Dense air???

[fletchersteel]

I am a little confused on the whole concept of the air is less dense the higher up you go, even though the air is normally colder as you go up. I understand that higher up there is not as much air above to compact the air and make it more dense, and that colder air molecules are closer together, therfore making it more dense. But, would the altitude mixed with the cold air tend to neutralize the density? Does that question make any sense? What I mean is would the rate at which the air cools with altitude (making it more dense) somewhere equal out to the air being less dense with altitude (which theoritically, would warm the air?)? Or, am I just reading into this whole thing way too much?

 

[Maurus]

Yes the cold does make the air more dense, but going up in altitude causes a much faster change in pressure which results in the air becoming less dense at altitude. The cold air doesn't make enough difference to keep it the same density at altitude as the ground.

 

[fletchersteel]

Thanks Maurus for replying. It is an area that I have always questioned. I suppose that it will have to be something that I will just half to believe. The cold air will not become more dense at the same rate that the increase in altitude decreases the density. Like you said, as you go up it gets less dense much faster.

 

[xpanterica]

The air cools because of decreasing pressure, and when you decrease pressure, you increase volume (the air expands). And, of course, volume and density are inversely related: If increase in volume, result is decreased density.

Think of Death Valley... below sea level... all that air pressure, and all that heat!

 

[tgrayson]

The air cools because of decreasing pressure

That's only true when air does work as it expands. Air just sitting there at a low pressure doesn't necessarily have to be cold.

and when you decrease pressure, you increase volume (the air expands).

I wish you'd tell that to my car tires; they tend to decrease in volume as the pressure decreases.

 

[CoffeeIcePapers]

This might help a little:

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

 

[mhcasey]

My meteorology prof used to always say "cooling causes sinking." Hence potentially huge downdrafts in precip on hot days (precip evaporates = cools). See Delta Flight 191.

Anyway, he never really explained why, but if I had to guess it's because the air has less energy to oppose gravity. So the cooling causes the air to sink more which contributes to greater density at those altitudes where the air sinks, but lesser density aloft.

Important thing to remember...there are a lot of variables out there (moisture also significantly affects air density), but pressure always decreases with altitude.

Anyone want to correct me on anything?

 

[tgrayson]

My meteorology prof used to always say "cooling causes sinking." Anyway, he never really explained why, but if I had to guess it's because the air has less energy to oppose gravity.

Why does a rock sink in a pool of water? The force to oppose gravity comes from the air around the cooled parcel of air, not from the parcel of air itself. The parcel will sink until the pressure below the parcel is sufficient to support its weight.

 

[Minuteman]

potentially huge downdrafts in precip

I have the idea that downdrafts in rain and hail were as much due to the mechanical interaction of the precipitation with the air as anything (i.e., stuff's falling, there's air resistance, so some of that energy gets transferred to the air in the form of momentum (downward, and also tiny little bit of heat)).

I recommend reading through AC 00-6A (after this post, I know I need to). Plus, it was made in the 70's and the picture below was the authors' depiction of "Pressure" at the beginning of Chapter 3.

 

[mhcasey]

That's only true when air does work as it expands. Air just sitting there at a low pressure doesn't necessarily have to be cold.

I'm not sure I agree. Temperature is a measure of average movement...if you have a parcel of air with low pressure, the molecules are pretty much by definition 1) moving slowly or 2) there aren't many of them. Maybe I'm misunderstanding?

 

[mhcasey]

Why does a rock sink in a pool of water? The force to oppose gravity comes from the air around the cooled parcel of air, not from the parcel of air itself. The parcel will sink until the pressure below the parcel is sufficient to support its weight.

Clearly stated. I guess what I meant was "the system as a whole doesn't have the energy to oppose gravity - i.e. - float the rock." Seems to me to be kind of a domino effect...molecule at level 1 (M1) cools and no longer provides the energy to hold M2, M2 falls and brings with it M3, which brings with it M4, which brings with it M5 etc. (but all the while when these things are falling they're gaining kinetic energy and heating up).

If my analysis is correct, this pretty clearly explains why cooling from below creates stability (a given parcel, when dislodged, will tend to settle back down), whereas cooling from above creates instability (less pressure up top pressing down that otherwise would have kept the potential towering cumulus from towering). But maybe I'm completely wrong...I probably think aloud too much in the tech talk forum.

 

[tgrayson]

I'm not sure I agree. Temperature is a measure of average movement...if you have a parcel of air with low pressure, the molecules are pretty much by definition 1) moving slowly or 2) there aren't many of them. Maybe I'm misunderstanding?

You answered your own question. Temperature depends on average velocity of the molecules. Pressure depends on average velocity and density. You can keep the same temperature, but have less dense air, and the pressure will be less.

As examples, at the tropopause, the temperature stops dropping with altitude, yet the pressure continues to fall. In the stratosphere, the temperate increases with altitude, while the pressure continues to fall.

High altitudes are generally colder because the sun heats the ground, rather than the air. The ground heats the air from the bottom up.