Wind, storms, cyclones, tornadoes
and lightning!
The storm warning comes over the radio. Outdoor furniture and loose
materials are put away, windows closed and all are prepared for the
onset of the winds and rain. A familiar occurrence in coastal Australia,
storms form a very important component of our weather systems. Their
violence is often destructive, their rain welcome and power awe inspiring.
Severe storms are more common than any other natural hazard and they
occur everywhere in Australia. On average, unclassified storms are responsible
for more damage each year than tropical cyclones, earthquakes, floods
and bushfires. Each year between 5 and 10 people are killed by lightning.
Tornadoes have caused at least 41 deaths in Australia's short recorded
history.
The anatomy of a storm
A storm is produced when hot air and cold air mix,
releasing huge quantities of energy and creating strong winds, rain,
hail, lightning and thunder. Sometimes very large storms are driven
by massive quantities of hot moist tropical air, these types of storms
are known as cyclones in Australia or as hurricanes and typhoons overseas.
Tropical
cyclones occur in northern Australia during summer when the overhead
Sun evaporates immense quantities of water creating huge reserves of
stored thermal energy. The warm moist rising causes wind to blow in
towards the centre of the storm while the rising air soon cools forming
storm clouds and releasing some of the stored energy from the moist
air. As more and more energy is released and more hot moist air is fed
into the storm, it becomes larger and stronger eventually reaching proportions
large enough to sustain itself and become a cyclone.
Tropical
cyclones soon die out when they move over land because the source of
energy, moisture from the sea, is no longer available to drive the cyclone.
Tornadoes and water spouts
Tornadoes occur when hot moist tropical air meets cold dry air. Tornadoes
are especially well known over the southern states of the U.S.A. but
also occur in Australia, mainly in south-eastern Australia.
A tornado is a very violent windstorm in which the air whirls rapidly
upwards in a vortex, forming a funnel shaped cloud. Tornadoes are associated
with larger thunderstorm activity and form in the edges of the storm
clouds and descend until they reach the ground. They can range in width
from a few metres to hundreds of metres, their winds have been measured
at more than 450 km/h.
There is no generally agreed theory for the formation and maintenance
of tornadoes, waterspouts, and other vortices. Eyewitness accounts associate
particularly bright blue lightning in the eye of tornadoes and theories
involve both electrical activity and the formation of circulating winds
formed by the enormous energy released when large quantities of warm
moist air are suddenly cooled when two air masses meet.
The
largest and most dangerous tornadoes definitely occur in the southern
U.S.A. with over 1000 tornadoes recorded in some years. In Australia
we quite often see much smaller vortices such as 'Willy-Willys', 'Dust
Devils' or 'Whirl Winds'.
Waterspouts are tornadoes that occur over water. The whirling air
sucks up water creating a very well defined column of rising water.
Fish caught in waterspouts have been known to fall to the ground many
kilometres from the sea having been carried by the waterspout and associated
storm.
Lightning
The lightning flashes that accompany thunderstorms are enormous electrical
sparks caused when electrical charges build up in the storm clouds.
Most lightning occurs within or between adjacent storm clouds. Friction
within clouds created by updraughts and the movement of air creates
the build up of negative and positive charge within a storm cloud. Lightning
within clouds occurs when enough charge difference is built up and an
electrical discharge jumps from a negative region to a positive region
within the cloud.
Lightning strikes from cloud to Earth occur slightly differently. Here
negative charge build up within a cloud repels negatively charged electrons
within the ground and induces (or creates) a positive charge on the
surface of the Earth. Lightning will tend to strike the nearest place
of accumulated charge on the ground, hence church spires, trees and
even golf clubs can act as lightning conductors as they allow the accumulation
of charge to occur.
Lightning itself involves a 'leader' stream of electrons descending
from the cloud towards the ground in a series of jagged steps and branches
as it tries to find the path of least resistance. Once the lightning
strikes the ground, it tries to dissipate its charge again along the
route of least electrical resistance. This may be back up to the cloud
along the original lightning strike path, so what at first appears to
be a single movement of the lightning can actually be a series of rebounds
back and forth between the cloud and the ground.
The
light from lightning is created as the electrons steaming along the
lightning path, smash into and ionise molecules in the air making them
emit light that we see as the flash.
Thunder
Thunder occurs when lightning dramatically heats nearby air, expanding
the air very quickly. This creates an area of low pressure around the
lightning flash. Upon cooling, surrounding air rushes back into the
low-pressure zone. The sound we call thunder is created as part of the
initial explosion of air and as the inrushing air meets again.
Wind categories
The Bureau of Meteorology uses the Beaufort Wind Scale to measure wind
strength. Using this scale, it is possible to estimate wind strength
and understand the effects that may be expected from a wind warning
in a forecast.
Category |
Wind Speed |
Description |
Calm |
0 km/h |
Smoke rises vertically. |
Light Winds |
0 less than 20 km/h |
Leaves rustle and wind felt in the face. |
Moderate Winds |
20 - 29 km/h |
Paper and loose dust stirred, small branches
move. |
Fresh Winds |
30 - 39 km/h |
Small trees begin to move. |
Strong Winds |
40 - 50 km/h |
Large branches move on trees. |
|
51 - 62
km/h |
Whole trees in motion,
difficult to stand in the wind. |
Gale |
63 - 75 km/h |
Twigs break from trees, cannot walk in
the wind. |
|
76 - 87
km/h |
Roofing dislodged,
large branches break. |
Storm |
88 - 102 km/h |
Trees uprooted, buildings damaged. |
|
103 km/h
or more |
Extensive and widespread
damage to buildings and infrastructure. |
|