The diurnal variation of warm‐frontal precipitation and thunderstorms

Abstract 1. Inasmuch as there is a direct relation between the amount of potential instability in the warm air and the amount of precipitation subsequently produced as the air ascends the warm‐frontal surface, the maximum precipitation should be experienced at the time when the maximum potential instability is being realized. Maximum potential instability is developed in the warm air in contact with the ground in the late afternoon. But this instability is realized only after lifting, so that the time of maximum rainfall is postponed for six to ten hours, occurring in the late evening and early morning. This maximum reaches the edge of the precipitation area eight or more hours after development of the instability, thus increasing the width of the area in the late night and early morning hours. The development of cyclonic curvature which accompanies the realization of the potential instability in the ascending current may also be a contributing factor. 2. Because latent instability is associated with potential instability, since if the latter is present, the former may develop through lifting, the maximum number of warm‐frontal thunderstorms likewise occurs in the late evening or early morning, six to ten hours after maximum potential instability has developed.