Tornadoes

Synoptic Conditions

Atmospheric instability is a prerequisite of tornado formation. For example, in the continental United States, starting each year in February, warm, moist air starts to flow into the land area of the southern states from the Gulf of Mexico. At the same time, cool, dry air flows eastward across the Rocky Mountains. Advection (horizontal air flow) combines with inversion of the temperature gradient by sinking of the cool, dry air, which warms until it traps the moist air underneath. The process will intensify if a cold front breaks into the inversion, causing cloud development to overshoot the tropopause (an atmospheric transition zone between the troposphere and the stratosphere). A squall line, also known as a derecho, may then develop.

Tornadoes result as the unstable moist air forces its way up through the inversion layers above, creating intense convective storms. As the continental land mass heats up from early spring until late summer, the focus of instability—that is, the area in which tornadoes are most common—migrates northward until it reaches the Canadian border in August. Similar conditions pertain in the Brahmaputra-Ganges delta area of Bangladesh as a result of the monsoon winds and associated rainfall. On a limited scale, coastal areas of the south of France, Italy, and southern England can also generate synoptic conditions for the formation of tornadoes. However, they tend to be weaker than the North American examples.

Convective cells form when rising, relatively warm air cools, sheds its moisture (releasing latent heat) and falls again, only to warm up as it reaches the atmospheric boundary layer. Microcyclones that spawn tornadoes show a hook-shaped echo on Doppler radar. Coriolis deflection causes the cyclone to break away from an area of increasing atmospheric instability and to grow to a diameter of up to 15 miles. The hook—i.e., the center of the depression—is characterized by tight convection and intense concentration of rainfall, and is the zone of tornado formation. Depending on the form of contrasting air masses, tornadoes can form singly, in lines (sometimes almost like a relay race), or in migrating families.

In the United States, they tend to follow a southwest-northeast trend, largely as a result of the general circulation of the atmosphere coupled with the Coriolis effect. Maximum concentrations of damaging tornadoes occur in parts of Texas, Oklahoma, Kansas, Missouri, Arkansas, and western Tennessee, an area colloquially termed tornado alley. The exact location of maximum activity depends on the span and type of data considered. However, this is the location of maximum tornado impacts; maximum activity occurs in New England, where the tornadoes tend to be weaker.

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