Canadian Geographic
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Extremes of weather

Anatomy of a thunderstorm


Thunderstorms begin when a parcel of warm, moist air begins to rise. As the air expands and cools, the water vapour within it condenses and forms a cloud (When air masses collide). If there is sufficient atmospheric instability, the heat released by condensation will keep the air inside the cloud warmer than the air surrounding it, enabling it to grow larger and higher. The power of the rising air, or updraft, keeps millions of water droplets in suspension until they become so heavy they fall as rain.
Above the freezing line (typically 12,000 to 15,000 m above the ground in summer), the droplets form supercooled ice crystals that can grow into hailstones. When the thundercloud reaches the cumulonimbus stage and hits the tropopause (where temperature stops decreasing with height, around 12,000 m in summer), the jet stream tugs the cloud into its famous “anvil” shape, and the rising air in the cloud falls back to earth in cool, dry currents of air surrounding the warm, moist core of the storm. These downdrafts can pool at the bottom of the thunderstorm and create microbursts — brief, violent gusts of wind and rain. When the static buildup between the clashing air masses in a thunderstorm (the downdrafts carry a positive charge; the updrafts a negative one) triggers an electrical discharge, lightning forks through the sky at 145,000 km/s. The lightning heats the surrounding air, which expands at supersonic speeds, creating the mighty crashes we recognize as thunder.

Synopsis
This animation illustrates the stages in the formation of a thunderstorm. Various elements of the storm cell are labelled, and arrows or graphics depict the direction of air currents, lightening, precipitation, and the possibility of a tornado.

Content (Narration)
Thunderstorms begin with warm, moist air rising.

Anatomy of a thunderstorm

As the air expands and cools, water vapour within it condenses and forms a cloud.

When the thundercloud reaches the cumulonimbus stage and hits the tropopause, the jet stream tugs the cloud into a recognizable anvil shape.

Static builds up between the cool, dry, positively charged downdrafts and the warm, moist, negatively charged updrafts, triggering an electrical discharge in the form of lightning.

This heats the surrounding air, which expands at supersonic speeds to create thunder.

The updraft keeps millions of water droplets in suspension until they become so heavy that they fall as rain. Above the freezing line, the droplets form supercooled ice crystals that can grow into hailstones.

A tornado may form if a downward-spinning column of air inside the thunderstorm touches the ground.

On the next page:

Terrifying twisters


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Quiz :

When was the coldest month recorded in Canada?

March
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