P = F/A         (pressure = force * area)

P/T = constant  (The ratio of pressure and temperature is constant
                 in a gas if the volume is fixed = [Charles' Law])

Earth's surface (that is, at a pressure of one atmosphere), if the 
temperature is 0 degrees Celsius (that is, 273 degrees Kelvin),

Majority of text TK.

http://plabpc.csustan.edu/general/tutorials/temperature/CharlesLaw/CharlesLaw.htm