cyclone The term ‘cyclone’ refers to large-scale rotatingweather systems which rotate with positive vorticity.Two major types of cyclones are tropical cyclones andextra- tropical cyclones. The latter are also known asdepressions. The mechanisms of formation and thestructures of these two forms are so different that theyshould be considered separately.
Tropical cyclones are also known as hurricanes and typhoons. They develop over tropical oceans and can produce extremely heavy rainfall and devastating winds with sustained wind speeds sometimes in excess of 100 metres per second (m s−1). Satellite pictures (Fig. 1) reveal a striking circular symmetry in tropical cyclones with a small (50 km diameter) cloud-free ‘eye’. The preferred regions for hurricane development are oceans where the winds are light, the humidity is high, and the surface water temperature is high (usually over 26 °C) over
Since these conditions exist in some places for only part of unfortunate that they share the same nameThe conditions required for tropical cyclones are also suitable conditions for thunderstorms, deep convective clouds with strong updraughts. Thunderstorms can be organized into a tropical cyclone in the presence of low-level convergence. The winds converging in one particular region increase the large-scale rotation in an anti clockwise sense in the northern
Because this rotation is an important factor in the development of tropical cyclones, and because no rotation is imparted at the Equator, no development of tropical cyclones is found within about 5 degrees of the Equator. As the air converges, the thunderstorms become more organized and closer together. Huge amounts of water evaporating from the warm ocean surface are carried aloft in the bands of thunderstorms. As the air rises it cools, and the water condenses releasing latent heat.
The latent heat released greatly enhances thebuoyancy of the air, producing even strongerupdraughts, which in turn draw in moreconverging air at the base. There is positivefeedback as more warm moist air being drawninto the base of the clouds produces evenstronger updraughts. At the tropopause the airspreads out in bands moving away from thecentre of the cyclone. When this divergence of airat high levels exceeds the convergence of air atlow levels, the surface pressure drops, forming alow-pressure centre around which the aircirculates and converges, bringing in even morewarm moist air to feed the cyclone. Extra-tropicalcyclones.
Extra-tropical cyclones are the middle latitude tropospheric circulation systems also known as depressions. The life cycle of a depression is often described by the polar front approach, in which the depression is seen as a disturbance which grows and modifies the front as it develops. Another approach is used here to illustrate the three-dimensional structure and the development mechanism by which an extra-tropical cyclone develops.
Fig.2 shows a wave depression at its most vigorous stage of development. The surface weather map shows an open warm sector with a deepening low-pressure centre. The low-pressure centre is situated below a region in the upper troposphere where a wave or trough of low pressure is lying slightly behind the surface low. This is exactly the configuration required to enable a low-pressure centre to deepen. The circulation of the wave depression is in
. (In the southern hemisphere the circulation would be clockwise, but the cold air would be to the south and so the mechanism would be the same.) The warm air at low levels in the warm sector is lifted above the warm front. This applies not only to air at the surface, but to all the air through a substantial depth. At the same time the cold air moving southwards behind the low-pressure centre is losing height. The net effect of warm air rising and cold air sinking is to decrease the ‘centre of gravity’ of the system because
By lowering the centre of gravity, some potential energy is removed from the system and converted to kinetic energy—the energy of motion. (When we release an object from a height and let it fall, we are converting potential energy into kinetic energy). In the wave depression the kinetic energy is manifested by the strength of the winds in the circulating system. Because it is these winds that are moving the warm air upwards and the cold air downwards, the process accelerates and feeds on itself. This unstable situation, known as baroclinic instability, continues until the warm air is lifted from the surface into the upper troposphere.
By moving warm, less dense, air into the column ahead of the upper trough the pressure ahead of the upper trough is reduced. Conversely, the cold air introduced behind the trough increases the pressure: so the trough minimum is moved from east to west. This means that the region of divergence is now no longer above the centre of low pressure at the surface. In fact, there is neither divergence nor convergence at upper levels, but convergence remains at the surface. The effect is thus to increase the surface pressure and complete the last phase
Cyclone and AnticycloneThe terms cyclone and anticyclone are used to describeareas of low and high atmosphericpressure, respectively. Air flowing around one or theother of these areas is said to be moving cyclonically inthe first case and anticyclonically in the second. In thenorthern hemisphere, cyclonic winds travel in acounterclockwise direction and anticyclonic winds, in aclockwise direction. When a cyclone or anticyclone isassociated with a wave front, it is called a wave, afrontal, or a mid-latitude cyclone or anticyclone.
Vertical air movements are associated with both cyclones and anticyclones. In the former case, air close to the ground is forced inward, toward the center of a cyclone, where pressure is lowest, and then begins to rise upward. At some height, the rising air begins to diverge outward away from the cyclone center.
In an anticyclone, the situation is reversed. Air at the center of an anticyclone is forced away from the high pressure that occurs there and is replaced by a downward draft of air from higher altitudes. That air is replaced, in turn, by a convergence of air from higher altitudes moving into the upper region of the anticyclone.Distinctive weather patterns tend to be associated withboth cyclones and anticyclones. Cyclones and lowpressure systems are generally harbingers of rain,clouds, and other forms of bad weather, whileanticyclones and high pressure systems are predictorsof fair weather.
One factor in the formation of cyclones and anticyclones may be the development of irregularities in a jet stream. When streams of air in the upper atmosphere begin to meander back and forth along an east-west axis, they may add to cyclonic or anticyclonic systems that already exist in the lower troposphere. As a result, relatively stable cyclones (or anticyclones) or families of cyclones (or anticyclones) may develop and travel
On relatively rare occasions, such storms may pick up enough energy to be destructive of property and human life. Tornadoes and possibly hurricanes are examples of such extreme conditions.