which the dew or frost point is most apt to be reached, and dew or frost often appears on the ground around this time. It takes a little time before the sun's rays become strong enough to begin heating the atmosphere, allowing the ground to start absorbing the heat again.

weathernotebook.org/transcrip ... old but erroneous proverb states that: it's always darkest just before the dawn. A related belief contends it is always coldest at the same time of day. Hi, I'm 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 dawn's early light. Observations and physical theory, however, show the coldest time of day generally occurs some time after sunrise. To simplify why, let's 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. Okay... that's pretty basic. Between sunset and sunrise, the Earth's 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.