Extreme Events
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Twister Sisters
Weather reports in western Canada may sometimes include references to wall clouds, funnel clouds, cold air funnels, waterspouts or tornadoes. Below are descriptions for these and other related types of weather phenomena, and the conditions under which they typically develop. Treat all such events seriously and take appropriate safety precautions if they are occurring or expected in your area.
Wall Clouds
A wall cloud is a portion of a thunderstorm cloud that appears to hang
underneath the main cloud base. It is associated with the main updraft
of the storm. If the wall cloud persists for more than a few minutes and
appears to rotate, it is a sign of a possible tornado, although only about
10-15% of rotating wall clouds generate tornadoes.
Tornadoes And Funnel Clouds
A tornado is a tightly spinning column of air in contact with the ground
beneath a thunderstorm cloud. The rotating column is physically connected
to the cloud base or wall cloud and is often visible as a cloud-filled
"condensation funnel". If the air is dry enough, the tornado may only
appear as a swirl of dirt on the ground without a visible connection to
the cloud above.
In contrast, a funnel cloud spins in mid-air without touching the ground.
To tell the difference between a funnel cloud and a tornado, look for
swirling dust or debris near the ground under the funnel. If present,
then a tornado is occurring. Most tornadoes and funnel clouds are preceded
by and form approximately in the middle of an accompanying rotating wall
cloud. The Fujita scale (F0 to F5) is used to rate the severity of tornadoes
after they occur by the extent of the damage they cause. The parent storms
often are observable on Doppler radar and can be forecast.
Cold Air Funnel Clouds
These funnel clouds are spawned by large cumulus clouds or weak thunderstorms.
Typically, the days when they occur are a bit cooler than normal with
large puffy cumulus clouds, showers or weak thunderstorms developing in
the late morning or early afternoon. Since the storms are not very strong
or well organized, cold air funnels are usually short-lived and normally
do not have the energy to reach the ground. However, a small percentage
may touch down briefly and can become destructive over a very small area.
In general, they are less violent than most other types of tornadoes.
These funnel clouds will normally appear with little or no warning. Meteorologists
can predict the days when cold air funnels are likely to occur over a
general area. When they are predicted or sighted, a special tornado watch
will be issued that outlines the need for caution but recognizes their
weak and short-lived nature.
Landspouts
Landspouts are a form of tornado that occurs in most parts of Canada.
Landspouts usually have a narrow, rope-like condensation funnel extending
from the cloud base to the ground, and are seen under small storms or
large, growing cumulus clouds. They are similar to a cold-air funnel that
has touched down, and while usually weak and short-lived, they can be
potentially dangerous. Like their cold air cousin, landspouts can form
from rather nondescript developing cumulus clouds or thunderstorms, often
before precipitation is visible on radar. Specific tornado warnings are
issued when landspouts are predicted or reported.
Waterspouts
The waterspout looks like a slender tornado but occurs only over water,
or the nearby shore, in the same conditions that bring cold-air funnels
or landspouts. Cool, unstable air masses passing over warmer waters allow
vigorous updrafts to form, which can tighten into a spinning column when
captured by a passing thundershower. Waterspouts are just as dangerous
on water and shoreline areas as landspouts are on land, but often collapse
after moving a few hundred metres inland, away from the warm water. A
true waterspout forms over the water and is not accompanied by a strong
storm. If conditions are cool and cloudy, with showers but no organized
storms, then the appearance of a tornado-like funnel over water can be
identified as a waterspout. If a severe storm with a tornado happens to
pass over a stretch of water, the tornado is sometimes called a tornadic
waterspout and would be just as dangerous as a tornado is over land.
Gustnadoes
"Gustnadoes" typically appear as a swirl of dust or debris along the
"gust front" of a thunderstorm. They are not directly linked with rotation
in the thunderstorm itself and can form a considerable distance away from
the parent storm. There is no condensation funnel or other visible connection
to the cloud base. Gustnadoes account for a large number of the weakest
tornado reports each year. Their localized impact and damaging effects
have allowed them to be counted as tornadoes but most are probably not
"true" tornadoes. The strong, straight line winds that can follow behind
the gustnadoes are likely to cause more damage than the gustnadoes themselves.
Gustnadoes are not visible on Doppler radar. Meteorologists do not forecast
gustnadoes, but can issue Severe Thunderstorm Warnings for the gust fronts
of stronger storms that are detectable on Doppler radar.
Straight Line Winds
A "plough" wind is a strong, straight-line wind associated with downdrafts
that spread out ahead of isolated thunderstorms or small clusters of thunderstorms.
These winds push across the ground like a blade in front of a snowplow
or bulldozer, damaging susceptible objects and scattering the debris.
The damage caused is often mistakenly attributed to a tornado. The wind
may roar as it passes by. Damage can be heavy and confined to narrow zones
like that caused by tornadoes. Plough winds (and the damage they cause)
are all in one general direction, rather than rotating as in a tornado.
A plough wind is termed "severe" if it exceeds 90 km/h. "Derechos" are
areas of greater wind damage associated with clusters of intense thunderstorms
or lines of closely-spaced thunderstorms. They are longer-lived and more
intense than plough winds.
Dust Devils
On sunny, dry days, heated air near the ground can rise in small, spinning
columns. If these columns occur over dusty ground, a dust devil may be
observable. The stronger ones become visible when loose grass, hay or
dust gathers into the whirl and rises up the column. These dust devils
can look like a weak tornado at the bottom but rarely extend higher than
100 metres. They are only seen in fair weather - sometimes without a cloud
in the sky. Larger dust devils can extend hundreds of metres high, toss
lawn furniture and lift objects weighing a hundred kilograms, but are
generally not a threat otherwise. Dust devils near a highway deserve caution
as vehicles passing through them can be difficult to control. Like other
weak circulations, meteorologists can tell which days and general areas
are most likely to have dust devils, but they cannot be forecast and are
not observable on Doppler radar. Severe Weather Warnings are not usually
issued for dust devils.
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