Hail Growth

To a great degree, many of the features of strong thunderstorms are related to the production of large masses of large hail within the convective storm. These features include cloud-to-ground lightning and heavy precipitation at ground level. But these features also include strong downdrafts, and outflow winds.

For a given buoyancy, it takes time for hail to grow to its maximum potential size given the strength of the updraft. If the updraft is "suppressed" quickly, it is less likely that the storm will become very strong or severe. In other words, even if the updraft is of large magnitude, say 30 m/s (dependent initially only on the CAPE), if it persists only for a short period, hail will not have time to grow to its maximum size. Around 30 m/s is the persistent updraft strength that could support hail growth to the size of golf balls. Storms that are characterized by updrafts of 30 m/s for only a brief time, will support large hail, but at least 15 to 30 minutes is needed for such updrafts to produce the maximum sized hail stone.

To a large extent, understanding why in certain situations the updraft is not suppressed (or is augmented) and becomes more long-lasting explains why storms become severe. This is the key concept in allowing us to transition from a discussion of common thunderstorms to severe thunderstorms.