Local storms (tornado, thunderstorm, hailstorm)

Thunderstorms are the result of vertical circulation in the atmosphere. Cold, heavier air sinks, causing the warm air in its path to rise. Especially when thunderstorms form on a cold front, the air streams down to the earth‘s surface from a height of several kilometres and shoots below the warm air in tongue-form.

As in the case of orographic storms, potential energy is converted into kinetic energy. Gusts are always particularly intense when a thunderstorm is accompanied by heavy rain or hail. As a result of the precipitation, the surrounding air also cools down and is finally dragged down, too. Near ground level, the stream of air veers into a horizontal plane, steering raindrops or hailstones into a sloping trajectory – sometimes at an angle of more than 45° from the vertical.

Tornadoes are small-scale storms that form in intense thunderstorm systems when cold, dry air passes over warm, moist air masses. Given suitable temperature differences, the cold air can plunge downwards in a violent whirling motion – similar to the action of liquid when a bottle is emptied quickly. On the edge of the whirling wind, the warm air moving up replaces the cold air moving down, condenses and thus makes the whirling wind visible from the outside.

Condensation often forms at the centre of the tornado, too, however. If the air pressure suddenly falls by as much as 10% below normal, this also leads to cooling and to droplet and cloud formation as a result of over-saturation. The rotation of the tornado funnel is determined, as a rule, by the rotation of the earth, as with tropical cyclones. Tornadoes therefore turn clockwise in the southern hemisphere and anti-clockwise in the northern hemisphere. However, there are also isolated records of tornadoes rotating in the opposite direction.

The average width of tornado funnels is about 100 m and the average track length a few kilometres, although widths of more than 1,000 m and track lengths of up to 300 km have also been observed. The maximum possible wind speed on the edge of the funnel is estimated to exceed 500 km/h – the highest speed of all windstorm types. Tornadoes usually have an average wind speed of just over 100 km/h and are most common between latitudes 20° and 60° north and south of the equator.

As in the case of tropical cyclones, there also are other names for tornadoes: in Japan they are called tatsumaki and in Germany Tromben. Waterspout is the term used when they form over water surfaces.

The formation of a local storm (tornado)

Of all wind systems, it is tornadoes that attain the highest wind speeds. They are generated whenever strong vertical air movements occur in the atmosphere and are therefore always accompanied by intensive thunderstorm cells. The schematic representation on the left demonstrates the air flows in and around a typical tornado. Vortex formation is strengthened particularly by the cold, dry air falling onto the warm, moist air below.