Download D4 – Extreme Weather: Hurricanes and Tornadoes

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D4 – Extreme
Weather: Hurricanes
and Tornadoes
The relative intensity of a hurricane is categorized by a
chart known as the Saffir-Simpson Scale. (See figure 1)
A hurricane begins to lose its intensity after landfall. This
is because it has lost its main source of energy: the heat
stored in the warm waters of the tropics.
Severe thunderstorms sometimes produce violently
rotating columns of air. When this whirling funnel of air
touches the ground, it is known as a tornado. The funnel
in a tornado spins rapidly around a very strong low pressure
center. Tornadoes typically rotate in a counterclockwise
direction in the northern hemisphere.
Guiding Question
How are hurricanes and tornadoes formed?
Key Concepts
• Severe weather conditions can result in atmospheric
disturbances of great violence and intensity.
• Hurricanes and tornadoes are violent storms that
form around strong low pressure areas.
Science Background
Hurricanes are intense tropical storms with a strong
rotating wind pattern. All hurricanes have a low pressure
center. Hurricanes must have wind speeds in excess of
119km/h (74 mph) in order to be classified as hurricanes.
Hurricanes are characterized by numerous thunderstorms
that are accompanied by strong winds and rain. They are
the most destructive storms on Earth.
A hurricane typically develops over the warm oceans of
the tropics. It initially starts out as a group of thunderstorms
that gradually begin to circulate around a low pressure
system. In the northern hemisphere, this results in a
characteristic counterclockwise pattern due to the Coriolis
effect. This storm gradually picks up energy from the
water that it travels over; the stored energy in the warm
water as it first evaporates and then condenses acts as
the energy source for future storm development. Such a
storm is known as a topical depression as long as its
wind speed does not exceed 61 km/h (38 mph). If the
storm continues to gain in intensity and achieves a wind
speed between 61 and 119 km/h (38–74 mph), it becomes
known as a tropical storm. The storm finally achieves
hurricane status when it achieves sustained wind speeds
in excess of 119 km/h (74 mph).
Tornadoes usually form in spring and early summer
when atmospheric conditions often support the formation
of violent thunderstorms. Tornadoes form when wind
speed and direction change suddenly with height. In
the United States, the most tornado-prone country in the
world, tornadoes usually form between April and July.
Tornadoes are usually only about 100 m (330 ft) wide.
They can, however, be extremely destructive because
winds in a powerful tornado can reach speeds up to
480 km/h (300 mph).
Lesson Summary
• A hurricane is an intense tropical storm with winds
in excess of 119 km/h (74 mph) circulating around
a strong low pressure area.
• Hurricanes are sustained by the energy in warm
tropical waters.
• The eye of a hurricane is the calm center of the
hurricane.
• A tornado is a violently rotating column of air
descending from a thunderstorm that touches
the ground.
• Tornadoes are extremely destructive because of
the speed of the rotating winds in the funnel cloud.
• The central plains of the United States, known as
Tornado Alley, experience more tornadoes than any
other place on the Earth.
The rotating structure of a hurricane is characterized by
a prominent physical feature known as the eye of the
hurricane. The eye is the calm center of a hurricane.
Figure 1. Saffir-Simpson Scale
Hurricane Category
1
2
3
4
5
Pressure (mb or hPa)
≥ 980
965–979
945–964
920–944
< 920
Wind Speed km/h
119–153
154–177
178–209
210–250
>250
mph
74–95
96–110
111–130
131–155
>155
Damage
minimal
moderate
extensive
extreme
catastrophic
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The Layered Earth | D4 Extreme Weather: Hurricanes and Tornadoes | 2
Review Questions and Answers
1) What is a hurricane?
2) Which way do hurricanes rotate in the northern hemisphere? Explain.
3) Why do hurricanes lose their intensity once they hit landfall?
4) What is a tornado?
5) Why are tornadoes so destructive?
6) Why is the area known as Tornado Alley so conducive to the formation of tornadoes?
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