NaturalHazards.org

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Mammatus clouds billow beneath a supercell thunderstorm at sunset in southern Wisconsin.

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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:

  • Torrential rain: rain produced by thunderstorms is usually intense, but short in duration - flash flooding is often associated with this type of precipitation. In fact, flooding is the greatest threat from thunderstorms, killing more people in the United States in an average year than tornadoes or lightning. Torrential rain can cause flooding many miles downstream of a thunderstorm's location, catching people off guard. Slope failures can also be triggered by the intense precipitation from thunderstorms in hilly and mountainous regions. Reduced visibility is common in the downdrafts of thunderstorms, where rainfall rates are the highest, creating hazardous travel.

  • High wind: inside a thunderstorm, air rises and descends rapidly, transferring vast amounts of energy. Such movement can create dangerous skies for aircraft. Winds at the surface beneath a thunderstorm can reach well over 80 kph (50 mph). Occasionally, severe thunderstorms produce derechos, which are inline windstorms with speeds greater than 58 mph (93.3 kph) and a path of damage that is at least 280 miles long (450 km).

  • Hail: falling at several meters per second, hail stones can result in extensive damage to crops and property in just a few minutes and can injure or kill people and other organisms. Almost $1 billion in damage is caused by hail in the U.S. each year.

  • Lightning: every year in the U.S., lightning is responsible for an average of almost 100 deaths and 300 injuries. Lightning is the primary cause of wildland fires, which threaten homes, businesses, and lives. Power and communication outages caused by lightning can result in large scale disruption of everyday activities.

  • Tornadoes: perhaps the most feared hazard associated with thunderstorms, these spinning winds can whirl at over 480 kph (300 mph). Please visit the separate section on tornadoes for more information.


WHERE do thunderstorms occur?

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 of thunderstorm activity per year. 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 the boundaries between warm and cold air masses 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. This movement of air can cause thunderstorms to form if enough moisture exists in the atmosphere. Windward sides of mountain ranges also experience numerous thunderstorms as a result of air being forced to rise over the mountains. 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.


WHEN do thunderstorms occur?

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 that prohibit thunderstorm development.

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.


HOW do we cope with thunderstorms?

The threats that any particular thunderstorm presents varies depending on its intensity, structure, and the ground below it. In many cases, people and their belongings simply need 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 the proper 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 United States, many people are killed every year attempting to drive their cars through floodwaters.

In the United States, the National Weather Service issues severe thunderstorm watches and warnings. A thunderstorm is considered to be severe if it produces hail larger than 2 cm (0.75 inch), winds greater than 93 kph (58 mph), 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 are imminent, or have already developed. As with tornado watches and warnings, these are broadcast via the media (radio and television), Internet, and NOAA weather radios. Reliable forecasting is essential to providing communities with adequate warnings about incoming thunderstorms and the specific threats that each storm possesses. Doppler radar has proved to be an essential tool for detecting and studying thunderstorms. A network of Severe Weather Spotters, who are citizen volunteers trained by the National Weather Service, provide critical local field observations.


WHO has more information on thunderstorms?