Hail

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.

As air rises in a large cumulonimbus or thunderhead cloud condensation occurs and all three forms of water are present.

Hail formation
A hailstone starts with a small nucleus. In this country this is 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 feet and a top as high as 40000 feet (even to 60000 feet in the tropics), sweep the ice crystal up and down As it touches a supercooled water droplet, the droplet flows over the ice crystal and freezes to form a layer of ice.

Hail types
There are three different phenomena which affect the British Isles that could loosely be described as hail.

• Snow pellets are beautifully white but are easily crushable between the fingers. They are occasionally called 'soft hail'.
• Ice pellets are quite moderate in size and are composed of clear ice, sometimes conical in shape.
• Hailstones are whitish in appearance and vary greatly in size. If you manage to get one large enough and cut it open, you will see it has the appearance of the inside of an onion and is made up of several layers of ice.

By counting the number of layers you can calculate how many times the hailstone has collided with a supercooled water droplet. 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.

Hail showers are quite common over the British Isles in westerly and northerly airstreams in spring, but really large hailstones tend to occur in the south and are very much a feature of Summer months.

Hail records
One of the most notorious places for hail is the area in the USA from Texas to Montana, and from the foothills of the Rockies to the Mississippi River, known as "Hail Alley". Total hail damage each year in the US costs over $500 million and some incredibly large hail stones have been reported.

The largest hailstone ever recorded in the US (measured by circumference rather than weight) fell from a storm in Aurora, Nebraska on June 22 2003. It measured 17.8cm wide and 47.6cm in circumference.

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.

The most damaging hailstorm to date in Australian history occurred in Sydney, New South Wales on the evening of 14 April 1999. This violent storm produced hailstones 9cm across! The insurance payout was approximately $1.5 billion.

The future
Because these storms can be so destructive a lot of research has gone into ways of modifying the clouds 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. By using this method in Kenya it has been claimed that loses to the tea plantations have been reduced by over 50%.

A company in the USA have now produced a gel that they claim is able to almost totally destroy a building thundercloud. Experiments with this product are now being conducted.

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