Re: Why does the temp go below 0 Celcius when you let out pressured air

This is something called the "Joule-Thomson effect"; you can find a lot of 
information on the Web if you search on "Joule-Thomson"

The cause of the J-T effect is the forces between molecules (an ideal gas, 
with no intermolecular forces, would not change temperature in such an 
expansion). Because of the weak attractive interactions between molecules, 
there will be a particular intermolecular distance where the potential 
energy for a pair of molecules is a minimum (this would be about the 
average intermolecular distance in a liquid, about one-and-a-half times 
the molecular diameter). When the gas is expanded, the molecules move 
farther apart on average, which (by itself) would give the system a higher 
energy. But if the expansion is done without any addition or removal of 
energy, the total energy must remain constant (actually, what stays 
constant in a J-T expansion is a thermodynamic variable called "enthalpy," 
but that's a detail you can learn about in college), so the higher 
potential energy due to the molecules being farther apart is compensated 
by a lower kinetic energy (which means a lower temperature).  This effect, 
observed back in the 1800s, was one of the first things that caused 
scientists to realize that there were attractive forces between molecules.

An extreme example of this effect occurs in a vapor-compression 
refrigeration cycle (like in your refrigerator or auto air conditioner). 
At some point in those systems the liquid refrigerant is expanded across a 
valve to such a low pressure that it vaporizes. This is a large increase 
in potential energy, which is accompanied by a large drop in temperature. 

I can also mention that the Joule-Thomson effect does not always produce 
cooling when the pressure is lowered; it depends on the state of the 
fluid.  Sometimes fluids actually get warmer by Joule-Thomson expansion, 
because the expansion puts the molecules in a state of lower potential 
energy.  Examples of this case include pressurized water near room 
temperature and helium gas above some fairly low temperature.