The theory of hailstone formation

Abstract

AbstractA fairly detailed mathematical analysis is presented of the generally accepted theory that the formation of large hailstones is due to the capture of under‐cooled water drops which lie in their path. It is shown that the principal factors which determine the ultimate size of a hailstone are its average density, the height at which its nucleus is formed, the average upward velocity of the air and the concentration of condensed water in the region of the atmosphere where the temperature is below 0°C. The interrelation between these various factors is shown by means of a number of curves.It is further demonstrated that the values of water content of the clouds and of upward air velocities, measured or deduced independently, are sufficient to account for the formation of hailstones at least 8 cm. in diameter.The matter of the disposal of the latent heat of the water which solidifies on the surface of the hailstone is also examined in some detail, and it is shown that this heat is disposed of quite effectively, partly by conduction to the surrounding atmosphere and partly by evaporation from the surface of the hailstone. Only in the case where the surface temperature of the hailstone approaches 0°C. does its incapability to get rid of its surplus heat act as a factor retarding its rate of growth, and consequently its ultimate size.