tornadoes

tornadoes Tornadoes are small and short-lived but highly destructive phenomena. Because of their severe nature and small size, comparatively little is known about their detailed structure: measurement and observation within them is difficult. They comprise elongated funnels of cloud which descend in a snake-like fashion from the base of a well-developed cumulonimbus cloud, eventually making contact with the ground beneath. They represent a zone of marked convergence into the cloud base generated by the intense updraughts that feed the cloud system, and are manifested as a narrow columnar zone of upwardly rotating violent winds, perhaps exceeding 100 metres per second. The vortex core is at very low atmospheric pressure, so that cloud develops within the funnel, and pressure gradients can approach an estimated 25 mbar (hPa) per 100 m. This compares to the most extreme pressure gradients of about 20 mbar per 100km around a larger-scale cyclone.

A tornado's impact as a hazard is extreme. There are three damaging factors at work. First, the winds are often so strong that objects in the tornado's path are simply removed or very severely damaged. The pronounced rotational element also tends to twist objects from their fixing, and strong uplift can carry some debris upwards into the cloud. More typically, the heavier material is thrown tangentially outwards by the system's rotational winds: the second source of hazard. The third source of damage is the very low atmospheric pressure near the vortex centre. When a tornado approaches a building, external pressure is rapidly reduced, and unless there is a nearly simultaneous and equivalent decrease in internal pressure, by opening windows or doors, the walls and roof may explode outwards in the process of equalizing the pressure differences.

Many tornadoes have a short life. The convectional activity which creates the source cloud is itself highly variable, and a single cloud can spawn a number of different tornado vortices, either simultaneously or in sequence, beneath different areas of the cloud, as parts of it develop and decay. Movement is generally with the parent cloud, perhaps with the funnel twisting sinuously across the ground beneath. Once contact with the ground is made, the track of a tornado at ground level may frequently extend for only a few kilometres, though there are examples of sustained tracks extending over hundreds of kilometres. The diameter of the funnel is rarely more than 200 m; track length and width are consequently modest, and the probability of being affected is comparatively remote—though this is no consolation to those afflicted.

Tornadoes, being associated with extreme atmospheric instability, show both seasonal and locational preference in their incidence. ‘Favoured’ areas are temperate continental interiors in spring and early summer, when insolation is strong and the air may be unstable, although many parts of the world can be affected by tornado outbreaks at some time or another. The Great Plains of the USA, including Oklahoma, Texas and, of course, Kansas, have by far the highest global frequency, and are particularly prone at times when cool, dry air from the Rockies overlies warm, moist ‘Gulf’ air.

Graham Sumner

Bibliography

Eagleman, J. R.,, Muirhead, V. U.,, and and Willems, N. (1975) Thunderstorms, tornadoes and building damage. Lexington Books, London.