Batten Down The Hatches, Dorothy, Tornado Season Is Here


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Tornadoes per 10,000 square miles per state (1990-2010). Bull's-eye: Kansas. Graphic courtesy of NOAA

Tornadoes per 10,000 square miles per state (1990-2010). Bull’s-eye: Kansas. Graphic courtesy of NOAA

Tornado Alley. Tornado season. Why there? Why now? In the spring, tornadoes form in a swath through Texas, Oklahoma, and (especially) Kansas at a rate not matched anywhere in the world.

The reason is the specific geography of the eastern United States, with a body of water nearby and mountains in the jet stream’s upstream direction.

What Is A Tornado?

Nature abhors a vacuum but loves a whirlwind — a swirling cylinder of air with low pressure at its center. Such rotating vortices can be of various dimensions and have multiple causes: large-scale systems such as tropical and extratropical cyclones; smaller scale vortices down to the size of a dust-devil. The most powerful of the smaller scale entities is the tornado, which can range from a few tens of feet to a mile wide.

How Does The Tornado Form?

The most destructive tornadoes form out of thunderstorm supercells. These have their origin in conditionally unstable air (see below) with vertical wind shear (a change of wind speed and/or direction with height). The instability creates a vertical turbulence and the wind shear creates a twisting motion, first horizontal, but eventually turned to the vertical by the instability.

Once a supercell about five miles wide has formed, the whole system contracts, creating an increase in the winds. An analogy is the skater, spinning slowly with arms outstretched, whose rotation rate increases when she pulls her arms closer to her body.

A similar system is a proto-solar system. As the material falls towards the central star, it speeds up. Our sun rotates once every 25 days.

Conditional Instability

Conditional instability occurs when the air near the ground is saturated and the air above it is dry. If some mechanism exists to lift the column of air (such as a cold front), moisture will condense in the saturated air. When water vapor condenses, it releases latent heat.

The atmosphere has a characteristic decrease of temperature with height, called the lapse rate; a normal lapse rate is around four degrees per thousand feet. Lifting dry air results in cooling at about five degrees per thousand feet, so dry air is stable. If you lift a parcel, it is cooler than its surroundings and will sink back where it came from. However, saturated air cools at about three degrees per thousand feet (because of the added latent heat of condensation). So a lifted parcel of saturated air will be warmer than its surroundings and will continue to rise.

When this happens, violent overturning of the atmosphere is possible.

Perfect Conditions For Tornadoes

Though tornadoes have been observed on every continent except Antarctica, the United States heartland provides the most favorable conditions in the world for tornadoes. Warm air streaming north and northwest from the Gulf of Mexico provides a layer of saturated air near the ground. Above the surface layer streams dry air which has lost much of its moisture in the Rockies. The stage is set: moisture-laden southerly  or southwesterly flow at the surface; dry west wind aloft. Bingo: tornado alley.

Tornadoes can occur in tornado alley at any time of year, but spring offers the most favorable conditions. In winter, cold air often penetrates far into the Gulf of Mexico; in summer, the dry westerlies aloft are weaker and there are fewer cold fronts to provide lift.

What’s The Trend?

The United States averages about 1,200 tornadoes per year. Though there has been a slight increase in tornado numbers and average strength in the recent past, it is not enough to establish a reliable trend.

There is also a weak correlation with El Niño, but on the whole, what you see is what you get: high tornado risk in a wide swath from Texas to Nebraska and east through the deep south and midwestern states, especially in the spring.

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