Extreme Weather: Hurricanes and Tornadoes

The Layered Earth | D4 Extreme Weather: Hurricanes and Tornadoes | 1

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

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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?