Tornadoes are of two types and, in the Guinea lands of West Africa, a violent storm often known as an 'African Tornado' occurs. It consists of a squall (very strong wind) blowing from a severe thunderstorm, often starting quite suddenly and usually lasting a short period only. Often causing considerable damage on land and sea, it also is usually accompanied by torrential rain. More common during the day, when the surface area is heated and is most frequent during transition periods between wet and dry seasons. The 'African Tornado' is caused by the meeting of warm, damp monsoonal air from the south-west with the dry north-easterly or Harmattan air from the Sahara - it has a front several kilometres in length.
An extremely violent whirlwind covering approximately less than 0.5 km in diameter, it is also known as a tornado. Most frequent in the U.S. east of the Rockies, especially on the central plains of the Mississipi basin, when hot damp air from the Gulf of Mexico meets cool, dry air from the north. A local phenomenon, it travels in an approximate straight line at 30 - 60 km/h and often dying out after about 25-30 km. While it lasts though, it causes great destruction along it's narrow track with wind velocities reaching 300 km/h. Trees are uprooted and buildings are destroyed. The tornado is usually accompanied by heavy rain and thunder (oh yeah), and occurs most frequently during spring and early summer almost always in the afternoon when surfaced heating is at a maximum. It is an extremely low pressure system and usually occurs only over land.
Tornadoes are always associated with thunderstorm activity. It is the most violent and destructrive of all storms and has a relatively small horizontal extent - only a few hundred metres (average approximately 250 metres) and its movement is erattic but controlled by the parent cloud to which it is attached. It looks like a dark funnel-shaped cloud stretching from cumulo-nimbus cloud to the ground. The darkness is due to the amount of dust and debris which is suspended in this mass of whirling air. They are small intense low-pressure storms with winds sometime exceeding 400 km/h, which move in a clockwise motion in the Southern Hemisphere (anticlockwise in the Northern Hemisphere). Destruction from a tornado is caused by :
-speed of the winds;
-suction created by the rapidly expanding air
-explosive force as the intense low of the centre of the tornado passes over buildings. Buildings explode because of the strong pressure differences between the inside and outside.
Tornadoes can often be seen and heard but no warning is given that it is developing. Weather forecasters can predict conditions favourable for their development and therefore issue warnings, but cannot give exact predictions. Tornadoes cannot be seen on synoptic charts.
