There are a few meteorological
conditions necessary for tornado formation. Because of the scientific
this, the conditions are only listed and not further discussed
A deep layer of mid-atmospheric dry air above a moist
Steep moisture and temperature gradients
High surface temperatures
Low level convergence and upper level divergence
Vertical windshear (change in wind direction and speed
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.
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.
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.