Help! Why does it get colder the higher the altitude?

[Hayashi]

My son, who is in Primary 4 (4th grade in the US?) asked me this question recently. For once, me, the acknowledged "know-it-all" in the family, was at a loss.
Tried searching for this on the internet, no luck.
I suspect it's got something to do with the air density (it gets less the higher you go). But *why* does it get colder the higher up you go?
[img]graemlins/1ponder.gif[/img]

[Azred]

From physical chemistry, we get the classic formula PV=nRT, which means that pressure * volume = constant * temperature (nR is treated as a constant here). If we let volume = the volume of the entire atmosphere, it will also remain constant, so we can reduce the equation to a very simple P = k * T, which means that there is a direct relation between pressure and temperature. [img]graemlins/saywhat.gif[/img]

In short, as the pressure drops due to the altitude, so will the temperature. There are not as many molecules in the air bouncing off objects, so not as much kinetic energy is being transferred. That is what "temperature" really is--a measure of the kinetic energy of the molecules of some object.

Isn't science great? [img]graemlins/awesomework.gif[/img] [img]graemlins/petard.gif[/img]

[VulcanRider]

I found a pretty good explanation by going to www.askjeeves.com and ask "why are higher altitudes colder". I tried to post the link to the answer but it's a paragraph unto itself...

[Hayashi]

Ah yes! Charle's Law & Boyle's LAw and all that. How could I forget! *smacks forehead*
Now I just need o distill this into something a 10-yr old can understand...

[WillowIX]

Just a small note... It´s not quite correct to say that the temperature drops with the higher the altitude. When reaching the stratosphere(?) the temperature will start to rise again to about 0 degrees C, since the UV radiation is absorbed in this layer. We´re now around 25 miles up. Still further up the temperature will drop again, in the Mesosphere, to about -95 degrees C, 40-50 miles up in the blue . Above that comes the Thermosphere and as the name suggests it gets warmer again, up to 1,700 degrees C due to the Sun´s energy. I think NASA has some exact numbers somewhere on their site, not sure though but it seems likely.

[Lox]

In regards to PV=nRT: Can you really treat the atmosphere as one volume? You are comparing P and T differences within that volume. It's been a while since I took Chem, but I think you want to compare, say, one litre of air at 5000 meters with 1 litre of air at sea level. The answer works out the same (because we arbitrarly pick the same volume), but there is a subtle difference in the process. You still see a higher P, which leads to a higher T in the sea level air.

[Ronn_Bman]

Not to mention the T&A effect which always effects my normally constant tempurature.

[Azred]

For the purposes of this particular discussion, yes. We're just trying to determine the relationship between pressure and temperature, that's all.

WillowIX is correct, there are many layers at vastly differing temperatures in the atmosphere. Fortunately, Hayashi's son is in the 4th grade, so we don't have to dust off our graduate-level p-chem/physics books. *whew* [img]graemlins/laugh3.gif[/img] [img]graemlins/petard.gif[/img]