Some growth may occur during descent, but some melting also occurs near the surface, so the terminal velocity of the stone is probably not far from the peak updraft speed in the convective storm. It is now felt that some minor oscillations in height during the hailstone trajectory, plus inhomogeneities in the updraft thermodynamics likely could account for the multiple transitions from dry to wet growth phases and it is thereby not necessary for a real hailstone trajectory to include large height oscillations. Clearly, the issue of hailstone growth depends on microphysical factors that are not observed very often. Although having a strong updraft certainly is a necessary condition for the production of large hail, it is not a sufficient condition. Much remains to be learned about hail production in convective storms.
Tornadoes are relatively rare throughout most of the world.
A few places, notably the plains of North America, have many more tornadoes than the rest of the planet combined. Not only are North American tornadoes more common than elsewhere, but they attain intensities seldom seen outside of North America. Some of this might well be related to their relatively high frequency. Even in the United States, where tornado frequency is the highest in the world, tornado frequency decreases as the intensity scale increases. Thus, out of approximately 1000 tornadoes observed annually in the United States, only a handful are truly violent.