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Why do

storms

happen?

Storms are caused by the Sun heating Earth’s surface and generating powerful air currents in the lower atmosphere. These create swirling weather systems known as cyclones, which storę up huge amounts of water and energy in towering storm clouds. Eventually, all this water and energy is released as rain, hail, lightning, and wind.

Cyclones

01: Land and water heated by the Sun warms the air above it. This expands and becomes less dense than the surrounding air, so it travels upward.

02: The upward

movement of this warm air creates a zonę of Iow atmospheric pressure.

03: The surrounding air is under higher pressure, so swirls into the low-pressure zonę to replace the rising air. This is a cyclone.

04: Thebiggerthe pressure difference, the faster the airflow. So very Iow pressure at the cyclone's center generates high winds.

05: The rising air contains invisible water vapor that has evaporated from Sun-warmed oceans, lakes, and vegetation.

06: As the water vapor is carried higher, it cools and turns into the tiny water droplets that form clouds.

Building storm clouds

yfOn Water vapor evaporating from an ocean absorbs energy from the water and carries it up into the sky. So water vapor contains morę energy than water.

When the vapor turns into cioud droplets, the energy is released, warming the air. The warmth makes the air expand and rise, carrying the water with it.

lf all water vapor turns to ‘"T • cloud droplets, the process stops, forming the relatively smali clouds we see in fair weather.

But if massive evaporation TA generates enough water vapor, the process can continue, building colossal storm clouds containing huge amounts of water.

The cloud droplets “Apy gradually fuse together until they form much bigger drops that fali as torrential rain.

I don t I believe it ■

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hailstones and tornadoes

■ As water vapor turns to cloud droplets inside a storm cloud, it releases energy as heat. This generates powerful updrafts in the core of the cloud that can reach speeds of 100 mph (160 km/h) or morę!

■ The tops of storm clouds are madę of tiny ice crystals. These grow heavier until they fali through the cloud but are then hurled up again by the updrafts. As they are tossed around, they gather layers of ice and turn into hailstones.

■ The updrafts created by massive storm clouds may suck in so much air from below that they turn into spinning tornadoes, powerful enough to toss cars into the air and rip the roofs off houses.

A typical cloud may be hundreds of feet high, but a storm cloud may have a total height of 6 miles (10 km) or morel

on June 22, 2003 It

^Threco?d?1 ^hai,s‘°"e on recordfellin Nebraska

' ToT^ed7in(17-⁸⁰^)

across—almost as big ^{as a} soccerball!