Transcript Slide 1

Lesson 29
What are thunderstorms,
hurricanes and tornadoes?
A thunderstorm is a storm with
thunder, lightning, heavy rain,
and strong winds.
Thunderstorms usually happen
when a cold front and warm front
meet.
They start when huge cumulus
clouds form giant storm clouds.
Lightning is caused when giant
storm clouds give off electricity.
The electricity causes the air to
warm and expand quickly.
Thunder is caused by the
expanding and contracting air.
Did you ever notice that you see
lightning before you hear
thunder?
That is because light moves
faster than sound.
Sometimes, a very low pressure
area forms over the ocean near
the equator.
A tropical air mass picks up a lot
of moisture from the warm
waters.
Such low pressure causes very
strong winds and heavy rain.
When such a powerful storm
forms over an ocean, it is called
a hurricane.
A hurricane moves in a wide
circular path.
From an airplane, it looks like
bands of spinning clouds.
The winds spiral toward the
center of the storm, which is
called the eye of the storm.
Does it surprise you that the eye
of the storm is calm and clear?
Sometimes hurricanes reach
land and cause much
destruction.
Hurricane winds can reach close
to 325 kilometers per hour, or
200 mph.
Their rains cause much flooding.
In 1969, hurricane Camille killed
almost 300 people in the
southern United States. It
caused about half a billion
dollars worth of damage.
The most violent kind of storm is
a tornado. It looks like a dark,
curving funnel reaching down
from a black cloud.
It is a small storm and only lasts
a short time, but in its path it
causes tremendous destruction.
Its low pressure causes winds up
to 800 km per hour, or 500 miles
per hour.
It blows down buildings and
trees.
It tumbles automobiles and
trucks.
It can pick up trees and carry
them hundreds of feet.
One of the worst tornados in
history happened on March 18,
1925. This “twister” was much
larger and faster than most.
It killed 689 people as it roared
across parts of Missouri, Illinois,
and Indiana.