During the daytime hours, mixing occurs in the atmosphere thanks to sunlight.
The sun warms the Earth and everything on it (always remember everything on the Earth is heated differently).
As the surface warms from the sun, the warmed earth and everything on it warms the air directly above the surface.
Just like a hot air balloon, the air rises. As the air rises, it mixes with colder air aloft, warming the different layers of the atmosphere.
At night when the sun goes down, this mixing cycle shuts down. This allows what meteorologists call radiational cooling where the temperatures tend to drop.
This cycle continues on a daily basis but cloud cover, warm fronts and cold fronts can always interfere with this process resulting in unsettled weather.
Since the mixing doesn't start the moment the sun pops over the horizon the coldest temperature readings often occur near or even sometimes slightly after sunrise.
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