Why is it Colder at Higher Altitudes?

[BL4CKCR4Y0NS]

I don't know if I'm asking this in the correct section of the forum. *sorry*

Why is it that when you climb up (up a mountain?) it is colder?

Doesn't heat rise?
Aren't you close to the sun? (maybe not by much but you're still closer...)

I get the feeling that the answer is REALLY simple and I feel like an idiot for asking but I can't think of the answer...

 

[cepheid]

B4CKC4Y0NS,

In climbing a mountain, you do not get appreciably closer to the sun (not enough for it to make a difference).

It is true that heat rises. But the air is not being heated directly (at least, not much). Air is transparent to visible wavelengths of EM radiation, so that energy from the sun is not really absorbed until it hit the ground, which then heats up. So, the closer to the ground you are, the warmer it will be.

 

[BL4CKCR4Y0NS]

energy from the sun is not really absorbed until it hit the ground

Is this basically saying that heat doesn't rise until the energy hits the ground? In other words, heat will only start rising AFTER the energy hits the ground?

 

[cepheid]

Is this basically saying that heat doesn't rise until the energy hits the ground? In other words, heat will only start rising AFTER the energy hits the ground?

Er, I should have been more specific. "Heat" is not really a thing, it merely refers to the transfer of energy from a higher temperature area to a lower temperature one. Hot air is what rises, because it is less dense than cooler air. This large scale motion or flow of a fluid due to temperature gradients is called convection.

The EM radiation from the sun will heat up the ground. The ground, being at a higher temperature than before, will radiate more EM radiation. But since it is not nearly as hot as the sun, most of this radiation will be at infrared wavelengths. I would guess that it is mostly by this mechanism (infrared radiation) that the ground will heat up the layers of air immediately above it, and then that heat will be transferred to higher layers by convection.

 

[cepheid]

By the way, I should mention that what we're talking about applies only in the lowermost layer of Earth's atmosphere: the troposphere. In some of the higher layers (not all of them), the temperature trend reverses, and the temperature actually does get larger the higher you go. This article describes the atmospheric layers:

http://en.wikipedia.org/wiki/Atmosphere_of_Earth

For instance, in the stratosphere, there are molecules that are good at absorbing UV radiation from the sun. That's why this layer can be heated effectively, and therefore, the temperature does increase as you go higher. But, of course, even at the bottom of the stratosphere, we're already well above the height of any mountains!

 

[BL4CKCR4Y0NS]

Okay let's see... (I like to make sure I understand what I have read before continuing with my questions or else I get muddled up if I make a mistake.)

So the sun does NOT heat up the air? It only heats up the ground, which in turn radiates more electromagnetic radiation heating up the air above which would be convection.

So am I right to say this: The reason why it is colder up in the mountain is because the heat from the ground isn't strong enough to reach top?

 

[BL4CKCR4Y0NS]

Okay let's see... (I like to make sure I understand what I have read before continuing with my questions or else I get muddled up if I make a mistake.)

So the sun does NOT heat up the air? It only heats up the ground, which in turn radiates more electromagnetic radiation heating up the air above which would be convection.

So am I right to say this: The reason why it is colder up in the mountain is because the heat from the ground isn't strong enough to reach top?

 

[Yuqing]

Also, the so called greenhouse effect plays a large part in why the lower atmosphere is warmer. Think about a car left in the sun getting hotter. Although the mirror let's in visible light, infrared radiation is unable to effectively escape the car. The Earth's atmosphere does something similar. In fact, global warming is caused by the enhancement of this effect.

For an extreme example of atmospheric greenhouse effect, take a look at Venus which hovers at ~450 degrees Celsius, on the other hand, Mercury which lacks an atmosphere will not trap heat at all leaving the side not exposed to the sun to fall to almost 200 degrees below 0.

 

[Born2bwire]

Okay let's see... (I like to make sure I understand what I have read before continuing with my questions or else I get muddled up if I make a mistake.)

So the sun does NOT heat up the air? It only heats up the ground, which in turn radiates more electromagnetic radiation heating up the air above which would be convection.

So am I right to say this: The reason why it is colder up in the mountain is because the heat from the ground isn't strong enough to reach top?

It would probably be more accurate to say that the density of the air is insufficient to retain a large amount of the heat radiated off of the ground. The mountain is the ground, I'm fairly sure that the radiation per square meter from the Sun is rather independent of your altitude. As long as the composition of the ground is fairly the same then the amount of heat being absorbed at sea level versus 5 km up then the main factor that is going to change is the density of the air.

 

[Chelle12]

It would probably be more accurate to say that the density of the air is insufficient to retain a large amount of the heat radiated off of the ground. The mountain is the ground, I'm fairly sure that the radiation per square meter from the Sun is rather independent of your altitude. As long as the composition of the ground is fairly the same then the amount of heat being absorbed at sea level versus 5 km up then the main factor that is going to change is the density of the air.

It's called http://en.wikipedia.org/wiki/Thermodynamic_equilibrium"

 

[IttyBittyBit]

It is true that heat rises. But the air is not being heated directly (at least, not much). Air is transparent to visible wavelengths of EM radiation, so that energy from the sun is not really absorbed until it hit the ground, which then heats up. So, the closer to the ground you are, the warmer it will be.

What? As others have pointed out, a mountain is still 'the ground'.

The reason why it is on average colder the higher the altitude from sea level is (up to a point), is because warm air rising from the ground expands as it gets higher, cooling down (expansion cools gases). At the same time, the cold air from higher levels in the atmosphere flows down and it gets compressed, heating it up. So there's really a nice thermal convection equilibrium taking place.

This rule does not extend to above the troposphere because as you get to higher and higher levels the dynamics of the atmosphere completely change. At the highest levels (ionosphere) the dynamics are greatly influenced by electrical phenomena, causing the production of very high temperatures. Of course, if you stuck your hand out in the ionosphere it wouldn't burn because even though the temperature is hotter than a blowtorch, there's very little air pressure and thus very little thermal conductivity.

 

[D H]

Born2bwire and IttyBittyBit come closest to the mark here. The primary reason it gets "colder as you go up" is because pressure decreases with altitude.

A simple model of what is going on is that air is a fairly lousy thermal conductor. As a packet of air expands or shrinks it will do so more-or-less adiabatically. Pressure decreases with altitude because the atmosphere is more-or-less in hydrostatic equilibrium.
Thus as a packet of air rises/falls it will undergo something close to an adiabatic expansion/compression.

Those more-or-lesses are important. The true variation of temperature with altitude is sometimes greater than that predicted by a simple adiabatic model, sometimes less. Watch out for thunderstorms when the temperature drop per unit altitude is greater than that predicted by the adiabatic model.


Minor correction to Born2bwire's post:

I'm fairly sure that the radiation per square meter from the Sun is rather independent of your altitude.

Solar irradiation increases slightly with altitude. The atmosphere absorbs about 2% of the incoming solar radiation, with most of this absorption occurring in the lower atmosphere. This is why mountain skies have a slightly different, and slightly darker color than sea level skies -- and why you are recommended to make sure you wear sunglasses and suntan lotion when you go hiking in the mountains.

 

[cepheid]

Thanks for the corrections and feedback guys. I really dropped the ball on this one. So, for the sake of the OP, would this be a good summary:

1. At higher altitudes (e.g. a mountaintop), EM radiation from the ground cannot transfer energy as effectively to the surrounding air because the air at these altitudes has a lower density and pressure, and hence is a poorer thermal conductor.

2. This air at higher altitudes is not heated much by warmer air rising from below, because when a packet of warm air rises, it expands and cools, and it does so more or less adiabatically (i.e. without transferring energy to its surroundings).

3. Furthermore, in addition to warm air rising, expanding, and cooling, there will be colder air from above sinking, compressing, and heating up. Therefore, the air temperature at any given altitude (in the troposphere where convection is important) will be determined by whatever convective equilibrium can be maintained.

How did I do?

 

[D H]

Thanks for the corrections and feedback guys. I really dropped the ball on this one. So, for the sake of the OP, would this be a good summary:

1. At higher altitudes (e.g. a mountaintop), EM radiation from the ground cannot transfer energy as effectively to the surrounding air because the air at these altitudes has a lower density and pressure, and hence is a poorer thermal conductor.

2. This air at higher altitudes is not heated much by warmer air rising from below, because when a packet of warm air rises, it expands and cools, and it does so more or less adiabatically (i.e. without transferring energy to its surroundings).

3. Furthermore, in addition to warm air rising, expanding, and cooling, there will be colder air from above sinking, compressing, and heating up. Therefore, the air temperature at any given altitude (in the troposphere where convection is important) will be determined by whatever convective equilibrium can be maintained.

How did I do?

1. is pretty much a red herring. Air is a poor thermal conductor, period. Think of it this way: Bats of fiberglass are used to insulate houses because they trap a lot of dead air. Spray foam insulation is even better because it seals the leaks and traps a lot of dead air.

A mountain does affect temperature, but opposite the way most of you are thinking. Suppose you fly in an airplane from sealevel to 14,000 feet, far from any mountain. The temperature at 14,000 feet will be considerably lower than the temperature atop a 14,000 foot mountain because the air at the top of a mountain is close to the ground and this will tend to warm the surrounding air somewhat.2. & 3. Rather than thinking about the dynamics that arise through rising and falling air, think about the conditions under which the atmosphere is very dry and vertically stagnant. Imagine a helium balloon that is lifting an insulated balloon filled with dry, unsaturated air. As the air is vertically stagnant, the temperature inside the balloon must be the same as the temperature of the surrounding air. The balloon expands as it rises, and since the balloon is insulated, the temperature drops adiabatically. This is the dry lapse rate of the air. The moist lapse rate is much the same thing, but with saturated air.

The real rate at which temperature varies with altitude, the environmental lapse rate, can be

• Greater than the dry lapse rate. Parcels of warm, moist air near the surface can rise far into the atmosphere and carry huge amounts of moisture aloft. This condition happens in the summer and is the source for those torrential summertime downpours.
• Between the dry lapse rate and moist lapse rate. The warm, moist parcels of air will rise to a certain level -- and stop. A cloud ceiling forms under these conditions -- the bottoms of the clouds form a more-or-less constant height "ceiling" in the sky.
• Less than the moist lapse rate. Parcels of warm, moist are near the surface stay near the surface. The sky remains clear because little if an moisture is pumped aloft.