LIQUID WATER IN SQUALL LINES AND HURRICANES AT AIR TEMPERATURES LOWER THAN −40° C.

Abstract

In convective clouds jet aircraft encounter icing at air temperatures lower than ?60° C. It may be reasoned that this results from (1) the melting of frozen droplets due to dynamic or frictional heating in the quasi-potential flow around the air foil; (2) the impact of precipitation consisting of ice spheres with liquid water cores; (3) the possibility that raindrops are carried upward by vertical currents so rapidly that the water temperature remains higher than the spontaneous nucleation temperature (?40° C.) while the air temperature is much lower; or (4) the possibility that the spontaneous nucleation temperature is pressure dependent or otherwise variable. This paper investigates the third of these possibilities and finds that the difference in temperature of liquid water and air at 12 km. can be no more than a few degrees except in the case of very large drops carried through deep layers of cloud by intense steady-state updrafts of the kind associated with hail-producing thunderstorms.