# Response to Deleted Thread that I've Worked on Too Hard to Let Go

Response to Deleted Thread that I've Worked on Too Hard to Let Go!!

This is one of my responses to the "Cold Air" thread under the "Earth" category.

The thread was deleted, but there was one really excellent response, to which I counter-responded, except that the thread was deleted before I could post my response.

As I said, I worked TOO DAMNED HARD on my response to let it go, so here 'tis, together with another response which i felt was excellent:

In atmospheric physics, there are two kinds of density.

One is mass density, that is, kilograms per cubic meter. The mass density of a volume of air depends upon its composition, its temperature, and its pressure. Colder humid air is less dense than slightly warmer dry air at the same pressure, because the mass of a vapor molecule is less than the mass of a dry air molecule. However, air at high elevations is less dense than air at lower elevations because the number density is less.

The second density is number density, that is the number of molecules per cubic meter. The formula follows Avogadro's Law and is n= P/kT. Here n is the number of molecules per cubic meter, P is the pressure in Pascals, k is Boltzman's Constant, and T is the temperature in Kelvins. Obviously, the number density of a parcel of air depends upon both the temperature and the pressure. The number density of high altitude air is less than that of low elevation air at the same temperature because the number density reflects the Maxwell molecular speed distribution for any given temperature. This is a common exercise in discussions of kinetic gas theory and statistical mechanics. That is, showing that the number density reflects the barometric formula.

Your answer is PERFECT!

But it's a little technical, and can be more easily explained thus:

Near the surface of the earth (let's say at sea level, which is where I live), the air is squished by the weight of the air above it. We physicists call this squishing "compression". The higher one climbs into the atmosphere, the less air there is above you to compress, or squish, the air around you. With less compression of the air at your higher altitude (say, the top of a 6,000 ft mountain), the easier it is for the molecules with make up the air to spread out lazily (in line with the Second Law of Thermodynamics, better known to laymen as the Law of Entropy). Thus, the air up there, because it is thin, enjoys low density because there's much less air above it to press down upon it. In terms of physics, the air at that altitude needs to resist far less gravitational potential energy trying to become gravitational kinetic than does the air further down, which has so much more air bearing down upon it. This allows the higher-altitude air to easily vent any heat, to which we physicists refer as "Thermal Energy", by just moving a little faster for a little while, until the energy, usually derived from solar radiation, is dissipated via the increased motion of the air molecule.

Now, further down, any piece of air that receives a lot of Thermal Energy from the sun has to expend this energy by increasing the movement of its molecules to an extent that forces these molecules further apart from one another, to the point that they must often infringe upon the portion of the Earth-touching atmosphere which has colder, less energetic air. Thus, hot air at the surface creates space for the more rapid motion of its molecules by pushing cooler air aside, keeping its own density as low as possible to accommodate the greater need for freedom of motion for its high-energy molecules. Another way of saying this is that there are two types of air pressure: air pressure due to the weight of the air above the air mass you're standing in, which might be called "Gravitational Air Pressure", or "High-Density Air Pressure", and air pressure due to the average kinetic energy of the molecules (which is the real meaning of the word "temperature") which make up the air mass you're in, which might be called "Thermal Air Pressure", or "Low Density Air Pressure".

Now, as hot air always rises (because it just HAS to squeeze itself out the confines of the higher-density cooler air), it eventually reaches an altitude at which its thermal energy can be more easily vented, as there are fewer molecules around to interfere with this process. This is something like the way a refrigerator or an air conditioner (these two devices actually being, in principle, the same device) works, by taking hot air with air molecules at at a high level of average kinetic energy and, by compressing this air by means of the exertion of mechanical energy (which means squeezing the hot air by squishing it with a piston driven into a cylinder, with the piston being powered by an electric motor) until the average kinetic energy of the air's molecules is severely reduced, with the result that, when the compression is released, the resulting air is much colder than it was when it entered the cylinder. Where does the heat go? Look at your refrigerator or your air conditioner: it certainly has a radiator in the back, which vents the heat into the atmosphere. And, where does the heat of a rising hot air mass covering a large portion of the surface of the earth go at it rises to a higher altitude? How about outer space? The possibility exists that outer space might just be larger, and, thus, better able to absorb thermal energy, than your apartment!

## Answers and Replies

Evo
Mentor

First, members are not allowed to continue or restart closed or deleted threads without prior approval of a mentor. If you have a question about a thread you should either PM a mentor or you can ask about the thread in Forum Feedback.

The thread was apparently temporarily deleted because the OP is a new member that mistakenly used his e-mail address as his member name and is at risk of being attacked by spam bots. We are waiting for him to choose a new name.

This thread is closed because discussions can continue in the original thread once the OP's issue is resolved.

So, next time ASK A MENTOR!