# Altitude and air density

by sgstudent
Tags: altitude, density
 P: 645 As we go higher up a mountain, the air pressure decreases. But why would that cause the air density to decrease? Since pressure is hpg, so if h decreases pressure decreases but what causes the air density to decrease as well? Thanks for the help :)
 P: 4,238 Gases are compressible. More pressure means they are compressed into a smaller volume. Liquids not so much. Water has about the same density in a column.
P: 645
 Quote by A.T. Gases are compressible. More pressure means they are compressed into a smaller volume. Liquids not so much. Water has about the same density in a column.
Oh so at the surface of the earth, because the pressure is greater so it is more compressed while higher up the pressure is less so the compression is smaller?

But then again now with only one formula, P=hpg now that h and p decrease so the pressure will decrease disproportionately?

 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,750 Altitude and air density It's a direct consequence of PV = nRT.
P: 645
 Quote by SteamKing It's a direct consequence of PV = nRT.
How would you apply that formula here?

Thanks
Emeritus
PF Gold
P: 5,196
 Quote by sgstudent How would you apply that formula here? Thanks
If you define N as number of particles (as opposed to number of moles), the ideal gas law is PV = NkT where k is the Boltzmann constant, a fundamental physical constant. Divide both sides by V and you get

P = nkT

Where n = N/V is the number of particles per unit volume. Multiply n by m, the mass per particle, and you get the mass per unit volume (aka density) rho. Hence n = rho/m and the ideal gas law becomes

P = (rho/m)kT

For air, there is more than one type of particle so m is a weighted average of the masses of the different molecules.
P: 645
 Quote by cepheid If you define N as number of particles (as opposed to number of moles), the ideal gas law is PV = NkT where k is the Boltzmann constant, a fundamental physical constant. Divide both sides by V and you get P = nkT Where n = N/V is the number of particles per unit volume. Multiply n by m, the mass per particle, and you get the mass per unit volume (aka density) rho. Hence n = rho/m and the ideal gas law becomes P = (rho/m)kT For air, there is more than one type of particle so m is a weighted average of the masses of the different molecules.
Oh so from this equation how will we show that the density and pressure changes with altitude?
Emeritus