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Severe Storms
• Classified under
severe storms are
thunderstorms,
tornadoes, and
hurricanes.
Thunderstorms
• Thunderstorm – produces thunder and lightening and
possibly heavy rain, gusty winds, and hail.
– Can be local, single cloud storms, or multiple cells along a cold
front.
• Development of thunderstorms involve 3 stages.
– Cumulus Stage: warm, moist air is moved upward.
– Mature Stage: (about 1 hour later) heavy precipitation begins
with gusty winds, lightning, and hail.
– Dissipating Stage: cooling of the air  the storm to dies down.
Tornadoes
• Tornadoes – violent windstorms
that form a rotating column of
air (vortex) that extends
downward from a
cumulonimbus cloud.
• Most form during
thunderstorms.
• Begins with a strong wind high
up in the atmosphere that causes
winds lower in the atmosphere
to roll horizontally strong
thunderstorm updrafts will cause
the mesocyclone to tilt until it is
standing upright.
• The intensity of a tornado is
based on the wind speed and
the estimated damage.
– F0  F5 (see page 574)
Hurricanes
• Hurricane – whirling tropical cyclone that produces winds of at least 119
km (74 mph) in the U.S.
• Develop in late summer when the water temperatures are warm
enough to provide enough heat and moisture to the air.
– Tropical Depression (disturbance) – first stage with several thunderstorm
clusters and little or no cloud rotation.
– Tropical Storm – winds begin to rotate and thunderstorms progress.
– Hurricane – rotating cloud with winds of 119 km/hour or more.
• Intensity of a hurricane is based on the wind speeds.
– Category 1 is lowest with wind speeds of 119 km/hr.
– Category 5 is the most intense with wind speeds above 249 km/hr. (see
page 577)
Hurricanes (cont’d)
• Eye – center of the hurricane where air is subsiding (sinking) and being
warmed.
• Eye Wall – the layer of clouds that surrounds the eye, where winds are
fastest and rainfall is greatest.
• Rain Bands – clouds that extend outward from the eye wall and
circulates and rotates around the storm cell.
• Storm Surge – a dome of water directly underneath the eye of the
hurricane which can cause severe flooding as the storm hits landfall.