Why is it that a propane tank has water residues freezing on the outside?

the liquid propane as whats in your tank and is what is delivered to you is minus 44 degrees. what you burn in your house is a vapor at half a pound or 10.5 inches water column. the vapor in your tank comes from vaporization or the boiling of the l.p as it is being drawn through the regulator. while the liquid state in your tank is -44 degree's condensation occurs on the outside at the liquid level the same as pouring yourself a ice cold drink and the condensation appears on the outside of your gl.