Hail is a type of graupel
(a form of precipitation) composed of balls or irregular lumps
of ice. It occurs when supercooled water droplets (remaining in
a liquid state despite being below the freezing point, 0 °C/32
°F) in a storm cloud collide with some solid object, such
as a dust particle or an already-forming hailstone. The water
then freezes around the object. Depending on the wind patterns
within the cloud, the hailstone may continue to circulate for
some time, increasing in size. Eventually, the hailstone falls
to the ground, when the updraft is no longer strong enough to
support its weight.
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Hail often forms
in strong thunderstorms, often along a cold front, where the layer
of air on top is much colder than that on the bottom. The smaller
hailstones can bounce up and down between the warm and cold layers
due to updrafts and gravity. The longer the stones bounce around,
the larger they grow. For the same reason, larger hail can occur
in warmer regions in the world due to stronger updrafts. These
strong, severe, or even supercell thunderstorms can also occur
in summer, even without a cold front. Hailstones, while most commonly
only a few millimetres in diameter, can sometimes grow to several
centimetres or occasionally even bigger. Such big hailstones can
do serious damage, notably to automobiles, skylights, and glass-roofed
structures. Pea or golfball-size hailstones are not uncommon in
severe storms. Rarely, massive hailstones have been known cause
concussions or to kill people by causing head trauma.
Above Images Are From The NOAA
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