Deth

When I was a child and truly interested in the weather, I would either look at a thermometer to 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. Back then, it seemed counterintuitive to me. 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, all other things being equal. That means the air closer to the ground is the coldest if there’s no 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 air.

When the sun first comes up, it’s weak, with the light being mostly scattered before it reaches the ground since it passes through much more atmosphere. That’s why sunrises (and sunsets, for that matter) tend to be orange. So, it is not really warming the air yet, but it is starting to warm up the ground. The ground is darker and absorbs more of the energy in the sun’s light. The atmosphere is transparent, so it mostly passes through. 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. When the sun is so low in the sky it just doesn’t have enough energy yet to offset the mixing. It also isn’t able to warm the ground level air enough yet to do much more than some slight mixing.

Sometimes, if it’s clear and calm, you can even feel that it’s colder if you walk into a small valley. You can also feel it with a mountain breeze after the sun sets.Did you ever notice that 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 about the air being coldest near the ground.