Severe Storms

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Transcript Severe Storms

Severe Storms
D 64-77
What are thunderstorms?
The most common kind of severe
• Form from clouds called thunderheads
(cumulonimbus clouds)
• Can produce huge electric sparks called
lightning; The lightning heats up the air
which causes thunder
• Usually have heavy strong winds along
with strong rain showers and possibly
First Stage
Intense heating causes air to rise very
A cloud forms where there is an upward
rush of heated air (an updraft)
• As more and more warm moist air is carried
upward the clouds grow larger and larger
Strong updrafts keep droplets of water
and ice crystals in the cloud so they grow
in size
Second Stage
Once the rain falls it causes
downdrafts in the clouds
• As it moves downward it builds up static
electricity causes lightning
Lightning is unpredictable and may jump
from cloud to ground or ground to cloud; it
can also jump from one spot in the cloud to
 It superheats the air which begins to expand
causing thunder
Third Stage
The storm dies when the downdrafts
become stronger than the updrafts
 Heavy rain lightens up and stops
 Thunderstorms usually form in the
warm air just ahead of a cold front
How and where do tornadoes
The most violent thunderstorms
often spin off even more dangerous
• Tornadoes are violent whirling winds
that move across the ground in a
narrow path
How they happen?
Late in the day
when the Earth’s
surface is very
warm convection
can get very
strong and can
lead to tornadoes
A tornado is a
convection cell
Updrafts in a convection cell are strong
and the air rushes in from all sides at
high speeds
The air curves in and lowers the pressure
even more; air rushes faster and faster
and the pressure continues to fall
causes the tornado to spin faster
As the tornado gets stronger a funnel
forms and starts to drop to the ground
 In the center of a tornado the speeds can
reach about 300 mph
The speed of the wind in a tornado is
not the speed at which the tornado
moves across the ground
• Tornados can continually change
Where they happen?
Most tornados
occur in the
Midwest and the
• They form where
there is dry cold air
masses that mix
with warm moist
• Waterspouts
tornados over water
How do hurricanes form?
A hurricane is a very large swirling
storm with very low pressure at the
center (the eye of the storm)
• They form over tropical oceans near the
• They forms many thunderstorms
Strong heating and evaporation over the
ocean cause a large low pressure center
to form; winds begin to blow towards the
center causing it to move upward and
form rings of tall thunderstorms
The Coriolis effect causes the winds to
spiral counterclockwise; clusters of
thunderstorms merge forming a single
large storm
As water vapor in the storm
condenses heat is released and the
air is warmed; this decreases the
air pressure
The lower air pressure the faster
the winds that blow toward the
center of the storm
 When the winds reach 75 mph it’s
considered a hurricane
As the moist air in the storm rises
and cools condensation occurs; the
clouds thicken; heavy rain falls
Hurricanes can grow to be more
than 400 miles in diameter
• Hurricane Fran was almost as large as
How do hurricanes affect ocean
Hurricane winds whip up large waves
in the ocean that move outward form
the storm and pound against the
shore for days before the storm
actually arrives
• Storm surge a great rise of the sea
along the shore; caused by low air
Air pressure normally presses down
on the surface of the sea like a giant
hand, when the pressure drops its
like the hand lifting up
 Hurricane winds push the water
ahead of the storm forcing waster
inshore and adding to the storm
For thunderstorm, tornado, and
hurricane safety rules look at D 7475