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View submission: Why does it get cold at night ?
"...the atmosphere absorbs a lot of IR already so it's not that."
It's *exactly* that.
Other than escaping gas molecules (which are an extremely minor factor), the only way that heat has to leave the earth is through radiation. All of energy that arrives from solar radiation, as well as all the energy that's produced by nuclear reactions on earth, must eventually leave in the form of radiation, or else our planet would continually get hotter.
Now, it's true that the atmosphere absorbs some of this IR radiation and slows the rate at which it leaves. That's why it's not freezing cold every night. But "some" is not "all". The temperature of the earth equilibrates at the temperatures where it's hot enough to radiate all the incoming radiation out into space. Incidentally, this is why greenhouse gasses are such a big deal. More CO2 and methane in the atmosphere increase the amount of IR radiation the atmosphere catches, which means the planet still warms up during the day, but can't cool down as well at night. This means that the planet has to be hotter into order to shed the same amount of heat. It shifts the equilibrium temperature to something different than we've built our entire civilization around.
On a planet with no atmosphere (or with very little atmosphere), the dark side cools off very quickly when the sun goes down, and the dark surface can become very cold. The dark side of the moon, for example, gets as cold as -250 F. The atmosphere holds the heat in, to some degree, and regulates how cold nights become. Because water vapor is potent greenhouse gas, and because clouds tend to reflect heat back down to the surface, drier areas tend to have bigger temperature swings between day and night. When I lived in the Middle East, one of the warnings about the desert is that you could suffer heatstroke during the day and hypothermia that same night.
By contrast, a planet with a very thick atmosphere would heat up and a heat up until it's finally hot enough to shed the incoming heat through the thick blanket of atmosphere around it. That's also not theoretical. Venus has a surface temperature in the neighborhood of 870 F. With so much IR-absorbing gas, the planet heats up and heats up, until it's finally so hot that not even that thick atmosphere can stop all the IR radiation.
But, yeah, the reason it gets cold at night is because we're radiating heat out into space, and don't have an influx of sunlight to make up for it. How cold it gets depends on a bunch of factors, but when you're facing away from the sun, you're constantly losing energy to the void.
There's nothing here!