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WHAT are thunderstorms? Thunderstorms are usually small, organized parcels of warm and moist air that produce lightning and thunder. They are one of Nature's frequent ways of balancing the amount of energy in the atmosphere - it is estimated that over 40,000 thunderstorms occur each day around the world. Although most last only 30 minutes, thunderstorms can create several dangerous phenomena:
Thunderstorms are one of the most common natural hazards throughout the world. In the United States, approximately 100,000 thunderstorms occur each year. The southeastern U.S. has the greatest number per year, with some areas in Florida experiencing an average of 90 days or more of thunderstorm activity. This is mostly due to the proximity to the Gulf of Mexico and the Atlantic Ocean, which provide an ample amount of warm, moist air. The Midwest and Great Plains regions of the U.S. average between 40 and 60 days of thunderstorms per year. This central area of U.S. is home to the most severe thunderstorms in the world. Frontal thunderstorms are common along air
mass boundaries in the mid-latitudes. At an air mass boundary,
warm air (which is less dense than cool air) is forced to rise over
a mass of cool air. Windward sides of mountain ranges also experience
numerous thunderstorms as a result of air being forced to rise over
the mountains (a process referred to as orographic lifting).
Near the equator, converging surface winds and vast volumes of warm,
moist air result in frequent thunderstorms throughout the year. Occasionally,
thunderstorms in this area may organize into larger storms, resulting
in the development of tropical storms and hurricanes
Spring and summer bring the greatest risk of thunderstorms to most of North America. In central and southern California, thunderstorms are most common during winter and spring, a result of temperature inversions in valley areas during the summer. Whenever warm, moist air collides with cool, dry air, the potential for the development of thunderstorms exists. For most of the world, this time occurs during the spring and summer seasons. In the equatorial region of the tropics, where temperatures are warm all year, thunderstorms are common throughout the year.
The threats that any particular thunderstorm presents varies depending on its intensity, structure, and the ground below it. Many thunderstorms simply require people and their belongings to seek shelter inside a sturdy building. However, severe thunderstorms can be very dangerous and require seeking shelter underground because of the damage they can cause to buildings. Seeking shelter before a thunderstorm arrives, and remaining sheltered after the storms passes, is essential because lightning can strike up to 10 miles away from the edge of the storm. Hail-resistant roofs can reduce property damage, as can properly attached roofs. As always, learning about what safety measures to take during a thunderstorm is the first and most important step in coping with thunderstorms. Staying away from streams and ditches is also essential as flood waters can rise to dangerous levels in seconds. In the U.S., the National Weather Service
issues severe thunderstorm watches and warnings. A thunderstorm is considered
to be severe if it produces hail larger than 3/4 of an inch (2 cm),
winds greater than 58 mph (93 kph), or tornadoes. In the U.S., only
about 10% of all thunderstorms are classified as severe. A watch is issued
when atmospheric conditions are favorable for the development of a severe
thunderstorm. A warning is issued when severe thunderstorms
have developed. As with tornado
watches and warnings, these are broadcast via the media (radio and television),
Internet, and NOAA weather radios
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