Tropical disturbances are classified into four categories: (1) easterly waves, (2) tropical depressions, (3) tropical storms, and (4) hurricanes or typhoons.
(1) Easterly waves:
These are the migratory wavelike disturbances of the tropical easterlies. The easterly waves represent a pattern of wave perturbations which move from east to west with the current, but with a slower speed.
The source of their occurrence lies between 5° to 30° latitudes in both the hemispheres. These waves form in the easterly current particularly over the tropical oceans. They never form exactly over the equator.
They originate at a certain height from the earth’s surface. Because of these waves, isobars on the weather charts develop indentations. These atmospheric disturbances are quite unlike the depressions of the middle latitudes.
These are mild pressure troughs which generally slope eastward with height. These waves travel in a east- to-west direction at the rate of 320-480 km per day. Ahead of the trough line there is divergence of air currents.
Since in this part of the disturbance there is subsidence, the weather tends to be dry and clear. On the contrary, behind the waves, warm and moist air currents tend to ascend which leads to the development of cumulonimbus clouds and thundershowers.
When such a type of transverse wave passes an observer, the following weather sequence is generally produced: There is a ridge of high pressure ahead of the trough. This is invariably associated with fine weather.
There are scattered fine-weather cumulus clouds floating in the sky. The lower air may have some haze, but close to the trough line there are well-developed cumulus clouds with occasional showers.
The visibility near the ground gradually improves. Behind the trough, veering of the wind is observed. Because of convergence the heavy cumulus or cumulonimbus clouds develop yielding moderate to heavy precipitation.
This part of the wave is I also associated with thunderstorms. There is a slight decrease in temperature.
Most of the easterly waves formed over the Atlantic Ocean make an inverted-V wave-form in the low-level wind field. These disturbances in the easterlies have a cyclonic circulation at about 600 mb level.
In view of the paucity of observational data, it is rather difficult to throw much light on the origin and development of wave disturbances over the tropical oceans and in continental areas.
But, broadly speaking, about 80 percent easterly waves develop between 5° and 20° north latitudes, poleward of the equatorial trough, towards the western part of the oceans.
First of all, the easterly waves of the Caribbean Sea were studied and described. Since then investigations have been made into the origin and growth of similar waves in the easterlies in other tropical regions.
It was found that these wave disturbances occur near the boundary of the trade winds and the doldrum belt. They travel westward rather sluggishly and are always associated with extensive cloud areas and rain areas.
It is a common characteristic of all such wave perturbations that bad weather follows the waves, whereas good weather precedes them. Sometimes acute instability develops in these waves and they intensify into hurricanes.
It may be pointed out that when easterly waves pass over the land areas the associated weather may largely be affected by the configuration of landforms.
As regards the regional distribution of easterly waves, they tend to develop in the Caribbean, when during summer and autumn the trade wind inversion happens to be weak or absent.
The winter and spring seasons do not favour the formation of easterly waves. Sometimes easterly waves originate in the North Pacific as well. During summer, in the Caribbean and the Pacific easterly waves originate after every three and two days respectively.
The easterly waves form in the equatorial troughs which are elongated in shape and extend up to hundreds of kilometers.
They extend in the north-south direction lying across the trade wind circulation. They travel long distances as distinct entities. Near the surface of the earth these waves appear to be feeble, but they are fully developed at a height of, say, 4000 meters.
Their climatologically significance lies in the fact that such areas receive heavy rainfall by them as would have remained dry in their absence.
(2) Tropical depressions:
Petterssen defines the tropical depressions as the centers of low pressure around which the wind velocity hardly exceeds 40 km per hour. These depressions are liable to occur anywhere in the warm humid air of the tropics, but they are quite frequent in the vicinity of inter-tropical convergence zone.
However, they are very rare in the trade wind belt. Many of these weak depressions never grow into storms of hurricane intensity; rather they die out as weak disturbances.
(3) Tropical storms:
These low pressures centers are surrounded by winds having their velocities in the range of 40 to 120 km per hour. Most favourable atmospheric conditions for their occurrence exist during the summer season.
The Bay of Bengal and the Arabian Sea offer ideal conditions for the origin and growth of these storms. They are also present in the West Indies and in the vicinity of the Philippines.
These storms produce heavy precipitation and bring about a change in the existing weather. Many storms of this type develop into more violent and destructive type of tropical storms.
(4) Tropical-Cyclones (Hurricanes or Typhoons):
Hurricane is a severe tropical cyclone having a maximum wind speed in excess of 119 km per hour. The name ‘hurricane’ is given to the tropical cyclones in the North Atlantic and the eastern North Pacific oceans.
The tropical cyclones of hurricane force in the western North Pacific are known as typhoons. In Australia this type of storm is given the name willy-willy, whereas in the Indian Ocean they are called cyclones.
These storms are known as baguio in the Philippine Islands. In olden days the Japanese called these severe storms as nowaki, but now they are called taifu. Elsewhere they are just called tropical cyclones.
Hurricanes represent the most violent, most awesome, and most feared of all the atmospheric disturbances.
Since these storms cause widespread damage over land areas, and are dangerous to shipping, the weather scientists have given considerable attention to them.
Aircraft reconnaissance flights during the periods of maximum hurricanes, radar observations of cloud and precipitation, and satellite photography have all been helpful in the investigation of such a very complicated and difficult weather phenomenon.
According to Byers, the following are some of the most prominent distinguishing features which make the tropical cyclones different from the middle latitude cyclones:
(1) Tropical cyclones are found at certain seasons in well-defined areas of the tropics.
(2) They form only over oceans having a high surface temperature (27°C).
(3) They do not have fronts, nor are they associated with moving anticyclones.
(4) They are many times more intense than the extratropical cyclones.
(5) They do not form with any regularity, and they can exist only on oceans.
(6) They have a central core of calm or light winds. This is called the ‘eye’.
(7) They derive their energy from the latent heat of condensation.