Anyone who lives or vacations in the tropics knows that the weather is usually warm with gentle breezes and occasional thunderstorms. It seems surprising that these quaint conditions can turn into a ferocious storm that can potentially disrupt the lives of millions of people. How does this happen?
Formation of tropical cyclones Tropical cyclones represent still another example of air-sea interactions. These storm systems are known as hurricanes in the North Atlantic and eastern North Pacific and as typhoons in the western North Pacific.
The winds of such systems revolve around a centre of low pressure in an counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. The winds attain velocities in excess of km 71 miles per hour, or 65 knots, in most cases.
Tropical cyclones may last from a few hours to as long as two weeks, the average lifetime being six days.
The oceans provide the source of energy for tropical cyclones both by direct heat transfer from their surface known as sensible heat and by the evaporation of water.
This water is subsequently condensed within a storm system, thereby releasing latent heat energy. When a tropical cyclone moves over land, this energy is severely depleted and the circulation of the winds is consequently weakened.
Such storms are truly phenomena of the tropical oceans. Most tropical cyclones are spawned on the poleward side of the region known as the intertropical convergence zone ITCZ. More than two-thirds of observed tropical cyclones originate in the Northern Hemisphere.
The North Pacific has more than one-third of all such storms, while the southeast Pacific and South Atlantic are normally devoid of them.
Most Northern Hemispheric tropical cyclones occur between May and November, with peak periods in August and September. Conditions associated with cyclone formation The formation of tropical cyclones is strongly influenced by the temperature of the underlying ocean or, more specifically, by the thermal energy available in the upper 60 metres about feet of ocean waters.
This temperature requirement, however, is only one of five that need to be met for a tropical cyclone to form and develop. The other preconditions relate to the state of the tropical atmosphere between the sea surface and a height of 16 km about 10 milesthe boundary of the tropical troposphere.
They can be summarized as follows: A deep convergence of air must occur in the troposphere between the surface and a height of 7 km about 4 miles that produces a cyclonic circulation in the lower troposphere overlain by an anticyclonic circulation in the upper troposphere.
The stronger the inflow, or convergence, of the air, the more favourable are the conditions for tropical cyclone formation. The vertical shear of the horizontal wind velocity between the lower troposphere and the upper troposphere should be at minimum. Under this condition the heat and moisture are retained rather than being exchanged and diluted with the surrounding air.
Monsoonal and trade wind flows are characterized by a large vertical shear of the horizontal wind and so are not generally conducive to tropical cyclone development.
A strong vertical coupling of the flow patterns between the upper and lower troposphere is required. This is achieved by large-scale deep convection associated with cumulonimbus clouds.
A high humidity level in the middle troposphere from 3 to 6 km 1. All these conditions may be met but still not lead to cyclone formation. It is thought that the most important factor is the presence of a large-scale cyclonic circulation in the lower troposphere.
The above conditions occur for a period of 5 to 15 days and are followed by less-favourable conditions for a duration of 10 to 20 days. Once a tropical cyclone has formed, it usually follows certain distinct stages during its lifetime.
In its formative stage the winds are below hurricane force, and the central pressure is about 1, millibars, or mm The formative period is extremely variable in length, ranging from 12 hours to a few days. This stage is followed by a period of intensification, when the central pressure drops rapidly below 1, millibars.
The winds increase rapidly, and they may achieve hurricane force within a radius of 30 to 50 km 19 to 31 miles of the storm centre. At this stage the cloud and rainfall patterns become well organized into narrow bands that spiral inward toward the centre.
In the mature phase the central pressure stops falling and, as a consequence, the winds no longer increase.Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.
The severity of a tropical cyclone is described in terms of categories ranging from 1 to 5 related to the zone of maximum winds.
An estimate of cyclone severity is included in all tropical advices. Climate - Formation of tropical cyclones: Tropical cyclones represent still another example of air-sea interactions.
These storm systems are known as hurricanes in the North Atlantic and eastern North Pacific and as typhoons in the western North Pacific. The winds of such systems revolve around a centre of low pressure in an counterclockwise direction in . Tropical Cyclone Genesis is the technical term for the process of storm formation that leads ultimately to what are called hurricanes, typhoons, or tropical cyclones in various parts of the world.
Ranking and naming a cyclone Intensity scales. A wide range of wind speeds is possible between tropical cyclones of minimal strength and the most intense ones on record, and tropical cyclones can cause damage ranging from the breaking of tree limbs to .
The climatology of tropical cyclones is limited by uncertainties in the historical record. Patterns in storms imagery are best recognized by the human eye, so we need your help analyzing these storms.