Here is a basic explanation:
(P1*V1)/T1=(P2*V2)/T2
P=pressure, V=volume(basically the same,small change, negligible), T=temperature
1 is those before and 2 is pressure after.
Example:leaving volume constant
(100psi*1cubic foot(just for example))/100 deg=1
so say it is 50 deg outside you would...