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Precipitation
It is late afternoon. The white puffy clouds that have been growing all day are replaced by a greenish sky. A distant rumble is heard...then another. It starts to rain. A flash of light streaks the sky, followed by a huge BOOM. Welcome to a thunderstorm.
Thunderstorms are one of the most thrilling and dangerous of weather phenomena. Over 40,000 thunderstorms occur throughout the world each day. Thunderstorms have several distinguishing characteristics that can cause large amounts of damage to humans and their property. Straight-line winds and tornadoes can uproot trees and demolish buildings. Hail can damage cars and crops. Heavy rains can create flash floods. Lightning can spark a forest fire or hurt you. Safety during a thunderstorm is really important.
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Types of thunderstorms
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There are two main types of thunderstorms: ordinary and severe. Ordinary thunderstorms are the common summer storm.
They are usually multi cell. Multi cell storms consist of a line of thunderstorms in different stages of development.
Ordinary thunderstorms last about one hour. The precipitation associated with them is rain and occasionally small hail.
An ordinary thunderstorm cloud can have a vertical extent up to 12 kilometres. Severe thunderstorms are the most
dangerous weather phenomenon. They are capable of producing baseball-sized hail, strong winds, intense rain, flash floods,
and tornadoes. Severe thunderstorms can last several hours and can reach a vertical extent of
18 kilometres. Several phenomena are associated with severe thunderstorms. These include the gust front, microburst,
super cell thunderstorm, and the squall line. |
Gust front
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Thunderstorm form cycle
Most thunderstorms form by a special cycle. This cycle has three stages: The Cumulus Stage, Mature Thunderstorm Stage, and Dissipating Stage. See here below to learn more about how each cycle works.
Cumulus Stage
| The sun heats the earth's surface during the day. The heat on the surface and warms the air around it. Since warm air is lighter than cool air, it starts to rise (known as an updraft). If the air is moist, then the warm air condenses into a cumulus cloud. The cloud will continue to grow as long as warm air below it continues to rise. | 
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Mature Thunderstorm Stage
| When the cumulus cloud becomes very large, the water in it become large and heavy. Raindrops start to fall through the cloud when the rising air can no longer hold them up. Meanwhile, cool dry air starts to enter the cloud. Because cool air is heavier than warm air, it starts to descend in the cloud (known as a downdraft). The downdraft pulls the heavy water downward, making rain. This cloud has become a cumulonimbus cloud because it has an updraft, a downdraft, and rain. Lightning and thunder start to occur, as well as heavy rain. The cumulonimbus is now a thunderstorm cell. | 
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Dissipating Stage
| After about 30 minutes, the thunderstorm begins to dissipate. This occurs when the downdrafts in the cloud begins to dominate over the updraft. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cloud disappears from bottom to top. The whole process takes about one hour for an ordinary thunderstorm. | 
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Tornadoes and severe storms
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Tornadoes form from severe thunderstorms. They have a very high energy density which means that they are very destructive
to a small area. They also don't last very long which makes them hard to study. Without much information about how they
develop, meteorologists have a hard time forecasting when they'll form. The general public knows even less about tornadoes,
which is why there are so many inaccurate myths about them. 75% of the world's tornadoes occur in the United States, but they can (and probably have) developed anywhere. Most tornadoes in the U.S. form in an area of the Great Plains known as Tornado Alley. There are also some other interesting facts about tornadoes. People with an interest in tornadoes sometimes attend classes held by the National Weather Service and become spotters for their community. Sometimes people travel out to Tornado Alley to chase tornadoes first hand! When a tornado touches down, scientists try and figure out how strong it was by using the Fujita Tornado Scale. |
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Engergy
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A tornado is the most destructive force in nature; that doesn't mean it has the most energy. Thunderstorms which produce
tornadoes can have 40,000 times as much energy as a tornado! Tornadoes are so destructive because they have a higher
energy density than thunderstorms; the energy is concentrated in a small area while the energy in a thunderstorm is
spread-out over a much greater area. Energy and energy density can be compared to weight and force. Energy density is energy per unit volume and force is weight per unit area. If a 100 pound person in flat shoes stepped on your foot, you'd feel it. If the same person stepped on your foot while wearing very narrow high-heels, you might end up with a broken toe. That's because the person's weight is concentrated in a small area. The same holds with tornadoes. They may not have as much total energy as thunderstorms, but the energy they do have is concentrated in a very small area. So small, in fact, that tornadoes have destroyed houses while not damaging the neighbours. |
Tornado
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Mesecyclone
| Most tornadoes form in a part of a super cell thunderstorm called a mesocyclone. The mesocyclone draws energy into the storm so it can last for hours. Scientists aren't sure why, but some can create tornadoes. Mesocyclones can be detected by conventional radar as a hook echo. In the mesocyclone, air is drawn into the storm. Scientists believe a vertical wind sheer (wind that changes direction with height) causes the tornado to begin spinning. Most tornadoes spin cyclonically but a few spin anti cyclonically. Strong fronts develop between cold polar air and warm tropical air and when the atmosphere is unstable tornadoes can form. Tornadoes form through the year but most occur in May. Though, the most damage is usually caused in April which means that the more dangerous tornadoes form then. The more north you go, the later the main tornado season becomes. The atmosphere in the northern plains is cooler and more stable earlier in the year; it takes longer for the sunlight to heat it up. |
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Forecast and detection
| The short duration and complicated nature of tornadoes make them nearly impossible forecast. Meteorologists don't really know the specifics of how they form, but they do know what atmospheric conditions were present during past tornadoes. They use this knowledge to try and identify tornado threats before they happen. The earlier they realize that a tornado is going to strike, the earlier they can notify residents of the areas which will be hit. The more warning people have, the fewer people will be killed. To know the current atmosphere conditions, meteorologists send up weather balloons every twelve hours to take soundings of the upper atmosphere. The equipment on the balloons measure conditions such as the atmospheric stability, temperature, and relative humidity. Two conditions tornadoes need to form are high instability and a high dew point. When these conditions exist, they issue a tornado watch. A new type of weather radar can greatly improve the time between tornado detection and tornado touchdown. Conventional radar could only be used to detect a tornado after it had formed, usually by seeing a shape such as a hook echo on the radar return. The echo doesn't appear with every tornado, so forecaster can't rely on just that information. They also need observations from tornado spotters in order to assure a tornado warning. |  |
Tornado strength scale
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Before 1971, there was no way to categorize tornadoes by their strength. Because they're so intense, it's impossible to
measure the wind speed or the pressure like you can do in a hurricane. Without a standard system to rank tornadoes, there
was no way to keep an accurate storm record; everybody had their own idea of how strong the storm was. T. Theodore Fujiata, a professor at the University of Chicago, came up with a system that equated wind speed with tornado damage. The scale Fujita came up with connects the Beaufort wind scale with Mach 1 in twelve steps. Tornadoes are only observed in categories F0 to F5. There could theoretically be an F6 tornado, but the damage would be so incredible that it would be nearly impossible to tell the difference between an F5 and an F6 tornado. Scientists have to figure out how strong a tornado was after it is over. Because the scale is based on the damage caused by it, they can't predict how strong a tornado would be before it strikes like they can predict a hurricane's strength using the Saffir-Simpson scale. |  |
Super cell thunderstorms
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A super cell thunderstorm is a huge rotating thunderstorm. It can last for several hours as a single entity. These
storms are the most likely to produce long-lasting tornadoes and baseball-sized hail. A squall line consists of
several thunderstorms banded together in a line. They are generally associated with a cold front. Squall lines can
be just as severe as a super cell thunderstorm. |
Supercell thunderstorm
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Lightning and thunder
| During thunderstorm conditions the turbulence in the cloud causes the charges to separate in such a way that the negative charges concentrate in the base of the cloud. Since like charges repel, some of the negative charges on the ground are pushed down away from the surface, leaving a net positive charge on the surface. With other words: It's all about static electricity! So the lightning happens when the negative charges in the bottom of the cloud are attracted to the positive charges (protons) in the ground. | 
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| Opposite charges attract, so the positive and negative charges are pulled toward each other. since the negative charges (electrons) are many thousands of times smaller than the positive charges (ions--charged atoms) they move much more easily and cover most of the distance. This first, invisible stroke is called a stepped leader. The accumulation of electric charges has to be great enough to overcome the insulating properties of air. When this happens, a stream of negative charges pours down towards a high point where positive charges have clustered due to the pull of the thunderhead. | 
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| As soon as the stepped leader connect there is a conductive path from the cloud to the ground and the negative charges rush down it causing the visible stroke. The channel created by the stepped leader is full of relatively static charge, like a line of cars at a red light. When the two parts join, it is like that light turning green, and just as the cars near the light start moving first, so do the charges near the join. Since it is the fast-moving charges that create the light, the visible stroke actually travels upwards, even though the charges are moving downward! The connection is made and the protons rush up to meet the electrons. It is at that point that we see lightning and hear thunder. | 
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Thunder
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The temperature of lightning is over 22,204 degrees Celsius (40,000 degrees Fahrenheit), so a bolt of lightning heats the air along its path causing it to expand
rapidly. Thunder is the sound caused by the rapidly expanding gases. Light is moving faster then sound, so that's the reason you see first the lightning and later the thunder.
If the lightning and the thunder are at the same time, be aware! The thunder is right above you. You can easily calculate the approximate distance between you and the thunder. Sound travels with a speed of approx. 330 meter per second. So the only thing to do is count the seconds between the lightning and the thunder. The best thing to do this is with a stopwatch. This is how it goes: You see the lightning, start to count immediately: 1...2...3...Booooom! Three seconds means: 3x 330 meter. The thunder is about 1 Km away from you. | 
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The images on this page and a part of the explanation are provided by the National Center of Atmospheric Research and Windows to the Universe.
Hurricanes
| Hurricanes are severe tropical storms that form in the southern Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern Pacific Ocean. Hurricanes gather heat and energy through contact with warm ocean waters. Evaporation from the seawater increases their power. Hurricanes rotate in a counter-clockwise direction around an "eye." Hurricanes have winds at least 74 miles per hour. When they come onto land, the heavy rain, strong winds and heavy waves can damage buildings, trees and cars. The heavy waves are called a storm surge. Storm surges are very dangerous and a major reason why you MUST stay away from the ocean during a hurricane warning or hurricane. |
Hurricane Isabel
 Hurricane Frances  |
Hurricane strength: Saffir-Simpson
Category 1: Minimal
Central Pressure: Greater than 980 mill bars (mb)
Wind: 74-95 miles per hour (mph)
Storm Surge: 4-5 feet (ft)
Damage: Damage mainly to trees, shrubbery, and unanchored mobile homes
Category 2: Moderate
Central Pressure: 965-979 mb
Wind: 96-110 mph
Storm Surge: 6-8 ft
Damage: Some trees blown down; major damage to exposed mobile homes; some damage to roofs of buildings
Category 3: Extensive
Central Pressure: 945-964 mb
Wind: 111-130 mph
Storm Surge: 9-12 ft
Damage: Foliage removed from trees; large trees blown down; mobile homes destroyed; some structural damage to small buildings
Category 4: Extreme
Central Pressure: 920-944 mb
Wind: 131-155 mph
Storm Surge: 13-18 mph
Damage: All signs blown down; extensive damage to roofs, windows, and doors; complete destruction of mobile homes; flooding inland as far as 6 miles; major damage to lower floors of structures near shore
Category 5: Catastrophic
Central Pressure: Less than 920 mb
Wind: Greater than 155 mph
Storm Surge: Greater than 18 ft
Damage: Severe damage to windows and doors; extensive damage to roofs of homes and industrial buildings; small buildings overturned and blown away; major damage to lower floors of all structures less than 15 feet above sea level within 500 yards of shore
Photos of a Severe Thunderstorm in The Netherlands on the 17th of July 2004
