Transcript Severe Storms

Severe Storms
D 64-77
What are thunderstorms?

The most common kind of severe
storm
• 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
hail
First Stage


Intense heating causes air to rise very
quickly
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
another
 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
happen?

The most violent thunderstorms
often spin off even more dangerous
storms
• 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
runaway
convection cell
1.
2.
3.
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
directions
Where they happen?

Most tornados
occur in the
Midwest and the
South
• They form where
there is dry cold air
masses that mix
with warm moist
masses
• 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
equator
• They forms many thunderstorms
1.
2.
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
3.
4.
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
5.

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
Florida
How do hurricanes affect ocean
waves?

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
pressure
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
surge
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For thunderstorm, tornado, and
hurricane safety rules look at D 7475