Tornadoes are one of the most destructive forces of nature. They occur frequently in the United States. Every year there are on average 800 tornadoes reported nationwide with at least 80 people dead and 1500 injured. Winds in most tornadoes can reach 100 mph, travel along the ground for a few miles and are usually less than 100 yards wide. It is very rare to have "monster" twisters with speeds of 300 mph or more, that travel for at least 50 miles, leaving behind a mile wide damage paths. A few significant tornadoes also happen annually in Europe, Asia, southern Africa, and southeastern South America.
Most tornadoes do not come unannounced. Certain weather conditions have to be present for a tornado to develop. It usually happens during a thunderstorm. A thunderstorm begins when warm, humid air and a fast moving cold front meet. The increase in wind speed and changes in its direction can create severe thunderstorms, called supercells. Supercells often produce intense tornadoes, ranging from 3 to 5 on the Fujita Scale (explained below), along with hail, rain and lightning. The trigger is usually converging winds, which start low near the ground and flow in upward motion picking up the moisture.
Warm, humid air shoots up meeting colder, dryer air. Since warm air is lighter than the cold air, the combination makes a strong updraft within the thunderstorm. As it keeps moving up, it meets winds of different directions at different altitudes. If these winds are positioned just right and have sufficient speed, they will embrace the uprising air and start spinning it. It is similar to spinning a pencil with your hands. When you hold a pencil in the palms of your hands facing each other and move them in the opposite direction the pencil will spin. The storm will show a visible rotation. That can give a start to a mesocyclone, which is a vortex of air that rises and rotates around a vertical axis. As the updraft keeps rotating, it may form a wall cloud, a spinning layer of clouds descending from the mesocyclone. The area of rotation can be as large as 6 miles wide. As the wall cloud descends, a funnel-shaped cloud may form. This is the first stage of the tornado formation. Only about 30% of mesocyclones produce a tornado, but the presence of a mesocyclone is believed to be the key factor in the formation of strong tornadoes.
Tornadoes usually form when the thunderstorm reaches its peak. The momentum and vacuum built up of large masses of air, shooting into the upper atmosphere cause a siphoning effect at the ground level. Smaller pockets of remaining warm air at the ground are sucked up into the storm. Once the warm air at the ground lessens, the large wall cloud siphons air from a mile or less diameter region at the ground, forming a tornado. If the supply of warm air is available, the tornado will keep on going. Once the warm air runs out, the storm will gradually die down.
Most tornadoes appear as a column or a narrow funnel of air, spinning violently and picking up debris from the ground. However, they can take different shapes and even colors depending on the environment in which they start. In a dry environment a tornado can be almost invisible, unless it carries a lot of dust and debris along. In higher humidity, condensation funnels do not pick up much debris and tornadoes are usually white or gray in color. During lighting, which often accompany tornadoes, the same funnel can look almost black. Dust, heavy rain or hail can obscure tornado's visibility, and only radar observations can warn of its path.
Tornadoes vary in intensity and size. The Fujita and Pearson Scale, usually called the Fujita Scale ranks tornadoes by their wind speed, path, length, and width. The ranking ranges from F0 (very weak) to F6 (inconceivable). In the States 80% of tornadoes are F0 and F1. Less than 1% are stronger than F4. They usually emit loud roaring or whooshing sounds, although audibility depends on atmospheric conditions and land topography. Occasionally, during an intense tornado, multiple vortexes can form and work as separate tornadoes. During their last stage they resemble narrow tubes that twist into complex shapes.
Waterspouts are another kind of tornadoes. They can either form over warm water and be relatively weak, or start as a strong tornado on land and move onto water and, in result, turn into a water tornado. Waterspouts are most common along the Gulf Coast and southeastern states. They occur during cold, late fall or winter storms, when tornadoes are least expected.
Landspout, or dust tube tornado, is similar to a waterspout. It has a short life span and is relatively weak. Its funnel does not always reach the ground, but when it does, it creates a cloud of dust and can cause serious damage.
There are several air circulations that resemble tornadoes, but are not associated with the cloud base, for example: gustanado, fire whirl (which can happen during wildfires), dust devil, or steam devil. They are not tornadoes by definition, but they look alike and can cause severe damages, as well.
Tornadoes can occur any time of the year, however the frequency changes with seasons. In the States during early spring, tornadoes occur in the central Gulf states. In March and April the activity spreads to the southeastern Atlantic states and continues until August. May is the most dangerous month for tornadoes in the United States and they usually happen in the southern Plains states. In June until August they appear in the northern Plains states and Great Lakes area.
Most of the information about thunderstorms comes from data collected by Doppler radar. Strategically located radars across the country can detect air movement towards or away from the radar. Early detection of increasing rotation within a thunderstorm allows to predict a tornado before if actually forms. However, as the weather cannot always be predicted, a tornado can start without any warning at all. There have been many programs and projects undertaken to better understand and predict tornadoes. For example, the VORTEX projects (in 1994, 1995 and 1999) involved sending several vehicles that were equipped with customized instruments to measure and analyze the weather around tornadoes. The movie 'Twister' was partially inspired by the VORTEX project.
It is important that the research continues, as peoples lives depend on early warnings.