Eli5 why the upper atmosphere is cold

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Mostly I’m wondering why it gets colder when you go higher if heat rises? Does it, like, dissipate?

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In order to measure heat, we need to measure the thing that is hot. The upper atmosphere is a lot less dense than the lower atmosphere, so *in a sense*, yes, it does dissipate, but only because the things that are hot (the air molecules) are more spread out.

The upper atmosphere is hot. It’s been heated by the Sun and reaches thousands of degrees.

The lower layers of the atmosphere are cold. You’re probably thinking of being at sea level and then going up a mountain. That’s still low in the atmosphere.

The difference between the top of a mountain and the ground towards sea level is that the lower atmosphere is mostly heated by the ground. The Sun warms up the ground and the ground then heats up the air it’s in contact with.

Light travels to earth from the sun. Our atmosphere or rather air doesn’t absorb visible light as it’s transparent. The earth absorbs the light, causing it to heat up. Conductive heat is the process of something hot heating up something else by directly touching it (like you grabbing a hot pan). So the hot earth directly touches the air molecules above it heating it up. Those molecules heat up the air molecules above them and so forth. So the hottest air is on bottom and gets cooler as it rises. Though air is turbulent (chaotic and moves around) so it’s not perfectly measurable that easily. But as a system, air is typically hotter the closer it is to earth.

Notes: this conductive heat transfer isn’t the only thing responsible for heating you up. You also absorb radiation, aka the sunlight, like the earth does, which heats you up. So you get conductive heat from the air and radiation directly from the sun.

The thermosphere is also hot. Its chemical makeup makes it a protective layer for our earth. Meaning it absorbs uv rays (invisible light). This causes the thermosphere to heat up, similar to how the earth absorbing light causes it to heat up.