can’t find anything on the net about it.
It is colder at higher altitudes because the air carries less heat, since the air is less dense.
does that mean its less humid?
I think this is due to something called adiabatic cooling.
Adiabatic means, I think, that no heat energy is added or removed. So if you take air at sea level at 1 atmosphere of pressure and raise it say 3000m to where the pressure is significantly lower, the air will expand.
This expansion requires energy and since no external energy has been supplied, the temperature of the air drops.
After googling:
http://daphne.palomar.edu/jthorngren/adiabatic_processes.htm
I find it interesting how sharp the cutoff point can be between snow starting, and not settling: the snowline, I believe it is called.
replying to myself, but after reading that again, I realised it may seem counter-intuitive… why should the air cool down when it expands? well it is because, i think, although somebody may correct me, it is doing work on the air around it.
the opposite is also true, for example, when using a bicycle pump, you are doing work on the air by compressing it and it heats up, even though you haven’t directly added any heat to it.
I may have got the wrong end of the stick, thermodynamics wasn’t really my thing.
It cools down due to the ideal gas law, which states that:
PV=nRT
P=Pressure
V=Volume
n=number of moles of gas (atom count)
R=Gas constant
T=Temp.
So, if P and/or V goes down, T must decrease to balance the equation.
That is… if I remember my chemistry/physics, and thermodynamics correctly myself.
OK. So why do you hit an iostherm at 36K ft? The pressure and desnsity continue to drop.
The thing to remember is that temperature and heat are related but distinct concepts. For instance, you can put your hand in a 400 degree oven for a short period of time, and you just feel warm. But if you touch the 400 degree metal pan in that oven you’ll be burned in less than a second. Both were the same temperature, so why did 400 degree metal burn you but 400 degree air didn’t?
This is different from the effect mentioned in the OP.
In the case of the oven, metal is a great conductor of heat whereas air is a poor conductor. They are both at the same temperature, but the heat is transferred quicker by the metal to your hand.
In the case of the OP, think of a big bunch of air at a certain temperature. As the air rises it expands and the air molecules become more spread out. This particular bunch of air contains the same heat but the temperature has decreased because the energy is spread out over a larger volume. You can think of temperature as the concentration of heat. Temperature is the average kinetic energy of the air molecules - how quickly they jiggle around.
In my first reply I said the air carries less heat at higher temperatures. To be more precise the air carries less heat per unit volume.
Now think ‘why does the air expand as it rises?’
No, it is usually more humid. As the air cools it capacity to hold moisture falls which is why clouds form (i.e. relative humidity reaches 100%).
A couple of other factors, one minor, one major.
Minor: With less atmosphere above it, and typically low humidity, the ground at higher elevation radiates more heat at night, so nightime lows are lower. The average surface temperature is thus lowered.
Major: The above noted adiabatic cooling causes weather systems to selectivly “dump” thier moisture when they move over higher elevations. Isolated mountain ranges, or the range that faces the direction of prevailing winds (western slope of Rockies for example) will thus get about 5X precipitation than the foothills. This means it isn’t totally a matter of the lowlands getting rain, and the peaks getting snow. The peaks tend to get snow, and the lowlands comparitivly little moisture at all. In the spring, the deeper snow pack at elevation can take weeks to thaw even when daytime highs become sufficient to melt it, and of course this starts later in the spring/summer than at lower elevations.
You could say something similar about air: air at higher pressure (i.e. lower elevation) is a better conductor of heat than air at lower pressure (i.e. higher elevation). Although really, air is a pretty poor conductor of heat anyway so I’m not sure if that contributes very much to the effect.
It can be just a matter of a few feet. On time we drove north from here toward Lone Pine. As we got further along it began to rain. Then we came to a stretch of road where there are a series of gullys maybe 20 to 50 ft. deep. The road follows the contour of the ground and at the bottoms of the gullies it was raining while at the tops it was snowing with a pretty sharp transition between the two…
Wouldn’t this be Boyle’s Law?
(P1V1)/T1 = (P2V2)/T2
Given a constant (V)olume of water, let’s take the §ressure and (T)emperature at ground level for P1 and T1. We then climb a local mountain and take the Pressure again (P2) and it is lower. Plugging all that into the formula, we would expect T2 to be lower and if it’s low enough, the water will freeze.
Or am I misapplying this here? It’s been a lonnnnngg time since high school chemistry and this is one of the very few things I remembered from it. My chem teacher is probably dead by now, but if he were still alive, he’d dance a jig to know that he actually hammered something through my thick skull 
Perhaps spinning in his grave. 
Boyles law does not concern temperature. It is simply P1V1=P2V2. When you combine it with Charles law then you get the ideal gas equation.
I think you mean constant volume of *gas *in the first sentence, not water. This equation is an approximation for gases.
Boyles law explains why a balloon expands as it gains altitude. Air pressure decreases as you gain height because there is less mass of air pushing down at higher altitudes, not because the temperature is less.
What role does being more distant from the Earth’s molten core have in all of this? I had a vague understanding that mountains can be cooled by the air masses more easily because of less radiated heat from below - correct or incorrect?
With the exception of volcanoes, I’d imagine it dosn’t have much to do with distances from the Earth’s center.
Think about if you start digging down from sea level in a non-volcanic area: the underground temperatures cool off pretty quickly before they warm up again as you get closer to the mantle.
This is one of those Yes and No types of things. As the temperature of the air decreases, the amount of water it can hold per a specific volume decreases. So if the actual quantity of water stays the same as the temperature decreases, then the Relative Humidity increases. It’s like a glass of water: if a 2 oz glass has 1 oz of water in it, it’s only half full. But if that glass shrinks to 1 oz while the amount of water stays at 1 oz, then the glass becomes full. That’s why thunder clouds have flat bottoms; that’s the altitude at which the air becomes cold enough to become “full” of water.
So while air at high altitudes can have a higher relative humidity (it’s holding capacity is filled up), the absolute humidity is unchanged since the actual amount of water in the air is the same.