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SEVERE WEATHER
Thunderstorms and Lightning
Thunderstorms
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Form in warm, moist, unstable air when a
“trigger” such as a mountain or cold front
forces the air to rise quickly.
Storm clouds may be up to 20 km high in
the atmosphere.
Life Cycle of a Thunderstorm
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Cumulus Stage – Air rises and a
cumulonimbus cloud forms. The rising air
is an updraft. The updraft prevents
precipitation from reaching the ground.
Mature Stage – the precipitation becomes
heavy enough to fall through the updraft
and reach the ground. The falling
precipitation creates a downdraft.
Life Cycle of a Thunderstorm
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Dissipating Stage – the downdraft
weakens the updraft, cutting off the supply
of moist air rising in the cloud. The cloud
begins to evaporate.
Lightning
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Lightning is a discharge of electricity
Typically occurs during thunderstorms
In the atmospheric electrical discharge, a bolt of
lightning can travel at speeds of 60,000 m/s
(220,000 km/h), and can reach temperatures of
30,000 °C (54,000 °F). This is hot enough to
fuse sand into glass
There are some 16 million lightning storms in
the world every year
Lightning
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The top of the cloud becomes (+) charged
and the bottom (–) charged from friction
and other processes.
This charge is then discharged as
lightning
Lightning
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Discharge of electricity.
The temperature may
reach 50,000oF. (5X
hotter than the sun’s
surface)
Thunder is produced
by the shock wave
sent out due to the
heated air expanding
so violently.
Hail Formation
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Hail is produced during
some thunderstorms
under these
conditions:
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Condensation nuclei is
available (dust)
Super cooled (<0oC)
water droplets are
carried through the
updrafts and
downdrafts of a
cumulonimbus cloud.
Types of Thunderstorms
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Single Cell: A single thunderstorm with
one main updraft
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Rarely severe
Result from local unstable atmosphere
Typical summer thunderstorm
Types of Thunderstorms
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Multicell cluster:
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Formed from a cluster of storms
Each cell is it’s own separate updraft
Can develop into squall lines
Squall Line (Multicell line):
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Form when multicell clusters from a line
Can be hundreds of miles long
Supercells
Largest and most
severe
thunderstorm that
can last for hours
Have a
mesocyclone, a
rotating updraft
Produce large
amount of rain,
hail, and strong
winds
About 30%
produce tornadoes
Most supercells and
tornadoes rotate
counter-colockwise in
the Northern
Hemisphere
Supercell Video