FIGURE2 How a thunderstorm works
E
B
A
F
Cold downdraft
Warm updraft
D
C
A Formation of static electricity: As air currents in a cumulonimbus cloud become more violent, ice crystals and water droplets collide more forcefully. These collisions create friction, building up large amounts of static electricity in the cloud. B Separation of charges: Lighter particles with a positive charge drift upwards in the cloud. heavier particles with a negative charge sink to the bottom of the cloud. C Ground charge: The ground below the cloud carries a positive charge. D Energy difference and lightning: A huge energy difference builds up between the positive charge on the ground and the negative charge at the cloud’s base. A lightning bolt helps to balance this energy difference. E Path of lightning: Lightning travels to the ground via the shortest route, often striking tall objects like buildings or trees. F Multiple flashes: A single lightning bolt consists of several flashes moving up and down between the cloud and the ground, but this happens so quickly that we see it as one flash.
SkillBuilder discussion Geographical inquiry, Concluding and decision-making 1. What causes the warm updraft shown in FIGURE2 ? 2. What do the ‘+’ and ‘–’ symbols represent in the cloud? 3. How does the cold downdraft affect the storm?
9.11.2 Severe thunderstorms The Bureau of Meteorology classifies a thunderstorm as severe if it has any of these features: • Flash flooding: Slow-moving thunderstorms can drop heavy rain or hail in one area, leading to flooding when the ground can’t absorb all the water. • Hailstones: Hailstones must be 2 cm or more in diameter. The largest recorded hailstone was 20.3 cm wide with a 47 cm circumference. • Wind gusts: Wind gusts of 90 km/h or more occur when cold air rushes out of thunderclouds, spreading outwards when hitting the ground.
TOPIC9 Water in the world 317
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