Others have covered the fact it’s because of air pressure but haven’t fully answered why that is the way it is.
It’s simple really.
The force of gravity is also at play. As you go higher up, gravity gets weaker as you get farther from the earth’s centre.
And it is that gravitational force that increases the air’s density, same reason why if you keep going.g down in the water, the water gets denser.
For the heat to move around you need to be in a sort of goldilocks zone of density.
It needs to be dense enough that the fluid molecules can move around and spread the convention energy around… but not so dense they can’t move much either.
Furthermore there’s actually a couple different layers of our atmosphere.
First at our level is the troposphere, where heat is absorbed into the ground itself and radiation back out as well as the perpetual heat from the earth’s core, and reflected off the ground too (visible light).
The troposphere is warm and gets colder as you get farther away from the earth’s surface, naturally. That heat is absorbed by the air itself so, as you get farther away it gets colder as it has more air to travel through.
Up higher is the Stratosphere, where it’s ice cold and the air this out.
However we get a sudden uptick in temp as we go even higher into what is called the Stratopause, ack to briefly warm temperatures between the Stratosphere and the Mesosohere. Why? How?
Simple, this is the little sweet spot Ozone molecules hang out, forming a protective convenient bubble around the earth. Ozone absorbs Ultraviolet light from the sun and turns out that stuff is HOT, so there’s a band of a hot zone right above and below the Ozone layer. Think of it as a toasty little bubble around us.
Above is the mesosphere which cools off again and gets back to being really frosty quickly.
Then we hot the mesosphere, which is effectively the point when the atmosphere is so thin it stops protecting and is the “outside” of our protective blanket.
You can imagine this like earth being wrapped in a blanket, and the mesosphere is everything outside the blanket. Without any protection you are subject to the unbridled radiation of the sun which means you go back to being really toasty, as you get a bot higher you are effectively in space now and will soon enough hit Temps that just cook you alive in a minute or two. Really bad sunburn zone.
So to answer the question overall:
Hot air rises… but only when there is air to rise.
Top of the mountains just don’t have enough air anymore for it to really rise much more. It still dies but the hot air rising effect just gets weaker and weaker as the air gets thinner due to less gravity.
Boozilla@lemmy.world 1 year ago
Twitches@lemm.ee 1 year ago
This is great thank you! Very interesting
lurker2718@lemmings.world 1 year ago
While I agree in general, one point is a bit to simplified in my opinion
Less molecules mean less heat, it has nothing to do with the temperature, if you just decrease the density by removing half the molecules, you have the same temperature.
It cools down because it expands adiabatically. Consider a very thin balloon filled with air which is warmer than the surrounding. This now rises up, but as it does, the pressure decreases, causing the balloon to expand. During this expansion, the balloon transfers energy away from itself, because it has to push away air, to make room for expanding in the surrounding. This work cools the air inside the balloon. Assuming the air inside is dry, it would cool around 10 °C per km it rises. Now if you think about it, the balloon just stopped the inside from mixing with the outside. If you look at a large “piece” of air, it does not mix very fast, so you can remove the balloon and just consider what happens with warm air heated from the ground.
Now this does not mean, it has to be cooler when higher up. The same points hold, inside a house, but there it is often warmer when higher.
The best explaination is when looking where the heat comes from and goes too from the air. The atmosphere is mostly heated from the surface of earth, so the bottom and cooled from the upper layers. So naturally it gets hotter where it is heated. The question is now by how much? There are three modes of heat transfer in the atmosphere: radiation, conduction and convection. The first two are very slow. Connection is fast but has limits. Consider the piece of air, if it rises, it cools. So at some place it may be the same temperature as the surrounding air, so it stops rising. This means the convection works only when the air gets cooler by 10 °C/km going up (~6.5°C when the air is moist and precipation happens). So this temperature gradient is observable very often.
Turun@feddit.de 1 year ago
But does the energy to expand the balloon not just come from the pressure? Pressure has units of energy per volume btw.
mx_smith@lemmy.world 1 year ago
Here is a great visual representation