Environmental Engineering Reference
In-Depth Information
HAIL
Hail is a form of precipitation composed of spheres or irregular lumps of ice. It falls in
narrow bands associated with cumulonimbus clouds and so frequently misses the
observing stations. However, the destruction it can produce is dramatic. Crops can be torn
to shreds, glass-houses ruined and even cars dented by the weight of half a kilogram or
more of ice falling from the skies.
Splitting open a large hailstone will show that it is composed of alternating layers of
clear and opaque ice (Plate 4.5). It appears that the stone is involved in complex
movements within the cloud, being swept up to the higher, colder parts of the cloud
several times. When this happens, any moisture condensing on the stone will freeze
instantly, including any trapped air, producing opaque ice. At lower levels in the cloud
condensed water takes a longer time to
THUNDERSTORMS
human impact
When the atmosphere is very unstable, cumulonimbus clouds develop, sometimes
accompanied by lightning and thunder. At night an intense thunderstorm can be one of
the most spectacular displays of the atmosphere. Flashes of lightning shoot from cloud to
ground or within the clouds, accompanied by great crashes of thunder. How can such
dramatic manifestations of energy build up in a cloud? The question has puzzled
meteorologists for many years. Recent observations suggest that the main way in which
electrical charges are separated is by the formation, growth and electrification of pellets
of soft hail, with positive charges being carried into the upper parts of the cloud (Figure
1).
Figure 1
Schematic diagram of air movement and electrical charge
concentration in a thunder cloud.
As well as electricity, the thunderstorm is often accompanied by squalls of cold wind
blowing away from the cloud. They usually originate as downdraughts of air near the
main burst of rain (Figure 1). Even hail may fall near the centre of the storm, sometimes
causing great damage.
