A tornado is a violently rotating column of air in contact with the earth's land surface that originates from a thunderstorm. If it is not in contact with the earth's surface then it is a funnel cloud. If it is not rotating then it is a straight-line wind event. If it not does originate from a thunderstorm then it is a dust devil circulation. A typical significant tornado also originates from the updraft region of a strong thunderstorm. Those that occur along the leading edge of a straight-line wind gust fronts are called gustnadoes. A tornado over the water is called a water spout. A tornado that forms mainly from the stretching of an updraft is termed a land spout.

A tornado forms from the twisting of rising air. Radar and storm spotters are two primary sources that tornadoes are identified. The troposphere needs to have instability for a thunderstorm and a tornado to occur. Instability release produces the updraft of a thunderstorm. The closer the base of the updraft (known as Level of Free Convection) is to the surface then the better it is for tornadoes. The base of the updraft will tend to be closer to the surface when the dewpoints and relative humidity in the lower troposphere are higher.

The air needs to also be twisting. Twisting will not occur if the winds are too light or if the wind does not significantly change direction and speed with height. If the air in the lower troposphere is twisting from wind shear then when this air is forced upwards through the updraft it will spin like a top. If the balance between the twisting motion and instability release is just right a tornado will form.

Think of spinning a top. If the top does not spin fast enough it will fall over and not work. There is not enough of a twist in that situation. If the top is not spun correctly it will fall over and lose balance. There is not a good balance of twist and pull alignment in that situation. For a tornado to occur it helps to have an overlapping alignment between the updraft and the wind shear. This is a situation of significant Helicity and significant CAPE close to the surface occurring at the same time.

As twisting air accelerates upwards it will stretch. This stretching will enhance the rotation rate just like an ice skater twists faster when the hands are brought closer to the spinning body. Tornadoes produce the most powerful winds on earth. People that have seen tornadoes are often in awe of the fury and power of the wind.

The ultimate weather education website: