Water does exist on our planet in three forms. Liquid known as water, solid known as ice and the third form as a liquid but at temperatures below freezing.

Water doesn't necessarily freeze at 0 degrees Celsius and can exist in the atmosphere as a liquid down to temperatures as low as minus 40 Celsius; when it does it is known as supercooled water.

So as the air rises up in a large cumulonimbus or thunderhead cloud and condenses out, all three forms of water are present.

The hailstone starts with a small nucleus; in this country it is quite likely to be a salt particle or some other microscopic pollutant, on which the water condenses often forming an ice crystal. The up draughts and down draughts in this huge cloud, which can have a base at 2000 Ft and a top as high as 40000Ft (even to 60000Ft in the tropics) sweep this ice crystal up and down, and as it touches a supercooled water droplet, they flow over it and freeze forming a layer of ice.

This process of going up and down in the cloud can occur many times with several layers of ice forming on the original ice crystal. And it is only when the hailstone is too heavy for the vigorous up draughts to keep it in the cloud that it falls to the ground. In general the larger the cumulonimbus cloud, the stronger the up draughts in it and hence the larger the hailstones.

It is in the tropics where we see the largest hailstones, often the size of golf balls, which can do tremendous damage to us and to our crops and greenhouses.

The heaviest hailstones on record weighed in at over 2 pounds (1 Kilogram) and were reported to have killed 92 people in Gopalganj, Bangladesh, on the 14 April 1986.

In this country we do not get such large hailstones but on the 5 September 1958 at Horsham on West Sussex, hailstones of 5 ounces (142 grams) were reported.

Hailstones are generally white and if you manage to get one large enough and cut it open, you will see has the appearance of the inside of an onion and it is made up of several layers of ice. By counting the number of layers you can deduce how many times the hailstone has collided with a supercooled water droplet. And by studying whether each individual layer of ice is transparent or opaque, you can see whether it froze quickly, thereby trapping some air, giving opaque ice, or rather more slowly to give a layer of transparent ice.

Because these storms can be so destructive quite a lot of research has gone into modifying them to make them less potent. This has mainly been done by seeding the clouds with silver iodide which greatly reduces the size of the hail, and using this method in Kenya it has been claimed that loses to the tea plantations have been reduced by over 50%.

There is a company in the USA who have now produced a gel that is claimed to be able to almost totally destroy a building thundercloud, and experiments with this product are now being conducted.

So maybe in time destructive hailstorms could be a thing of the past.

Related Links:

- Weatherwise - Hail
- Weatherwise - Storms and Storm Clouds