If Heat Rises, Why are Mountain Tops Cold? 3

[Barry1492]

Sorry for such a seemingly juvenile question, but I am very weak in Thermo. I understand that heat rises and experience this effect in my house (ie, it is always warmer upstairs in my house). But why then are higher elevations colder than lower ones (ie take the tram to the top of the San Dias Mountains...cold on top).

The concept of heat moving from hot to cold may explain heat rising from an earth surface which radiates heat and moves to a colder higher atmosphere, but why then does heat move from a cold basement to a warm 2nd floor?

If anyone has a clear, simple explanation, I'd be grateful.

-Barry

 

[ivymike]

for one thing, it's HOT AIR that rises, not heat itself. The hot air rises due to buoyancy...

 

[Barry1492]

Thanks, that makes sense. But then, at what point does that rising hot air become not so hot anymore?

 

[Montemayor]

Barry1492:

Hot air rises and subsequently mixes adiabatically with much colder air in the upper atmosphere. The reason the upper atmosphere is cold (& not hot) is that it borders on the reaches of outer space - where absolute vacuum and cold predominate.

There is a hell of a lot of cold, upper atmosphere out there and the hot air rising due to natural convection doesn't have a chance to stay even warm.

Where have you heard or experienced that "heat move from a cold basement to a warm 2nd floor"? If the basement is cold, than so is the air in it; and as such, it will not rise to a warm 2nd floor. And that, as Ivymike has so succinctly inferred, is pure, factual science. Hot air will rise; colder air won't -it is denser and has no buoyancy.

 

[IRstuff]

basements are colder, because they're usually surrounded by earth that's at a relatively cool temperature.

2nd floor walls are surrounded by air and subject to solar load.

heat transfer coefficient for conduction is quite high compared to convection.

even ignoring the migration of hot air, basements are going to be cooler than 2nd floors.

TTFN

 

[katmar]

Hot air rises because it is less dense than the air around it. But the air on top of the mountain is even less dense because the pressure is lower up there. As the hot air rises and expands (because of the lower presure) it cools - just the opposite of how air heats up if you compress it.

 

[JStephen]

In the atmosphere, hot air DOES rise- that's one of the things that causes winds and storms.

 

[jsummerfield]

The atmosphere is a bit complicated. You can find published rates of temperature decrease as the altitude increases. A temperature inversion changes the reality. The actual increase is affected by humidity and other factors. However, this changes at the tropopause. This is the altitude that thunderstorms tend to cap. That altitude varies between the equator and polar regions. Liet's just call it around 40,000 foot in my neighborhood. Somewhat above the tropopause, I think that the temperature actually increases again. Look into the aviation literature for the troposphere, etc.

John

 

[imok2]

A basic physics law is that as pressure goes down so will the temperature that's why refrigerators do what they do, therefor as you go higher up the pressure will go down and the temperature will follow

 

[MintJulep]

Hot air rises only if it is less dense the the air around it. As a mass of air rises, it cools, generally following the dry adiabatic lapse rate. The mass of air will stop rising when it has cooled sufficiently to become the same density as the air around it. It is difficult to predict exactly where that will happen because many factors are involved.

Glider pilots plot the morning atmospheric sounding data on the lapse rate chart to provide an indication of the highest likely thermals on a given day. It's been many years since I've done that, so I can't recall the procedure from memory. Basically you assume that a mass of air at the surface of the earth will cool at the dry adiabatic lapse rate as it rises. If the atmosphere has a local temperature vs elevation profile that is less than the lapse rate there will be strong thermals. If the local atmosphere gets warmer with elevation (temperature inversion) then there will be no thermals that day.

 

[Barry1492]

So, based on given info, I can conclude the following:

Say I live in standard modern house and have not run the heater or air conditioner. The attic is hot as it is surounded by heated air and is being hit by solar radiation. The basement is cool as it is surrounded by earth and is shielded from solar radiation.

Now, I magicaly place a cold air molecule in the attic. This molecule will now be denser than the surrounding air and, thus, descend toward the basement absorbing heat along the way. Assuming it reaches the basement before reaching equilibrium temperature, it will absorb heat in the basement thus cooling off each room on its way down.

Next, I place a hot molecule in the basement and it will rise, possibly all the way up to the attic releasing heat on its way. Now I open a skylight in the attic. This molecule will continue to rise into the atmosphere releasing heat unitl, at some point its density is equal to that of the surrounding air (at the point the temperatures are in equilibrium).

In other words, air will only rise if it is hotter than its surroundings...NOT if it is hotter than air outside its surroundings (ie hot air in a desert does not rise to replace the colder air miles above it).

Am I correct?

 

[MintJulep]

Barry,

That's pretty much it.

Just add the following to your picture:

As your cold air molecule decends through your house is needs to displace other air molecules along the way. Upon reaching the basement it will need to kick one of the warmest molecules in the basement out, causing to start rising. That is how convective currents get going.

 

[zdas04]

Barry,
I think you re-stated the physics as I understand them very well.

David

 

[ko99]

The explanation is more accurate if you replace "molecule" with something like "a small volume of air".

Pressure, density, and temperature are properties that describe the interaction of molecules (at least for this discussion).

ko (

http://www.ecooling.biz)

 

[25362]

Beside all what was rightly said in previous posts about convection heat transfer by the bulk motion of air currents, let's add that a house located in the northern part of the United States, when the outdoor temperature is appreciably lower than the internal house temperature, would probably present a different picture than Barry's house.

Heat flows in the direction of decreasing temperature. A heated house in winter loses heat because of the temperature difference between itself and the environment, but this loss is balanced by energy input from a furnace, solar collector, electricity, or other source. The house is said to be in thermal energy balance.

Windows, walls and roof on the south side may admit solar energy by radiation while losing heat by conduction, radiation and convection. This loss of heat may result in cooler "exposed" house surfaces making the basement the warmer part of the house although it may lose some heat to the ground.

The atmospheric temperature in the troposphere (from the Greek for "sphere of change") decreases with altitude at a so-called lapse rate of about 6.5^oC per kilometer.

The main reason for the decrease is that sunlight first heats Earth's surface, which then transfers heat to the atmosphere. Air parcels start moving upward, being less dense (buoyancy force) than the surrounding air, as with gases belched out of smokestacks or automobile exhaust pipes.

As the air rises, the pressure of the surrounding air drops (mountain climbers and pilots use barometers to determine their altitudes) and the parcel expands to maintain pressure equilibrium. Since air is a good thermal insulator it exchanges little heat with its surroundings. Its expansion is therefore considered adiabatic, and thus, its temperature drops as it expands.

As MintJulep explained, atmospheric phenomena are quite complex if only due the fact that the rate of adiabatic cooling may be greater or smaller than the lapse rate. Thunderstorms, cloud formation, air plumes trapped with their pollutants at lower altitudes, water condensation and evaporation, winds, temperature inversion, etc., are apparently the result of these varying cooling rates.

Above about 16 km, where the stratosphere begins, the summit of the Everest -the highest point on Earth- is at about 8.8 km, temperature rises due to numerous chemical reactions caused by the sun's radiation, one of the most important being the formation, and eventual destruction, of ozone, to about 0^oC in the stratopause, at about 45 km above sea level.

Still above that height, in the mesosphere, the temperature again decreases as altitude increases up to about 80 km, the mesopause, to about -110^oC.

After that, in the thermosphere including the ionosphere, the temperature rises, and atoms, ions, and molecules reach speeds corresponding to 1000^oC.
It is in this zone where the layer of ionized gases reflects radio waves effectively, and makes possible shortwave radio transmission.

The region between 500 and 1000 km, called the exosphere or "region of escape", is the outermost region of the atmosphere; here the density is so low that the mean free path of particles depends upon their direction with respect to the local vertical, being greatest for upward-travelling particles.

Natural processes, still not fully comprehended, are being continuously studied by experimental and theoretical scientists.

 

[IRstuff]

Actually, the basement, wherever it is, tends to be a very consistent and constant temperature. That's why modern energy hounds like to build their houses directly into the side of a hill; likewise, the early settlers.

I don't remember the exact temperature value, but the ground, at least in the more temperature parts tends to be somewhere around 70ºF.

TTFN

 

[itdepends]

Just an addition to the post be 25632.

Even though the temperature at the ionosphere is 1000degC you would still freeze to death if exposed.

The individual molecules have a very high energy but the pressure is so low and they are so widely spaced that you would radiate far more heat than you would receive from the occasional interaction with one of these high temperature (read fast) molecules.

 

[25362]

In addition to what itdepends has said let us say the (average) temperatures mentioned are "kinetic" and follow some kind of maxwellian distribution.

 

[JKEngineer]

Barry1492 said:
"In other words, air will only rise if it is hotter than its surroundings...NOT if it is hotter than air outside its surroundings (ie hot air in a desert does not rise to replace the colder air miles above it)."

Not quite. In the case of a house, for example, there will be the "stack effect". Using a cold exterior temperature as the example: the change in pressure in the column of cold outside air from the ground to the top of the house will be different than the comparable change of the warm inside air from ground level to the top of the house. This is because of the differing densities of the air as a function of its temperature. As a result, in cold conditions, the outside air tends to push into the house (infiltration) at the lower levels and the inside air tends to push out of the house at the upper levels. In hot conditions, with the house cooler than the outside, the reverse happens. So, part of the reason that the upper floor is warm is that the air is coming from lower levels of the house, while the air in the lower levels is coming from outside. This is not to say that the other comments are incorrect, just incomplete.

Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[solidworkswell]

There is a pressure drop as your altitude increases. If there is a pressure drop there is a drop in temperature, have u ever noticed a compressor in a garage, the nozzle gets cold as u let the air out. Same principle applies here. And there is also radiation loss to the outer space.

A intresting fact, the altitude where Bowing 747 flies. its -55°C i believe or -55°F. Can mpt remember exactly