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WEATHER: What forces affect our weather?
Water Cycle
Powerful Storms
Ice and Snow
Changing Climate

According to the Worldwatch Institute, in 1998 alone, severe weather caused more than 30,000 deaths and close to $90 billion in damage. Hurricanes ravaged coastlines, tornadoes plowed through the United States with record force, and rain battered crops and left Do you have what it takes
to be a storm chaser? Identify tornadoes and other weather events in this activity. millions of people homeless worldwide. What causes such severe weather? Can we prepare ourselves for these disasters?

Powerful storms such as thunderstorms, hurricanes, and tornadoes are generated when warm, light air rises quickly into higher, colder levels in an unstable updraft that can reach over 100 miles per hour. Each type of storm forms under specific conditions; hurricanes occur over moisture-rich oceans and coastlines, for example. They draw their energy from warm ocean waters. Understanding the conditions that give rise to powerful storms is the key to preparing for their devastating effects.

Thunder and lightning

At any given moment, there are an estimated 2,000 thunderstorms in progress over Earth's surface. These storms can vary from relatively mild rainstorms to very damaging storms that feature hail and high wind. Thunderstorms form when warm air rises from Earth's surface and moves upwards quickly into the colder levels of the atmosphere. If conditions are right, tornadoes can form from this rapid updraft. Normally, however, the result is rain, wind, lightning, and thunder.

Without lightning, there would be no "thunder" in "thunderstorm." Thunder is the noise lightning makes as it travels through the air. Lightning occurs during all thunderstorms (though not every time it rains). During a storm, it strikes Earth 100 times each second. More than just a dazzling light show, lightning causes billions of dollars in damage each year.

Lightning forms when updrafts of air carry water droplets, which have a charge, upward to heights where some freeze into ice and snow particles. They form a cloud. As these particles begin to fall back to Earth, charges within the cloud become mixed. The differences in charge are released as lightning. You'll normally hear the sound of lightning a few moments after you see the sky light up. Light travels faster than sound, so if you are at a distance from the storm, lightning and thunder may seem oddly disconnected.

Ordinary objects can become dangerous missiles during a powerful storm. This damage was caused by a tornado.

Spinning air: Tornadoes and hurricanes

Both tornadoes and hurricanes are spinning columns of air capable of causing great damage. There are important differences between these two powerful storms, however. Tornadoes are more localized and typically found on land, while hurricanes can cover vast areas and draw their power from the warm tropical oceans.

Tornadoes range from only a few feet to one mile in diameter and are short in duration (normally only a few minutes long). Though these storms are localized, they can be extremely violent. The wind speed inside a tornado's funnel can exceed 200 miles per hour, enough to turn everyday objects into deadly projectiles. Tornadoes occur all over the world, at every time of the year, but they are most common in the summertime in the midwestern United States. This region's propensity for tornadoes has earned it the name Tornado Alley.

Tornadoes form from thunderstorms, though not all thunderstorms generate tornadoes. An unstable column of warm air rising within cumulus clouds can start to rotate because of changing wind directions at or near the ground. These updrafts alter the air's rotation from horizontal to vertical, creating conditions in which a funnel can develop. If conditions are right and the funnel forms, it can extend to the ground, forming a tornado.

All thunderstorms are capable of producing tornadoes, but detection is still a difficult task. Weather forecasters can identify the cloud features and conditions that normally precede these storms, and they know where they are most likely to occur. However, predicting the exact time, location, and intensity of tornadoes is still very difficult.

Tornadoes threaten areas the size of towns or counties, but hurricanes play themselves out on a much larger stage. These large storms can last for days or weeks and cover thousands of miles of territory. Hurricanes draw their strength from the warm tropical waters of the ocean. Unlike tornadoes, they lose their power source when they leave the ocean. Once on land, they gradually dissipate.

How do hurricanes form? Why are they so dangerous? Find out more in "Eye of the Storm: Inside a Hurricane.""

 "Weather" is inspired by programs from Planet Earth.

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