Violent, short-lived atmospheric disturbance, almost always associated with cumulonimbus clouds (very tall, dense rain clouds) and accompanied by thunder and lightning. Such storms usually generate strong, gusty winds and heavy rain, and occasionally hail or tornadoes. Thunderstorms have been known to occur in almost every part of the world, although they are rare in the polar regions. In the U.S. the areas of maximum thunderstorm activity are the Florida peninsula and the coast of the Gulf of Mexico (70–80 days per year).
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A thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder. It is usually accompanied by heavy rain and sometimes snow, hail, or no precipitation at all. Thunderstorms may line up in a series, and strong or severe thunderstorms may rotate.
All thunderstorms, regardless of type, go through three stages: the cumulus stage, the mature stage, and the dissipation stage. Depending on the conditions present in the atmosphere, these three stages can take anywhere from 20 minutes to several hours to occur.
Typically, if there is little wind shear, the storm will rapidly enter the dissipating stage and 'rain itself out', but if there is sufficient change in wind speed and/or direction the downdraft will be separated from the updraft, and the storm may become a supercell, and the mature stage can sustain itself for several hours.
In certain cases however, even with little wind shear, if there is enough atmospheric support and instability in place for the thunderstorm to feed on, it may even maintain its mature stage a bit longer than most storms.
In the dissipation stage, the thunderstorm is dominated by the downdraft. If atmospheric conditions do not support super cellular development, this stage occurs rather quickly, approximately 20-30 minutes into the life of the thunderstorm. The downdraft will push down out of the thunderstorm, hit the ground and spread out. The cool air carried to the ground by the downdraft cuts off the inflow of the thunderstorm, the updraft disappears and the thunderstorm will dissipate.
Thunderstorm cells can and do form in isolation to other cells. Such storms are rarely severe and are a result of local atmospheric instability; hence the term "air mass thunderstorm". These are the typical summer thunderstorm in many temperate locales. They also occur in the cool unstable air which often follows the passage of a cold front from the sea during winter.
While most single cell thunderstorms move, there are some unusual circumstances where they remain stationary. When this happens, catastrophic flooding is possible. In Rapid City, South Dakota, in 1972, an unusual alignment of winds at various levels of the atmosphere combined to produce a continuous, stationary cell which dropped an enormous quantity of rain, resulting in devastating flash flooding . A similar event occurred in Boscastle, England, on 16 August 2004 .
An unusually powerful type of squall line called a derecho occurs when an intense squall line travels for several hundred miles, often leaving widespread damage over thousands of square miles.
Occasionally, squall lines also form near the outer rain band of tropical cyclones. The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge.
This kind of storm is also known as "Wind of the Stony Lake" (Traditional Chinese:石湖風, Simplified Chinese: 石湖风) in southern China.
Supercell storms are large, severe quasi-steady-state storms which feature wind speed and direction that vary with height ("wind shear"), separate downdrafts and updrafts (i.e., precipitation is not falling through the updraft) and a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the cloud (or anvil) can break through the troposphere and reach into the lower levels of the stratosphere and can be wide. These storms can produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 inch or 10 cm diameter), straight-line winds in excess of 80 mph (130 km/h), and flash floods. In fact, most tornadoes occur from this type of thunderstorm.
A severe thunderstorm is a term designating a thunderstorm that has reached a predetermined level of severity. Often, this level is determined by the storm being strong enough to inflict wind or hail damage. In the United States, a storm is considered severe if winds reach over 50 knots (58 mph or 93 km/h), hail is ¾ inch (2 cm) diameter or larger, or if funnel clouds and/or tornadoes are reported. Though a funnel cloud or tornado indicates the presence of a severe thunderstorm, a tornado warning would then be issued in place of a severe thunderstorm warning.
In Canada, a severe thunderstorm is defined as either having tornadoes, wind gusts of 90 km/h or greater, hail of 2 centimetres in diameter or greater, a rainfall rate greater than 50 millimetres in 1 hour, or 75 millimetres in 3 hours.
Severe thunderstorms can occur from any type of thunderstorm, however multicell and squall lines represent the most common forms. Supercells are often the most powerful type of severe thunderstorm.
If the quantity of water that is condensed in and subsequently precipitated from a cloud is known, then the total energy of a thunderstorm can be calculated. In an average thunderstorm, the energy released amounts to about 10,000,000 kilowatt-hours (3.6 joule), which is equivalent to a 20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to 100 times more energetic.
Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth, an accolade which has also been bestowed upon Bogor on Java, Indonesia or Singapore. Thunderstorms are associated with the various monsoon seasons around the globe, and they populate the rainbands of all tropical cyclones. In temperate regions, they are most frequent in spring and summer, although they can occur along or ahead of cold fronts at any time of year. They may also occur within a cooler air mass following the passage of a cold front over a relatively warmer body of water. Thunderstorms are rare in polar regions because of cold surface temperatures.
Some of the most powerful and dangerous thunderstorms occur over the United States, particularly in the Midwest and the southern states. These storms can produce large hail and powerful tornadoes. Thunderstorms are relatively uncommon along much of the West Coast of the United States, but they occur with greater frequency in the inland areas, particularly the Sacramento and San Joaquin Valleys of California. Furthermore, in spring and summer, they occur nearly daily in certain areas of the Rocky Mountains. In the Northeast, storms take on similar characteristics and patterns as the Midwest, only less frequently and severely. Probably the most thunderous region outside of the Tropics is Florida. During the summer, violent thunderstorms are an almost daily occurrence over central and southern parts of the state. In more contemporary times, thunderstorms have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the visual and scientific aspects of storms and tornadoes.
Lightning is an electrical discharge that occurs in a thunderstorm. It can be seen in the form of a bright streak (or bolt) from the sky. Lightning occurs when an electrical charge is built up within a cloud, due to static electricity generated by supercooled water droplets colliding with ice crystals near the freezing level. When a large enough charge is built up, a large discharge will occur and can be seen as lightning. The temperature of a lightning bolt can be five times hotter than the surface of the sun. Although the lightning is extremely hot, the duration is short and 90% of strike victims survive. Contrary to the popular idea that lightning does not strike twice in the same spot, some people have been struck by lightning over three times, and skyscrapers like the Empire State Building have been struck numerous times in the same storm. The loud bang that is heard is the super heated air around the lightning bolt expanding at the speed of sound. Because sound travels slower than light the flash is seen before the bang, although both occur at the same moment. There are several types of lightning: