In the right conditions, tornadoes can also form. Violent, rapidly spinning updrafts caused by thunderstorms create a funnel-shaped column of air that touches the ground ( FIGURE3 ). Wind speeds within the funnel can exceed 300 km/h and cause significant damage. While severe tornadoes are rare in Australia, around 400 have been recorded. They are most common in Tornado Alley, a region in the central and southeastern United States. 9.11.3 When do thunderstorms occur? Thunderstorms can happen any time of year but are more common in spring and summer (see FIGURES4 and 5 ). This is because the sun warms the air, and warm air holds more moisture.
FIGURE3 A tornado is born when a funnel of air forms beneath a thunderstorm and makes contact with the ground.
SkillBuilder discussion Communicating 1. In which month does Darwin experience the highest number of thunderstorms? 2. How many thunderstorms occur on average in February? 3. Which months have the fewest thunderstorms in Darwin?
FIGURE4 Average monthly thunderstorm distribution in Darwin
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Month
Source: Bureau of Meteorology
SkillBuilder discussion Communicating 1. Which months have
FIGURE5 Average monthly thunderstorm distribution in Melbourne
2.5
the lowest number of thunderstorms in Melbourne?
2
1.5
1
0.5
0
Month
Source: Bureau of Meteorology
318 Jacaranda Humanities Alive 7 Victorian Curriculum Third Edition
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