PDP & ADP

The A.D.P. is related to temperature, while the P.D.P. is related to pressure. The relationship is inverse: Lower temperatures or higher pressures encourage condensation or, in your case absorption of water by the desiccant material. Higher temperatures or lower pressures do the opposite, which is why at high enough temperatures (and sometimes lower pressures also, as in a partial vacuum) the water is driven from the desiccant material thereby "re-newing" it to be used again.

The tendency for water to condense or evaporate is a factor of its vapor pressure, which is temperature related and the air pressure.

This link offers some insight:

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html

Greg