Mean monthly values of precipitable water were related to mean monthly values of surface dew point for 15 stations distributed over the continental United States. The degree of relationship was excellent, and a regression equation relating the two parameters was computed. Since surface dew point is so well related to precipitable water, this implies that the rate of decrease of moisture content through a deep layer tends toward a constant, which was approximately evaluated. Abstract Mean monthly values of precipitable water were related to mean monthly values of surface dew point for 15 stations distributed over the continental United States. The degree of relationship was excellent, and a regression equation relating the two parameters was computed. Since surface dew point is so well related to precipitable water, this implies that the rate of decrease of moisture content through a deep layer tends toward a constant, which was approximately evaluated.