Today I realized that I actually have no idea why the tops of mountains are cold. It’s not as though they get less sunlight than the surrounding area. There are higher and stronger wonds at the tops of mountains, but I (perhaps wrongly) was under the impression that wind chill has no impact on actual temperature. The best I can come up with off the top of my head is that there’s less atmosphere to insulate up there. Any ideas?
Temperature is more or less a measure of the average amount of heat energy in any place at any time. The way you determine that is to measure - indirectly - the number of molecules in motion at that place. The main way we do that is to put out a thermometer. When the active molecules in that area smack into the measuring device, they in turn jiggle the molecules of that device, in this case, the thermometer. They jiggle more and faster and they move farther apart and the liquid that they comprise expands - hence the temperature goes up. Or, if there are fewer molecules around the thermometer and they’re moving slower, they tend to absorb the activity of the thermometer molecules, and that causes that liquid to contract. Bottom line - the air is thin up there, so there is not much heat. Not much heat usually translates to cooler temperatures.
Thank you both.
The most simplistic answer is: it’s cold because it’s far from the ground. The earth’s atmosphere is mostly heated from the bottom. Sunlight may come from above, but it mostly passes straight through the atmosphere and heats the ground, which in turn heats the air right above the ground.
Of course the heated air does rise, but it cools as it rises, for the reason Squink mentioned.
And it works the other way as well. The bottom of the Grand Canyon is 20 degrees hotter than the rim.
But…if you’re on top of a very tall mountain, or a high plateau, you’re not far from the ground. You may be far from sea level, but not from the local ground…and yet it’s still cold up there.
Yes, the “heating from below” does count for something…but the adiabatic lapse rate would be there even without the effects of ground heating. It’s (almost) all about the pressure changes described so well above.
But if the atmosphere were heated from the top (e.g. if it were opaque to sunlight), would it still be warmer at the bottom?
I live at altitude. And see some strange things re temp.
It’s often and usually 5-10 degrees colder about 2000 feet BELOW me. I’ve been driving through it for 17 years. It’s consistently colder at 9,000 feet than at 11,000 feet.
Cold air is heavier? It cools above my house, and sinks into the valleys. That’s the only explanation I have.
The way I think of it is that heat in a gas is motion of the atoms. When atoms from one level are knocked upwards, gravity slows them down, so that when they get to a higher altitude, they are moving slower, hence have a lower temperature. Going the other way, atoms traveling downwards gain speed due to gravity, so have a higher temperature.
In short, some of the energy in the form of heat is converted to or from potential energy as atoms move to a higher or lower altitude.
I haven’t done the math yet, but I suspect that the change in potential energy as you move from Z1 to Z2 doesn’t account for the change in temp from T1 to T2.
Likewise, I suspect that adiabatic expansion/compression accounts for it nicely.
I will run the numbers in a spreadsheet this weekend and let you know which phenomenon provides a better fit with the U.S. standard atmosphere model.
Incidentally, temperature decreases with increasing altitude only for the first 30K feet or so. After that it’s steady at around -60F until you get to around 90K feet, then begins increasing to around 50F at 160K feet, before trailing off to “damn cold” as you approach space/orbit. There’s all kinds of weird interactions between sunlight, cosmic rays, and the air at high altitude that drive these temperatures in different ways depending on air density, intensity of radiation, and so on. The high-altitude ozone layer is one example of these effects: UV light at high altitude generates the ozone, and also destroys it
I came in to mention this. I live in a valley (only ~1000ft, I think), but my workplace is at an elevation of ~3000ft. Occasionally, it’s colder at home than at work (usually mornings in the wintertime…at least, that’s when I’ve noticed it). Someone who lives at higher elevation told me about the phenomenon of (temperature) inversion. I’d never heard of it before.
Dammit…edit timed out. “Cold air is heavier” isn’t the explanation, although it’s what I used to think.
Thanks for bringing it up JFF. I came in here to ask about the troposphere. I get the adiabatic lapse rate, and I can see how it works. So why does it get warmer in the trope? (I’ve never understood this one).
Given that the pressure difference derives from the height, mass of air, and gravity in the first place, I suspect that the “two effects” are actually two different explanations of the exact same effect. Consider the limiting case of a gas which truly has exactly zero intermolecular interactions (a situation approximated by ideal gases): In such a gas, a rising molecule of air can’t do work against the surrounding gas: The only way for a molecule to lose kinetic energy is to gain gravitational potential energy.
Heat is infrared light bouncing between air atoms. Where air is densest, the infrared light is trapped best. As air becomes less dense, the light escapes more and more, eventually venting out into space.
OTOH, it might be gravity compressing/accelerating atoms.
thin air.
While light (including infrared light) is one way to transfer the high-entropy energy commonly known as heat, the temperature of air is a function of the motion of the air molecules themselves, not of any light in between them. Nitrogen and oxygen at any density interact only very weakly with infrared light, and are lousy at trapping it (though carbon dioxide can trap it better).
I understood the nitpick about “is heat” when I wrote it, but do you know more about what extent do greenhouse gases, which are mixed into the atmosphere at all levels, contribute to the relation between temperature and elevation?
Well, I’m not a climatologist or atmospheric physicist, but I would guess that the effect due to greenhouse gases is small compared to the pressure-gravity effect. If you think about it, the Earth’s entire greenhouse effect is due to greenhouse gases in the entire thickness of the atmosphere, and from the bottom of a mountain to the top isn’t much compared to the entire thickness of the atmosphere. The top of a mountain has nearly as much greenhouse gas above it as the bottom of the mountain does.