Many people over the last several years have asked me if the coldest part of the day occurs just before sunrise.
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 makes things a little easier, let's consider a clear, calm night with no cold fronts and no temperature inversions.
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 30 minutes after sunrise.
Of course if cloud cover rolled in around midnight that would change things up a bit. If the clouds remained over the Valley well after sunrise, then the coldest part of this day would probably occur around the time or shortly after the clouds rolled through.
Cloud cover acts like a blanket trapping any heat that we gained from the day.
It's the same concept as pulling the covers over you in the middle of the night, keeping your body heat inside and nice and warm.
