Why are the top of mountains cold?

Discussion in 'Earth Science' started by John Connellan, Oct 23, 2009.

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443
Or the closest to reality.

3. John ConnellanValued Senior Member

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Yes. Pressure of the air cannot directly be correlated with it's temperature at least in sunlit conditions as can be seen from elevated plateau regions.

One thing I do agree with though is that elevated regions definitely cool off much faster than low lying regions when there is no solar radiation (e.g. at night). Less atmosphere to re-radiate heat to the ground.

5. Uno HooRegistered Senior Member

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Surface of Moon. Very low air pressure

. Astronaut suits designed with built in air conditioners to generate big time cooling for occupant. Excuse for cooling equipment high cost was that insolation raised temperature of objects in direct sunlight with very little natural cooling expected. Like Death Valley in summer.

Wait a minute. Moon = real low air pressure. Death Valley = real high air pressure compared to other places above sea level.

Maybe humidity and carbon dioxide and other greenhouse gas. Moon = really, really low humidity

and no greenhouse gas to speak of. Death Valley = higher humidity than Moon and lot more greenhouse gas.

Phenomena is not just about mountain tops. Some aviators have claimed that when they get high they run into cold air, and that the air gets colder the higher they get. I think that this has been studied by scientists and perhaps verified.

Apollo 13 lucky astronauts were famous for suffering really chilly ride when just about all systems were crapped out by equipment malfunction. Entire ride was in direct sunlight in conditions of no air pressure and no greenhouse gas. Sounds like Moon.

OK. Look at Hawaii. Temperature at seashore: 80 degrees all time. Temperature at top of Mauna Kea: snow on ground every day of year.

I give up.

7. Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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It really is not very difficult or complex. Part of the confusion about the effect of humidity is due to being an animal with body temperature which is usually greater than the surrounding air temperature and the fact that without wind, the heat transfer by conduction (skin to air) is not adequate to keep the body temperature from rising. Human bodies are constantly generating heat internally and must transfer it to the environment. So we sweat - call upon evaporative cooling. When the air humidity is near 100% there is very little evaporative cooling possible (compared to when relative humidity of air is only 50%) Thus, humans naturally associate high humidity with heating. This is false.

Humidity is H2O molecules in the air. They are no different from the N2 or O2 molecules as far as air temperature is concerned. All air molecules in any given cubic centimeter are all at the same temperature and the relative amount of them has no influence on what that temperature is (I am neglecting the green house gas effects when the air is in an IR flux, but even then it only effects the vertical distribution of temperature by absorbing radiant energy.)

The dominate reason why Death Valley air is hot is the same as why mountain top air is cold. It is adibatic heating (or cooling as the wind forces a chunck of air up the side of a mountain towards the top).

Summary: The realtive humidity makes humans feel warmer on warm days because it reduces the effectiveness of their evaporative cooling system, but has essentailly nothing to do with the air temperate or how it changes with altitude. Death Valley is hot because it is below sea level and the air That desended to that negative elevbation was compressed and that heated it to a higher temperature than when it was at sea level. Likewise the air moving to the mountain top was expanded and that cooled it.

If you get rid of your psychological associations between humidity and temperature and only look at the physics of expanding or compressing air you can easily understand, without any "humidity confusion," why Death Valley is hotter and Mountain top air is cooler than the sea level air.

Last edited by a moderator: Nov 22, 2009
8. John ConnellanValued Senior Member

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Well it's one reason why it is so hot. It's not that far below sea level (only 86m below).

It is also very dry allowing the sun to heat up the ground continuously. It is at a fairly low latitude meaning that the sun is almost directly overhead in June. And it is in a valley so the hot sand particles on the ground, as well as the mountain sides all radiate and conduct heat to the valley air ensuring that it is raised to the maximum.

9. D HSome other guyValued Senior Member

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The sun shines more intensely on the tops of the mountains around Death Valley than it does on Death Valley itself. The atmosphere between the mountaintops and the desert valley absorbs some of the incoming radiation.

10. TrippyALEA IACTA ESTStaff Member

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Death Valley is hot for a number of reasons.

It's in the global desert belt (persistent sinking air which warms).
It's in the rain shadow of the Sierra Nevada Mountains (Latent heat caused by precipitation (on the windward side) as the winds blow inland).
Because it's in the rain shadow of the Sierra Nevada Mountains, and because it recieves an average percipitation of 1.5 inches per annum, it has an absurd number of sunshine hours.
Finally, it's below sea level which means it recieves compressional heating.

Addendum - There's also it's shape to consider - long, narrow, and deep.

Last edited: Nov 22, 2009
11. CheskiChipsBannedBanned

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I'm not sure how much the emboldened statement is true. If you look at desert areas during the course of the year - typically warmer areas have lower pressure during summer months. Which means pressure during the hottest parts of the year is low.

It's actually related to your first first statement. Typically low elevation high temperature experience down-welling as a result of thermal lows. Thermal lows occur when extreme surface heating reduces the atmospheres density capacity.

Typically I think the following relationship holds: (d(alpha) density, dT temp, dp pressure)
$|\frac{d\alpha}{dT}|>|\frac{dp}{dT}|$

In which case, the equation of state is a general form that holds rather true.
$\alpha p= R_dT$

This causes a rapid geopotential thickness decrease (The central low) above the extreme temperatures which typically results in cyclonic motion. The secondary circulation of the cyclonic motion results in down-welling.

12. DRZionTheoretical ExperimentalistValued Senior Member

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I was going to point this out as well - in the upper troposphere there are hardly any air particles, but their average kinetic energy would imply a very high temperature (something like 2500 degrees).

This is true, but it should not lead to an overall cooling effect, instead it would make mountains more of a desert environment with intense heat during the day and bitter cold during the night.

But when the air has already risen, it should then have the ability to heat up again - especially due to the increased intensity of sunlight.

It is true however that absorption of sunlight via the atmosphere is dependent on collision frequency and hence pressure (as has been mentioned already), and it is true that mountains are not a very good absorber of sunlight, so maybe most of the heat falling on a mountain gets bounced away and only a small fraction is converted into heat that would later be transferred to the air particles.

And since there is only a very small amount of surface area at high elevations (as compared to the surface area of low-lying lands), any horizontal movement in air would lead to the thinning of the warm air around mountain peaks (if that is in fact how the atmosphere works).

In conclusion, I would say that cold air at high altitudes is a result of the thin atmosphere's reduced ability to absorb sunlight as well as the distance from the surface from which it could gain energy via conduction and convection... but this still doesn't explain why mountaineers don't get excessively overheated when exposed to that high-altitude sun.

Last edited: Nov 22, 2009
13. John ConnellanValued Senior Member

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3,636
Well because they lose a lot of heat to the surrounding cold atmosphere

14. CheskiChipsBannedBanned

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$\alpha p = R_d T$
The specific density and gas constants are almost homogeneous for the troposphere. Temperature is almost a direct function of pressure.

The diatomic gas which is the main constituent of the atmosphere has a specific pressure, which is the product ratio of the specific gas constant in the atmosphere. Therefore, its lapse rate is a direct function of its height, height being mostly affected by gravity.

$\frac{dT}{dz} = \frac{-g}{C_p}$

It works out...see...
$g = \frac{m}{s^2}$
$C_p = C*R_d = \frac{(kJ)}{kgK}$

$\frac{g}{C_p} = \frac{m}{s^2} \frac {kg K}{kJ} = 1000 \frac{m}{s^2} kgK \frac{s^2}{kg m^2}=\frac{K}{1000 m} = \frac{K}{km}$

[note - this assumes a perfectly dry atmosphere. Wet atmospheres are similar with numerous other terms irrelevant.]

Last edited: Nov 22, 2009
15. TrippyALEA IACTA ESTStaff Member

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It's one of several explanations that i've seen for the heat in Death Valley - note that I attributed no specific importance to it (in fact, I would personally have classed it as a relatively minor feature).

I'm also not referring to desert areas in general, but Death Valley specifically (Yes, I mentioneld the global desert belt, but the fact that it's in the global desert belt is a seperate thing altogether).

Ultimately, I'm simply making the point that there is no one single reason that Death Valley is so hot, but it's a combination of factors.

If I were a betting man, I would place money on latitude and geography being the two most important factors.