May generally has more tornadoes than any other month, but April's twisters are sometimes more violent. Farther north, tornadoes tend to be more common later in summer.
Although they can occur at any time of the day or night, most tornadoes form in the late afternoon. By this time the sun has heated the ground and the atmosphere enough to produce thunderstorms. The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. The updraft will begin to rotate if winds vary sharply in speed or direction. As the rotating updraft, called a mesocycle, draws in more warm air from the moving thunderstorm, its rotation speed increases.
Cool air fed by the jet stream, a strong band of wind in the atmosphere, provides even more energy. Water droplets from the mesocyclone's moist air form a funnel cloud.
The funnel continues to grow and eventually it descends from the cloud. When it touches the ground, it becomes a tornado. Twisters are usually accompanied or preceded by severe thunderstorms and high wlnds.
Hail is also common. Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours. The average twister is about feet wide and moves about 30 miles an hour.
Most don't travel more than six miles before dying out. Massive tornadoes, however—the ones capable of widespread destruction and many deaths—can roar along as fast as miles an hour.
These measurements are scientists' best estimations. Anemometers, which measure wind speed, cannot withstand the enormous force of tornadoes to record them. Using units F0 to F5, the Fujita scale measures a tornado's intensity by analyzing the damage the twister has done and then matching that to the wind speeds estimated to produce comparable damage.
The United States now uses the EF Enhanced Fujita scale , which takes more variables into account when assigning wind speeds to a tornado. Every year in the United States, tornadoes do about million dollars in damage and kill about 70 people on average. Extremely high winds tear homes and businesses apart.
Winds can also destroy bridges, flip trains, send cars and trucks flying, tear the bark off trees, and suck all the water from a riverbed. High winds sometimes kill or injure people by rolling them along the ground or dropping them from dangerous heights.
Scientists have even observed dust devils on Mars and spotted solar tornadoes whipping out from the sun. In a tornado, the same sort of thing happens as with our bathtub example, except with air instead of water. A great deal of the Earth's wind patterns are dictated by low-pressure centers, which draw in cooler, high-pressure air from the surrounding area. This airflow pushes the low-pressure air up to higher altitudes, but then the air heats up and is pushed upward as well by all the air behind it.
The air pressure inside a tornado is as much as 10 percent lower than that of the surrounding air, causing the surrounding air to rush in even faster.
It's kind of like when a rock concert erupts into a riot. Conditions were already volatile; they merely escalated into something even more dangerous. However, if the updraft continues, this cloud mass will continue to grow and rise 40, feet 12, m or more up into the troposphere , the bottommost layer of the atmosphere that we live in.
A typical thunderstorm cloud can accumulate an enormous amount of energy. If the conditions are right, this energy creates a huge updraft into the cloud, but where does the energy come from?
Clouds are formed when water vapor condenses in the air. This change in physical state releases heat, and heat is a form of energy. A good deal of a thunderstorm's energy is a result of the condensation that forms the cloud. Every gram of water condensed results in about calories of heat -- and another 80 calories of heat per gram of water results from freezing in the upper atmosphere.
This energy increases the updraft temperature, as well as the kinetic energy of upward and downward air movement. The average thunderstorm releases around 10,, kilowatt-hours of energy -- the equivalent of a kiloton nuclear warhead [source: Britannica ]. In supercell thunderstorms , the updrafts are particularly strong. If they are strong enough, a vortex of air can develop just like a vortex of water forms in a sink. This precursor to the tornado is called a mesocyclone , and is typically 2 to 6 miles 3 to 10 kilometers wide.
One a mesocyclone forms, there's a roughly 50 percent chance that the storm will escalate into a tornado in around 30 minutes. Some tornadoes consist of a single vortex, but other times multiple suction vortices revolve around a tornado's center. These storms-within-a-storm may be smaller, with a diameter of around 30 feet 9 meters , but they experience extremely powerful rotation speeds.
The tornado reaches down out of a thundercloud as a huge, swirling rope of air. Wind speeds in the range of to mph to kph aren't uncommon. If the vortex touches ground, the speed of the whirling wind as well as the updraft and the pressure differences can cause tremendous damage, tearing apart homes and flinging potentially lethal debris.
The tornado follows a path that is controlled by the route of its parent thundercloud, and it will often appear to hop. The hops occur when the vortex is disturbed. This happens in the same way that figure skaters spin faster when their arms are drawn in rather than when their arms are outstretched.
This is called conservation of angular momentum. The rotating air moves horizontally across the land, and can be tilted vertically by the force of the rising, rotating air. That allows a tornado to form. Most tornadoes form during supercell thunderstorms, but not all supercell thunderstorms produce tornadoes.
Enhanced, Operational Fujita Scale. Light damage: Branches broken off trees; minor roof damage. EFO mph.
Moderate damage: Trees snapped; mobile home pushed off foundations; roofs damaged. EF1 mph. Considerable damage: Mobile homes demolished; trees uprooted; strong built homes unroofed. EF2 mph.
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