A tornado is a violently rotating column of air which descends from a thunderstorm to the ground. No other weather phenomenon can match the fury and destructive power of tornadoes. Tornadoes can be strong enough to destroy large buildings, leaving only the bare concrete foundation, or lift 20-ton railroad cars from the tracks. A tornado might not have a visible funnel until it picks up debris from the ground. The strength of a tornado is measured by the Enhanced Fujita Scale.
The truth is that scientists don't fully understand how tornadoes form. A parent supercell thunderstorm is needed. Beyond that, each storm is different. Some research suggests it has to do with how strongly the wind changes direction with height, how much moisture is in the air, or the difference between the temperature of the surrounding air and the temperature of the cold downdrafts coming from the storm.
Figure 2. Reflectivity of the supercell that produced the Moore, Oklahoma tornado on May 3, 1999.
At the time of this radar scan, a tornado was on the ground. Image courtesy NWS.
A supercell is an organized thunderstorm that contains a very strong, rotating updraft. This rotation helps to produce severe weather events such as large hail, strong downbursts, and tornadoes. Supercells usually form isolated from other thunderstorms because it allows the storm more energy and moisture from miles around. These storms are relatively rare, but always a threat to life and property.
A tornado begins as a rotating, funnel-shaped cloud extending from a thunderstorm cloud base, which meteorologists call a funnel cloud. A funnel cloud is made visible by cloud droplets, however, in some cases it can appear to be invisible due to lack of moisture. A funnel cloud is not affecting the ground. If the funnel extends far enough down to begin affecting the ground, then it becomes a tornado.
Tornado paths range from 100 yards to 2.6 miles wide and are rarely more than 15 miles long, although some strong tornadoes on record have crossed through multiple states (e.g. the Tri-State Tornado of 1925. They can last from several seconds to more than an hour, however, most don't exceed 10 minutes. Most tornadoes travel from the southwest to northeast with an average speed of 30 mph, but the speed has been observed to range from almost no motion to 70 mph.
Most tornadoes occur in the deep south and in the broad, relatively flat basin between the Rockies and the Appalachians, but no state is immune. Peak months of tornado activity in the U.S. are April, May, and June. However, tornadoes have occurred in every month and at all times of the day or night. A typical time of occurrence is on an unseasonably warm and sultry Spring afternoon between 3 p.m. and 9 p.m.
Figure 3. Tornado Alley is so active with severe storms because of its unique location.
Tornadoes form under a certain set of weather conditions in which three very different types of air come together in a certain way. Near the ground lies a layer of warm and humid air, along with strong south winds. Colder air and strong west or southwest winds lie in the upper atmosphere. Temperature and moisture differences between the surface and the upper levels create what we call instability, a necessary ingredient for tornado formation. The change in wind speed and direction with height is known as wind shear. This wind shear is linked to the eventual development of rotation from which a tornado may form.
A third layer of hot dry air becomes established between the warm moist air at low levels and the cool dry air aloft. This hot layer acts as a cap and allows the warm air underneath to warm further, making the air even more unstable. Things start to happen when a storm system aloft moves east and begins to lift the various layers. Through this lifting process the cap is removed, thereby setting the stage for explosive thunderstorm development as strong updrafts develop. Complex interactions between the updraft and the surrounding winds may cause the updraft to begin rotating-and a tornado is born.