What are hurricanes?
"Hurricane" is one of the names given to a tropical storm whose
winds revolve around a centre of low pressure, called the eye, at
a minimum speed of 120 kilometres per hour. In the Caribbean and
on the west coast of Mexico, this phenomenon is called a "hurricane";
in the South China Sea, the Philippines and Japan, it is a "typhoon";
and in the Indian Ocean, it is a "cyclone".
In the low-pressure systems which constitute hurricanes, the pressure
difference between the centre and the outer edges is very great,
generating strong winds. Initiated by the Earth's rotation, the
Coriolis effect causes these winds to turn in a counterclockwise
direction in the northern hemisphere and clockwise in the southern
hemisphere.
At the centre of a hurricane's vortex - the eye - there is a calm area of blue sky. The diameter of this
clear-weather area is between five and 50 kilometres. Around this central column, there are very strong
winds accompanied by torrential rains. The wind strength is moderate at the outer edges of the hurricane,
but wind speeds increase rapidly as one approaches the centre.
How do hurricanes occur?
Hurricanes form over warm waters of the oceans between 5° to 25°
north latitude and 5° to 25° south latitude. Most hurricanes affecting
North America originate off the west coast of Africa.
They get their energy in the following way: the sun's heat warms the water in the ocean, which in turn
heats up the air above the water by convection; the warm air rises, carrying with it water that has been
evaporated. The process of evaporating water uses up a lot of energy; the rising water vapour therefore
contains huge quantities of energy that will be released when the vapour cools and condenses to water.
The rising air column generates an area of low pressure. Once aloft, the air cools and the water vapour
condenses, releasing its energy.
Every year, several hundred tropical storms form over tropical oceans, but usually fewer than ten become
hurricanes. However, when conditions in a tropical storm permit, the process can build on itself with the
result that winds blow violently in a spiral around the eye. These winds are accompanied by columns of
cloud of the cumulus type which generate torrential rainfall.
Paths of Hurricanes
In the tropics, a hurricane moves slowly from place to place, but
its speed tends to increase as it heads northward. When a hurricane
reaches an area of cooler water, its thermal energy declines and
it tends to abate. Moreover, when a hurricane passes over land,
the effect of friction and the loss of energy contribution by a
surface of warm water reduces its energy considerably.
Hurricanes normally travel northwestward and then turn northeast, but their paths vary considerably and
are often erratic. The further a hurricane moves from the equator, the more it is deflected by the prevailing
west winds, and the more its path changes direction towards due north or northeast. This is why it is rare
for hurricanes to hit Canada with their full force. Even Canada's most destructive hurricane - Hurricane
Hazel - moved in a northeast direction, but as its path was well to the west of most hurricanes, it passed
overland before hitting the Great Lakes area. This is why this hurricane was Canada's most destructive, because even though it had lost strength it brought violent winds and torrential rain fall.
The Impact of a Passing Hurricane
The radius of a hurricane, as measured from its eye outward, is only about 30 kilometres. In this area
occurs the main system of extremely strong rain and wind. However, a large area beyond the system is
affected by violent winds and heavy, continuous rain. It is usually this outer area that affects the eastern
parts of Canada. However, the most destructive effect of a hurricane is the surge of sea water onto the
land, causing serious flooding.
In Canada, where and when do hurricanes occur?
In North America, hurricanes strike between June and November,
but most of them occur between August and October, when the ocean
surface is warmest.
Hurricanes which form in the Gulf of Mexico and in the western
Caribbean Sea in early summer rarely reach Eastern Canada. However,
those along Florida's Atlantic coast in late summer can affect the
Atlantic provinces.
Even though hurricanes rarely vent their full fury on Canada, the
eastern part of the country (from the Atlantic provinces westward
to the eastern part of Lake Superior) is exposed every year to the
final effects of a hurricane. As they move along the Atlantic coast,
these hurricanes exhaust their energy, discharging the rest of their
heavy rain and violent winds on this region and causing floods as
they pass.
Canada's best-known hurricane was Hurricane Hazel, which hit southwestern
Ontario on October 14 to 15, 1954. Its main impact was flooding
- the seven inches of rain which suddenly hit Toronto washed away
entire streets in river floodplains and caused more than 80 deaths.
For its part, British Columbia can also receive the final effects
of typhoons originating off Asia as they pass over the Pacific.
An example was Typhoon Freda which hit Victoria and Vancouver on
October 12, 1962 and caused seven deaths and extensive damage.
Descriptions for each of the major hurricanes shown on the map are found in the
document Major Hurricanes.
|
