Definitions

What is El Nino?
El Nino is a disruption of the ocean-atmosphere system in the Tropical Pacific having important consequences for weather and climate around the globe. These consequences can include increased rainfall in some areas (which can lead to flooding) and extreme drought in others.

What is La Nina?

La Nina is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Nino, which is characterized by unusually warm ocean temperatures in the Equatorial Pacific.

What is a tornado?

A tornado is officially defined as an intense, rotating column of air extending from the base of a thunderstorm cloud to the ground. Most tornadoes form under the southwestern section of the thunderstorm cloud. Air moves very rapidly upward around a tornado center. This distinguishes tornadoes from microbursts, which often do tornado-like damage and are often mistaken for tornadoes. Microbursts, on the other hand, features air blasting downward from thunderstorms. The large hail that often precedes tornadoes forms as a result of the intense updraft feeding the thunderstorm.

The United States is the world capital for tornadoes as conditions favorable for tornado development most often occur over the Plains during spring and summer. A typical tornado outbreak often features an intense upper-level disturbance moving across the Plains during spring. This disturbance provides the strong vertical wind shear that gives an updraft its twisting motion, turning a normal thunderstorm into a potentially tornado spawning supercell. Although, the United States has the most tornadoes of any nation in the world, tornadoes do occur in other locations such as Australia and Europe.

What are favorable conditions for tornado formation?

A convective cap is a layer of hot, dry air in the middle layers of the atmosphere above the surface. Often, temperatures increase with height in this layer and relative humidities are extremely low. As you can see in the graphic above, warm humid air in the lower layers of the atmosphere near the surface is heated by the sun, but is not allowed to rise and initiate clouds and precipitation because of the hot, dry air above it. As the air near the surface continues to heat up, it builds up an enormous amount of energy much the same way boiling water in a pot with a heavy lid on it would. If a triggering mechanism, such as a cold front or dryline, moves into the area, the convective cap may weaken enough to allow the heated, humid air near the surface to burst through the cap and initiate extremely violent convection. Supercells, along with intense tornadoes, often form as a result of this violent convection.

How powerful/dangerous do tornados get? (Tornado Facts)
Tornado strength and intensity are measured in the Fujita Tornado Damage Scale, that ranges from F0 (weakest) to F5 (most powerful).

Category F0: Light Damage (<73 mph); Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.

Category F1: Moderate Damage (73-112 mph); Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off road.

Category F2: Considerable Damage (113-157 mph); Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

Category F3: Severe Damage (158- 206 mph); Roofs and some walls torn off well-constructed houses, trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.

Category F4: Devastating Damage (207- 260 mph); Well-constructed houses leveled; structure with weak foundations blown off some distance; cars thrown and large missiles generated.

Category F5: Incredible Damage (261- 318 mph); Strong frame houses lifted off foundations and swept away; automobile sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur.

How do tornadoes form?
Many of the strongest tornadoes are produced in supercell thunderstorms. A supercell storm consists of one very large cell with dimensions of up to 20km high and 20-50km in diameter. These cells have a much longer life than ordinary thunderstorms; they can persist for many hours. Because of their size, supercells are quite complex in nature. Wind being lifted in the updraft may start off from the south, but then shift to the west at a higher elevation, causing a rotation to form. The area of rotating air is called a mesocyclone, and this is where tornadoes often form. Mesocyclones can become nearly vertical, and stretched lengthwise, reducing girth, which increases wind speeds. As the mesocyclone extends downward in the storm, part of it may protrude from the bottom of the cloud. If this 'funnel cloud' touches the ground, it is classified as a tornado.

Tornado Structure
As mentioned, tornadoes are a rotating column of air extending downward from a cumulonimbus cloud. Weaker tornadoes may consist of only one vortex, but in many larger tornadoes, several smaller 'suction vortexes' are present within a main vortex. These suction vortexes are very concentrated, and often no larger than 10 meters in diameter. Extremely low pressures are located within a tornado. Some estimates are on the scale of 10 percent less than the immediate surrounding area. Because of this steep pressure gradient, we can experience wind speeds of up to 300 miles an hour.

Figure - Tornado structure with multiple suction vortex.


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