Hailstones start life as a small nucleus, such as a salt particle or other microscopic pollutant, around which frozen condensed water particles attach themselves. This fusion, which takes place in thunderclouds high in the atmosphere, steadily increases the size of these ice particles until they fall to the earth as hailstones (Anon, 2010).
Despite being composed of water; the same as snowflakes and rain, hail is harder, heavier and larger. So what process does hail undergo to make it so different?
How does hail form?
There are loosely three different categories that could be described as hail, which are, 1) Snow Pellets, 2) Ice pellets and 3) Hailstones. These similar forms of hail are different in size, shape and hardness but all form in much the same way – via the freezing of small water droplets in supercooled thunderclouds (The Met Office, 2008).
1) Snow pellets
Snow pellets are the smallest forms of hail (<2-5mm) and are brilliant white in colour. They are easily crushed underfoot and are also sometimes referred to as ‘soft hail’ (Anon, 2007), graupel and ‘tapioca snow’. They are mainly round in shape but may sometimes be conical or cone-shaped (The Met Office, 2008).
Snow pellets usually fall in shower form, often before or at the same time as snowfall, and when the temperature is slightly below freezing (0oC). Snow pellet hail is formed when water droplets collide with supercooled cloud particles in the lower reaches of clouds (The Met Office, 2008).
2) Ice Pellets
Ice pellets are moderately sized hail (<5mm) composed of clear pieces of ice, which are sometimes conical in shape, but more often than not are spherical or irregularly shaped (Anon, 2007). Ice pellets are in affect frozen raindrops that fall to earth as sleet and bounce when they strike the ground (The Met Office, 2008).
3) Hailstones
Hailstones are off-white and irregularly shaped varying greatly in size, from small granular particles 5mm in diameter (The Met Office, 2008), to the mammoth lumps of solid ice several inches across.
Hailstones that are cut open reveal a structure of layers, similar to those of an onion, in which larger specimens may constitute several layers of clear and opaque ice (Anon, 2007). By counting these layers it possible to calculate how many times the hailstone has collided with a supercooled water droplet. By studying whether each layer is either clear or opaque, you can determine whether it froze quickly, thereby trapping some air, resulting in opaque ice, or whether the freezing was slower, giving rise to transparent layers of ice (Anon, 2010).
Large hailstones fall from deep cumulonimbus clouds, from which the cloud base may be 3,000 feet above the ground with tops as high as 60,000 feet (Anon, 2007). Most of the cloud structure is composed of supercooled water droplets as in snow pellet formation. However, the difference with hailstones is that they are formed when particles are continually caught in an updraft within the cloud, which leads to a further accumulation of ice, making hailstones larger and more abrasive than snow pellets (The Met Office, 2008).
As the hail particles fall they will collect additional water droplets, which freeze instantly, forming more and more layers of ice (Anon, 2007). This process of vigorous updrafts, and the falling of the hail particles through the cloud will continue until the particle is so big it will act under gravity and fall to earth as hail (The Met Office, 2008).
The largest ever recorded hailstone found on the United States mainland was measured at a massive seven-inches in diameter, roughly the size of a soccer ball (Anon, 2003).
The cost of Hailstone damage
Every year hailstones cause millions of dollars worth of damage around the world, but can also be fatal if they hit someone. Unsurprisingly, a lot of research has gone into finding away to disperse the volatile thunderclouds from which hailstones originate (Anon, 2010). Several initiatives such as seeding clouds with silver iodide have been tried to reduce the size of hail within the cloud. A U.S. company is also trailing a gel, which they believe could prevent a thundercloud developing in the first place (Anon, 2010). Science and technological advances could therefore one day consign destructive hailstorms to the history books.
References:
Anonymous. (2003). Largest Hailstone in U.S. History found. From http://news.nationalgeographic.com/news/2003/08/0804_030804_largesthailstone.html (Accessed 12/11/10).
Anonymous. (2007). National Meteorological Library and Archive: Fact Sheet No.3 – Water in the atmosphere. From http://www.metoffice.gov.uk/corporate/library/factsheets/factsheet03.pdf (Accessed 11/11/10).
Anonymous. (2010). BBC Weather: Hail. From http://www.bbc.co.uk/weather/features/understanding/hail.shtml (Accessed 12/11/10).
The Met Office. (2008). Weather (Eye Witness Companions). Dorling Kindersley, London.
