Raindrops are limited in size because the chance of disruption increases with size. One cause of breakup is the aerodynamically induced circulation of water in the drop. Kamabayasi et al. (1964) have given data on the probability of this "spontaneous" breakup as a function of drop size, and the size spectrum of the small drops produced. Their results show that once a diameter of approximately 3 mm is reached, it is not sure that surface tension can hold the drop together and that a drop of 6 mm diameter is unstable and can exist only briefly before breaking apart. Theoretical calculations of the growth of raindrops including spontaneous breakup have yielded size distributions that are flatter than those observed. Recent theoretical and observational evidence has indicated that water drops may actually reach diameters as large as 10 mm before spontaneous breakup occurs, and it is now thought that this mode of disintegration may be unimportant in the development of raindrop spectra in the atmosphere.
Another cause of breakup is collisions between drops. In a laboratory study of water drops of diamater 0.3 to 1.5 mm colliding at an angle with relative velocities ranging from 0.3 to 3 m/s, Brazier-Smith et al. (1972) found that permanent coalescence becomes less likely for increasing values of drop size, relative velocity, and impact parameter. Collisions at grazing incidence produce a spinning, elongated drop which may quickly fly apart, resulting in the formation of satellite drops. Disruption occus when the rotational kinetic energy for the coupled drops exceeds the surface energy required to produce separate drops....
