Tornadoes

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Tornado!

Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries. A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!

What causes tornadoes?

Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.

During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.

Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.

Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.

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To have a tornado you need a thunderstorm. To have a thunderstorm you need three things:

instability
an uplifting mechanism
moisture in the mid to lower levels of the atmosphere

An unstable air mass is warm and moist near the ground and relatively cold and dry in the upper atmosphere. If an air mass is unstable, air that is pushed upward will continue upward.

Lift is the mechanism that pushes the air upward. Sources of lift can be cold fronts, mountains, converging air (as in Florida), or differential heating. The most common of these is differential heating. Differential heating takes place when portions of the earth's surface warm more readily than other nearby areas. These "warm pockets" are less dense then the surrounding air and will then rise.

The final ingredient is moisture. As air rises in a thunderstorm updraft, moisture condenses into small water drops, which form clouds. When the moisture condenses, heat is released into the air, making it warmer and less dense than its surroundings. This lets the updraft continue rising.

Thunderstorm Types

The Single Cell Storm

Single cell thunderstorms have lifespans of 20-30 minutes. They are usually not strong enough to produce severe weather. A true single cell storm is actually quite rare.

The Multicell Cluster Storm

This is the most common type pf thunderstorm. It consists of a group of cells, moving as one unit, with each cell in a different phase of the thunderstorm life cycle. The mature cell is usually found at the center. Although each cell may last only 20 minutes, the cluster may last several hours. These can produce heavy rainfall, downbursts, moderate-sized hail, and occasional weak tornadoes.

The Multicell Line Storm or "Squall Line"

This consists of a line of storms with a continuous, well-developed gust front at the leading edge of the line. This line of storms can be solid or it can have gaps. The main threats with these storms are golfball-sized hail, heavy rainfall, and weak tornadoes. However, they are best known for their downbursts.

The Supercell Storm

This is a highly organized thunderstorm. Although these are rare, they pose a great threat to life and property. This is like a single cell storm in that it has one updraft. However, the supercell updraft is extremely strong. This storm has a rotating updraft, or mesocyclone, that is the key to its ability to produce severe weather. This storm can produce large hail, strong downburst, and strong to violent tornadoes.

Convective Variables

Research has found that if the environment (uplifting, instability, or moisture) of a storm has changed then the type of storm favored to exist may also change.

The amount of vertical wind shear in a storm's environment is critical in determining the type of storm that will form. Vertical wind shear is defined as a change in wind direction or speed with height. If the vertical wind shear is weak, the multicellular storms with short-lived updrafts will be favored. If the vertical wind shear is stronger than the updraft, then storms with longer-lived updrafts will develop.

Closely related to the concept of vertical wind shear is veering. Veering is defined as a clockwise turning of the wind direction as we move up through the atmosphere. If there are two layers of clouds in the lower levels of the atmosphere and the direction turns clockwise between the lower and upper layers, then veering is present.

Computer simulations and observational studies have suggested that veering of the low-level wind is instrumental in the production of storm rotation. Once this vertical rotation is established, a mesocyclone can develop which may produce a tornado or other significant severe weather.

The amount of moisture in the air has an effect on storms too. If the amount of moisture is low, then the storms tend to have high bases. If the amount of moisture is high, the storms tend to have low bases. The higher the cloud base, the better the chance for microbursts. The lower the cloud base, the better the chance for flash flood-producing rains.

The Birth of a Tornado

As the mesocyclone strengthens it extends further downwards. At the same time, it is becoming more compact which is causing it to spin faster and faster. If this process continues, then the mesocyclone will reach to the ground, spawning a tornado.