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How Hail is Formed

The process of how hail is formed is not an unusual one. The easiest way to think of it, is to think of the sky as a vertical gradient which gets colder as you ascend (though this is not always the case if there is an inversion), but let us assume for this example that the air gets colder the higher you go.

This allows for two different scenarios for hail to form. You can either require an extremely large cloud with strong updrafts in a warm temperature environment or you can have a moderately large cloud with fairly low uplift in a cold air environment.

Summer thunderstorms which are formed from surface heating often tend to be created in a very unstable environment. This is the kind of environment where the hail formation process consists of rain drops being lifted to extreme heights within the cumulonimbus towers and unlike weaker updrafts in normal storms, the cloud can hold the weight of hail for a longer period of time without letting it fall. To put it in a way that’s easy to understand, picture laying on your back with your arms out straight performing a lifting motion, your arms and the force you generate representing the uplift of the storm, now imagine someone increasing the weight every 10 seconds, eventually your arms (uplift) won’t have enough power to maintain the weight they’re holding and the objects will fall, the same way the storm drops the hail.

If these storms updrafts are strong enough you will often see (once the hail falls) that the hail itself is visibly more than a single hailstone and rather a collection of hail stones, this is because hail stones in severe thunderstorms are rarely singular raindrops. You can think of the inside of a thunderstorm cloud as a washing machine and inside that washing machine you have magnets placed, now as the machine spins more and more- the magnets will touch each other and when they do they will bond together. This is the same process that takes place within a thunder clouds, hail stones merge with other hail stones and grow however large they can before the updrafts in the storm can no longer hold that weight. Hail stones can reach as large as softballs given that there is enough uplift in the storm.

In some areas such as Cape Town where thunderstorms are less common, the process of hail forming is somewhat the same but slightly different.

Winter hail is sometimes more like sleet, though only visually. Not to be confused with sleet in the UK and Canadian definition of the word. In the UK sleet is used to refer to a mixture of snow and ice that falls, while in the US it is ice pellets.

In the Cape Town winters rain is brought in the form of cold fronts, temperatures during cold fronts tend to be in the 10’C – 20’C mark, a lot cooler than the 30’C one can expect in summer thunderstorms. The formation of hail in these cold fronts occurs when isolated embedded cumulonimbus clouds have enough uplift to bring the rain to an altitude of freezing temperatures. Unlike the summer thunderstorms where the clouds have enough uplift to keep the hail in the cloud for an extended period of time, these cold front related cumulonimbus clouds tend to have weak uplift and hail will typically fall shortly after formation, the result is much smaller hail stones in comparison with the former storms. Hail from these storms are typically no bigger than a pea.



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