What is hail?
Hail is precipitation consisting of ice pellets with a diameter
of five millimetres or more which is formed in the core of thunderstorms
in the following way: water vapour in warm, rapidly rising air
masses (called convection currents) condenses to become water
at higher, cooler altitudes. Normally, this just produces heavy
showers. However, if the vapour or water is pushed up enough to
meet sufficently cold air, the product can be ice. Layers of ice
can then form around minute particles such as dust whipped up
from the ground. These nuclei increase in size as more water freezes
to their surfaces. When the ice pellets are too heavy for the
ascending air currents, they fall as hail. They may pick up more
water on the way down, becoming larger, heavier and more threatening.
As mentioned, hail pellets have a minimum diameter of five millimetres (or half a centimetre). Below that
size, droplets are defined as snow or ice pellets. Hail pellets can grow to be much bigger than the
minimum: they can exceed ten centimetres in diameter - the size of a grapefruit.
When and Where they Occur
Hailstorms can occur anywhere in Canada, but happen most frequently
in the west and in southwestern Ontario. Parts of the Prairies,
particularly near Calgary, can expect up to ten hailstorms each
year. The interior of British Columbia, southwestern Ontario and
some other parts of the Prairies receive an average of three hailstorms
per year. Storms with small hail pellets occur frequently in the
Atlantic provinces, eastern Quebec and near the Pacific coast, but
those with large pellets are more common in the continental interior
(from the lee - eastern slope - of the Rockies to southernmost Saskatchewan).
Hailstorms are most common in the May to July period, usually with
storms occurring in the afternoon. The hail portion of a storm (the
hailfall) usually lasts from six to ten minutes.
The Damage they Cause
For many Canadians, a hailstorm is an intriguing rarity, but for
farmers whose crops are crushed, or for other Canadians whose homes
and cars are damaged, a hailstorm is an unpleasant surprise, or
even a disaster. A sudden violent hailstorm, with large hail pellets,
is spectacularly damaging. But equally important are smaller storms
during the summer growing season as these cause serious damage to
crops. Because hail can hit the ground at up to 130 kilometres per
hour, it can cause severe damage to crops, houses and vehicles,
and can injure people and animals. To reduce the damage caused,
governments and insurance companies have sponsored cloud seeding.
This involves having aircraft release tiny particles into probable
storm clouds with the aim of creating smaller, softer hailstones.
Descriptions for each of the hailstorms shown on the map are found
in the document Major
Hailstorms.
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