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Once formed, a tornado will reach maximum intensity fairly quickly. Weaker
and stronger phases may alternate and the funnel cloud may lift briefly or change
in diameter. At peak intensity, you will usually see a dark mass of debris at
the base of the tornado, with the debris revolving upward in a tightly controlled
fashion. Low, wide tornadoes can be spread out,with several apparent contact
points or separate vortices within a broad circular region which show both debris
and condensing cloud matter mixing as they ascend.
In the waning stage,the tornado lifts and becomes thinner. Debris decreases
suddenly and become diffuse or spreads out horizontally. The condensation funnel
shrinks upward and becomes twisted, curved, or less vertical in orientation.
It continues to shrivel in this rope stage until it vanishes or retracts into
the cloud base. The wall cloud also loses structure and size but may rebuild
again in a while.
What it's like near a tornado
If a tornado is nearby and moving closer, forget about the sightseeing and
take immediate measures to protect yourself! (See Safety Tips, inside back cover.)
Tornadoes move with the storm cloud, at the same speed and direction, typically
30-50km/hr from the SW or W. Their motion is, therefore, reasonably slow and
predictable. Two factors can exist to alter this regular movement: the wall
cloud can be rebuilding (often eastward), causing the tornado, especially if
it is wide, to also shift and reform forward in steps; and, strong, persistent
tornadoes can move along a looping path where straight motion is replaced for
short periods by a tightening-up and northward twist in the motion. Tornadoes
are highly organized and quite well-behaved structures, and there are rarely
any sudden changes or surprises in the direction of movement.
Contrary to popular belief, it is not windy near most tornadoes. Within a few
kilometres, the air will be unusually calm since the storm's giant updraft is
overhead. Although weaker tornadoes tend to begin and end abruptly, some strong
tornadoes induce a steadily increasing wind outside of the destructive vortex
itself. Even without strong winds, it is dangerous outside near a tornado because
pieces of debris can fall from the sky without warning (most likely NW-NE of
the tornado).
In any one place, the severe tornadic wind will last from several seconds
to, at most, two minutes and be accompanied by a variety of sounds caused by
the damage to buildings, trees, etc. Some people also hear a roar, but many
tornadoes have no sound with them at all. If the sound is irregular it is likely
the result of damage occurring nearby; but if it is a steady, softer sound that
originates in the sky or toward the main storm cloud, it is more likely caused
by large hailstones hitting the ground or colliding in mid-air.
Most tornadoes occur near the back side of a worst-last storm, near a brightening
sky to the west, and without any precipitation. Heavy rain and hail may precede
its arrival but once the updraft core is overhead, only a few scattered hailstones
will interrupt the eerie calm. After its passage, some light rain or hail can
occur briefly,along with cooler W-NW winds,before clearing takes place.
Other rotating structures
There are several other rotating structures to watch for and report which
do not form under a typical wall cloud or storm updraft base.For all of these,
your description of their location in the storm, associated features, and general
situation will help the weather office determine the true risk and the need
for further action.
Cold-air funnels
Cold-air funnels are true funnel clouds by appearance but will not lead to
a damaging tornado. They are usually brief and thin, and often occur under the
rain-free base of weaker storms or large cumulus during relatively cool, moist
weather from spring to fall. A common situation,seen more often on the Prairies,
is a spell of cool, windy, unsettled weather in summer. Large cumulus fill
the sky and showers are scattered about. Some of the larger clouds will have
ragged bases and bits of scud below them. If an updraft is particularly strong,
the rising air can tighten into a small vortex which you see as a funnel. A
brief, weak touchdown is rare, but possible. The "cold-air" refers to the situation,in
which the temperatures aloft are relatively cold for the time of year.A special
weather bulletin is issued by MSC for these funnels because they can look so
much like a tornadic funnel.
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Created :
2002-08-26
Modified :
2002-12-19
Reviewed :
2002-12-19
Url of this page : http://www.msc.ec.gc.ca /education/severe_weather/page38_e.cfm
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