A tornado is a revolving column of air in a funnel-shaped cloud extending from the base of a cumulonimbus (thunderstorm cloud) to the ground. The entry to the funnel is only a few hundred meters wide but contains destructive winds with speeds exceeding 320km/h. The same phenomenon over water are called waterspouts.
During the mature stage of a severe thunderstorm, very strong updrafts feed parts of the insatiable cumulonimbus cloud. In other parts of the cloud, severe downdrafts occur. It is in this extremely turbulent region, between rising and descending air, where tornadoes are born. If the conditions are right, the turbulence becomes organized and the air starts to spin, just like the draining water in the bathtub. The rotation is very unstable and may last only a few minutes, long enough to cause immense damage on the ground.
Occasionally tornadoes form underneath tropical thunderstorms in cyclones. Severe thunderstorms with tornadoes are more frequent in the mid-latitudes, however, where air masses with significant temperature differences clash. The intensity of the clash increases when a region has mountain ranges funneling either air mass towards its adversary. The US has such mountain ranges. Warm and humid air from the Gulf of Mexico meets with cold Canadian air over the Central States, known as ‘Tornado Alley’.
While tornadoes occur worldwide, the US holds the record with 1,297 in one year (1992). Australia is second with several hundreds per year, but a sparse population prevents an accurate count. The plains of China and Argentina are also a favorite breeding ground for tornadoes. Waterspouts frequently cross the coast and develop into tornadoes in many coastal and island nations.
Despite their destructive powers, tornadoes don’t account for many deaths. Floods and even lightning cause a higher mortality rate. Nevertheless, approximately 100 US citizens die and about twenty times as many get injured every year. The economic losses are tremendous, though. Every year is different, but the damage is generally somewhere between A$800 million to over A$2 billion in the US. Because tornadoes accompany severe thunderstorms, hail and flood damage usually add to the figures.
Unless the tornado strikes buildings, farmers don’t usually suffer great losses. The tornado destroys everything within its narrow path, but has minimal impact on the rest of the crop. The wind damage is high in built-up areas where it cuts a swath of destruction. The destructive path can be so selective that one house gets completely demolished while the other next to it remains untouched. The type of building also influences the severity of the damage. Pre-fabricated housing and mobile homes suffer most.
Tornadoes strike without warning – almost. The weather forecasters know when severe thunderstorms are likely to develop and, because tornadoes come with severe thunderstorms, they can issue a tornado watch. Some weather stations are equipped with radar capable of detecting air in motion. Observed rotating air within the thunderstorm or actual witnessed tornadoes by weather service personnel or the public activate a tornado warning. Unfortunately, communities near the thunderstorm don’t have much time to react.