Why Does The Temperature Drop After Sunrise?

When I was a child and really interested in weather I would either look at a thermometer to try go see how cold it got or listen to NOAA weather radio and hear it throughout the area.

I noticed even then that the temperature tended to drop after the sun came up. It to me seemed counterintuitive at the time. Why should the temperature drop after the warming sun is above the horizon? Everyone has felt the warmth of the sun.

The first thing to understand about this phenomenon is that colder air is denser and heavier than warmer air with all other things being equal. That means the air closed to the ground is the coldest if there’s not any wind. Windy nights keep the air more mixed so there isn’t as much of a cold layer near the surface. Thermometers tend to be up off the ground so you can see them or in my case with my personal weather station it’s higher than that. That means they’re not necessarily measuring the coldest of the air.

When the sun first comes up it’s very weak, so it’s not really warming the air yet but it is starting to warm up the ground. That can get the air mixing a little and the coldest layer near the ground starts to mix with and cool the warmer air just above the ground a bit.

Sometimes if it’s clear and calm you can even feel that it’s colder if you walk into a small valley. Did you ever notice in the summer the dew tends to be lower to the ground or even more noticeable when there’s an early or late frost. The frost tends to be on the grass and things near the ground. That’s the same idea at work of the air being coldest near the ground.