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Where do they come from? Characteristics and Classification When and Where? Forecasting Tornadoes.

A tornado, from the Latin tornare ("to turn"), is a violent rotating column of air extending from a thunderstorm. Tornadoes are amongst the most violent and destructive of all weather phenomena, but despite the significant amount of research into the origin and prediction thereof, the phenomenon is still not fully understood or predictable. Field research on tornadoes is particularly difficult due to their short lifetime, the small area affected by them and the low probability of occurrence at a particular point. Where do they come from?

Tornadoes in South Africa are typically associated with very hot air masses and severe thunderstorms. To date scientists do not know exactly how tornadoes are formed, but all the evidence suggests that they develop as a combination of (1) strong spinning effects inside a thunderstorm or in the air surrounding the storm and (2) accelerated strong updrafts (wind moving vertically in an upward direction).

One of the more recent theories explains it as follows:

Before thunderstorms develop, a change in wind direction takes place. The wind speed also increase with height. These two factors combined give rise to an invisible, horizontal spinning effect in the lower atmosphere. The rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical (figure 1). An area of rotation now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

Figure 1
Figure 1

There are a few meteorological conditions necessary for tornado formation. Because of the scientific nature of this, the conditions are only listed and not further discussed here.

• A deep layer of mid-atmospheric dry air above a moist surface layer

• Steep moisture and temperature gradients

• High surface temperatures

• Low level convergence and upper level divergence

• Vertical windshear (change in wind direction and speed with height)

• Atmospheric instability (air continues to rise once it starts rising)

Characteristics and Classification

Damage Path: Not all tornadoes are strong enough to extend down to the surface of the earth. If a tornado touches the ground the damage path depends on the position within the thunderstorm where the tornado was generated and the topography. The length and width of the path also depends on the speed of the thunderstorm - when the thunderstorm slows down, the path of the tornado widens.

Wind Speed: It is very difficult, if not impossible, to obtain a direct and reliable wind speed record from a tornado. Even in those cases in which standard wind-measuring devices have happened to be present in the tornadic path, the actual wind speeds have been well above the limits of these devices and, besides this, the wind-speed anemometers have been damaged and/or displaced. The only actual measurements currently available are for USA tornadoes. Indirect methods of estimating the wind speeds have generally been used. One such estimate made for a South African event suggested a wind speed in the order of 350 km/h.

Classification: There are several different methods of classifying tornadoes. The most commonly used is the "Fujita-Pearson scale classification". This system classifies tornadoes in six intensities, ranging from F0 (no damage) to F5 (incredible damage). The intensity is based on the apparent damage to structures, the extent of the path and other descriptors from which wind speeds are then inferred. 65% of the South African tornadoes are classified as F0 or F1 (light damage), while more than 90% are classified as F0, F1 or F2 (considerable damage) or less. The tornado which occurred at Harrismith on 15 November 1998 was classified as F2 due to the severity of the damage.

When and where?

Tornadoes can occur basically anywhere where a thunderstorm is possible. From an analysis of the occurrence of South African tornadoes it became clear that most of them have been observed in Gauteng, the Free State, KwaZulu-Natal (along a line from Pietermaritzburg to Ladysmith) and the northern region of the former Transkei. In figure 2 the eastern part of the country is depicted, showing the more significant events (F2 and F3) from 1905 to 1997.

Figure 2
Figure 2

The seasonal distribution of South African tornadoes is given in figure 3. Most of the events occur in mid-summer from November to January, although a large number of tornadoes have occurred in spring and early summer (September and October) and in the late summer and autumn (February to May). It is also worth mentioning that most tornado events (for which the time of the day were available) occurred in the late afternoon or early evening, typically between 16:00 and 19:00.

Figure 3 Figure 3

Forecasting Tornadoes

Meteorologists rely on weather radar to provide information on developing storms. Currently in the USA, only a "20-minutes before touchdown" prediction is possible by identifying the so-called vortex signature of the tornado on radar.

Storm structure can provide clues about the existence of a tornado, clearly shown in the radar images for the Harrismith tornado. The Bow echo shape in the Harrismith area is typical of severe storms.

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