Think of the atmosphere as a layer cake. The bottom of the cake is very dense and the higher up you go, these layers get less and less dense. Up on a mountain, the air is very thin.
In other words, there are fewer air molecules per cubic foot (volume of air). The molecules are farther apart and can hold less heat energy. Because “heat” is what we say when we mean molecules are moving around. The more they move, and the more molecules there are, the more heat you have.
It’s also helpful to know that the air is heated by the ground and oceans. The heat from our sun mostly passes right through the atmosphere, not directly warming the air up very much. But the surface of the planet will warm up wherever the sun is shining on it. And in turn, the warm ground or the warm surface water then gradually warms the air from the bottom up. (This is because heat is transferred in different modes: radiant, convection, and conduction.)
And the warm air does indeed rise. As it rises, it gradually spreads out and cools off again. Some of the heat even radiates back out into space.
There are “fountains” of air constantly circulating throughout the atmosphere, and this creates weather patterns. It tends to snow on mountains because the warm air has carried some moisture with it on its way up. As it cools and thins, it can’t carry the moisture any more, and the moisture precipitates out. Which is why we call it precipitation whenever it snows, rains, sleets, etc.
So by the time air reaches a high mountaintop, it’s probably going to be cool or even frigid cold.
This is also why hotter regions, like the southern US, tend to get very humid in the summer. The warm air can carry a lot of moisture, and there is a lot of warm surface water. Our sweat is less efficient when the air is moist, because it takes longer to evaporate and carry the heat away with it.
Deserts have few water sources. So they also have hot dry air, and much less humidity, and therefore little to no precipitation. But they also get cold at night, because there’s very little humidity to hold the heat overnight.
All of this is to illustrate the complex interactions between the sun, the atmosphere, and water (or lack of it) on the surface, and humidity in the air.
Inside an older building you’re more likely to experience warmer air on higher floors than lower floors because the air is trapped in a nearly closed system and hot air rises. Of course, HVAC engineers try to compensate for this in modern buildings.
While I agree in general, one point is a bit to simplified in my opinion
In other words, there are fewer air molecules per cubic foot (volume of air). The molecules are farther apart and can hold less heat energy. Because “heat” is what we say when we mean molecules are moving around.
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.
But does the energy to expand the balloon not just come from the pressure? Pressure has units of energy per volume btw.
I think it is actually the other way around. You can consider the air inside the balloon to have internal energy from the heat. And additionally you have to make room for the balloon in the atmosphere, so you have removed the atmosphere from the volume the balloon takes, which also needs energy. If you consider both you arrive at the concept of enthalpy (H = U + pV), which is very useful for reactions in the atmosphere as pressure is constant. For this example it is not that useful as outside pressure changes when the balloon rises.
Another way to see it, the pressure has no “real” energy. In a ideal gas, the only energy comes from the kinetic or movement energy of the atoms. Each time a gas molecule is hits the balloon envelope it transfers some momentum. The cumulative effect of the constant collisions is the pressure of the gas. If the balloon is now expanding slowly, each collisions also tranfers some energy, in sum building the work the system has to do to the atmosphere. Leading to a decrease in internal, so “real” energy in the balloon. This corresponds to a decrease in temperature.
Each time a gas molecule is hits the balloon envelope it transfers some momentum.
I see! Thank you very much!
If we assume the balloon model and the sides expand then each collision of a molecule inside the balloon with the outer wall will leave it with less speed and therefore lower energy and therefore a lower temperature.
As a consequence, gas expanding in a vacuum does not cool off, because it has nothing to transfer the energy to!
Balloons are open. Most typically do not expand but the excess air just escapes out the bottom. Basically they will rise till the overall weight matches that if what they displace.
There are more efficient balloons that do expand and can attain same great heights. Far more than conventional aircraft even. But that expansion is mostly due to excess material in the construction and little from stretching. Thus the pressure difference is minimal while the volume increase significantly with altitude.
I can’t even criticize this comment, because you wouldn’t understand why.
So thanks for the effort I guess? But we’re not discussing real balloons.
This is great thank you! Very interesting
I saw a great one-liner, and two megalogs, but no Goldilocks-sized answer, so here’s my attempt.
As air rises, the weight of air above it (all the way to space) is less, so it’s less squashed, letting it expand.
It expands by pushing out on all the air around it, and every time an air molecule bumps a neighbouring bit of air away, but isn’t bumped back so hard (so it expands), it loses a bit of energy - i.e. heat.
So as some air goes up, it expands and loses heat; or as it sinks, it squashes and gets more heat.
This is adiabatic expansion.
Appendix:
This might beg the question of why higher air isn’t just heated by neighbouring expanding air, making up for its original loss. I think that can be answered by saying overall the top air is squashing the bottom air, so overall the top is cooler. Is that fully right? Right now I feel there’s multiple ways to think about it and I can’t write any clearly without long rambling!
Calvin’s Dad: “Because the hot air rose all the way to space and isn’t on the mountain anymore.”
That must be why it’s still warm in aeroplanes that fly higher than Everest.
Also why cold air balloons never took off
Oh, they did take off… They just went “ppthhlppppsssssssspplpssseeeeeeeethstsplpssseeeeeeeesssssssss” and fell to the ground again.
Fun fact: the temperature of space is actually thousands of degrees, but you would still freeze to death without protection.
(The actual answer is that atmospheric pressure is just as important as temperature in determining how “cold” something is)
The temperature of space actually is close to absolute zero, so quite cold. As the heat balance of an object there is mainly dominated by radiation, the object looses heat (~T⁴) but almost has no heat input from the surrounding if not directed to a star in sufficient proximity, e.g. the sun. The surface exposed to sunlight however, can become really hot.
While it’s true that hot air rises, causing lower temperatures at higher altitudes, the reason it’s colder on top of mountains is due to the decrease in air pressure with altitude. As air pressure decreases, so does the temperature, leading to colder conditions at higher elevations. Additionally, factors such as exposure to winds and proximity to polar regions can further contribute to colder temperatures on mountain peaks.
bad bot
proximity to polar regions? seriously?
Is it not colder at the polar regions?? Edit: downvotes for that?? Aight weirdos
Pretty sure the poles are colder because the farther you are from the equator, the less perpendicular the light. Light spread over a larger area means less heat per sqft. This is also why the seasons change with the tilt of the earth relative to the sun, and not the distance to the sun…
wtf I don’t get the downvotes all i said is that it’s colder on mt peaks bc a triangles angles all add up to 180 degrees
He said : bad bot, proximity to polar regions is a bad explanation in itself.
You replied: isnt it colder there?
Which can be taken as you saying there’s nothing wrong with what the bot said. It is colder there, but that’s not why the bot is bad.
Basically people will read into what you say and take it however they want to, i wouldn’t bother with people misunderstanding your intent. “Just asking” is a common tactic used for disinformation and steering conversations
You replied
No i didnt! I just got here!
That’s… what I thought as well but everyone’s shitting on every comment and downvoting any kind of discourse in ‘no stupid questions’ , best not to even trying having a discussion here I guess, learning bad, being asshole good 🤷♂️
It’s not? It allows hotter at the top of mountain. That is why volcanos are.
What utter gibberish is this comment?
It is explained why the volcano
That makes no sense though
Lemme c if I can xplan.
Hot air goes up.
Rocks ata top are also up.
So, more hot at the top makes the rocks hotter ad hotter, lika microwav, until they boil and melt tye tip of the mountain of, then, boom, volcano.
Makes sense.
Because down is hot and up is cold and up goes high enough that down can’t overtake it.
