PLANET EARTH - The Incredible Visual Guide Part 8 pps

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Intense solar heating can cause very high evaporation rates that make warm,

moist air rise unusually fast. This builds up huge cumulonimbus clouds that

cause thunderstorms and hail, and creates conditions of extremely low

pressure. Air swirls into the low-pressure zone, creating a

deep depression with very strong winds. In tropical

oceans, intense heating generates hurricanes.

In extreme cases the updrafts can give rise to

the destructive vortex of a tornado.

EXTREME WEATHER

HAILSTORMS

The giant cumulonimbus clouds that cause

thunderstorms are built up by powerful air currents with

vertical speeds of 100 mph (160 kph) or more. Ice crystals

hurled around by the turbulent air pick up water that

freezes onto them, and if they are tossed up and down

enough this builds up layer after layer of ice to form

hailstones. If the air currents are strong enough, they can

create huge—and very dangerous—hailstones like these.

￾LIGHTNING As the air currents inside a storm

cloud throw ice crystals around,

friction between the crystals

generates static electricity. It

charges up the cloud like a giant

battery, with the positive charge

at the top and the negative

charge at the bottom. If the

voltage reaches about one million

volts, it is discharged as a giant

spark of lightning. This heats the

air along its path to such a high

temperature that it expands

explosively, causing the

shockwave that we call thunder.

TORNADOES

These terrifying events are caused by air

swirling into the base of a very vigorous

storm cloud and spiraling upward. The

updrafts are powerful enough to rip houses

apart, and the winds around such tornadoes

are the most powerful ever recorded,

reaching at least 318 mph (512 kph)

on one occasion.

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