Cyclones, storms, typhoons: Why "rains" are different

             Oceans and seas have great influence on the weather  of continental masses. A large portion of the solar energy reaching the sea-surfaces expended in the process of evaporation. These water evaporated from the sea/ocean is carried up into the atmosphere and condenses, forming clouds from which all forms of precipitation result. Sometimes, intense cyclonic circulations occur which is what we call the tropical cyclones.

           Tropical cyclones are warm-core low pressure systems associated with a spiral inflow of mass at the bottom level and spiral outflow at the top level. They always form over oceans where sea surface temperature, also air temperatures are greater that 26 degrees Celsius. The air accumulates large amounts of sensible and latent heat from the sea and the exchange can occur rapidly, because of the large amount of spray thrown into the air by the wind. The energy of the tropical cyclone is thus derived from the massive liberation of the latent heat of condensation.

        Tropical cyclone is defined as a non-frontal, synoptic-scale cyclone developing over tropical and sub-tropical waters at any level and having a definitely organized circulation. In other parts of the world, these are referred to as hurricanes, typhoons, or simply  tropical cyclones depending on the region. In the North Atlantic, Eastern North Pacific and South Pacific Ocean, they are called “hurricanes.” In the bay of Bengal, Arabian Sea and Western South Indian Ocean, the name is “cyclonic.” In the eastern part of the Southern Indian Ocean, it is “willy-willy,” and in Western North Pacific Ocean, they are called “typhoons.”

         Tropical cyclones can only form over oceans of the world except in the South Atlantic Ocean and the south eastern Pacific where a tropical cyclone could never be formed due to the cooler sea surface temperature and higher vertical wind shears. They develop at latitudes usually greater than 5 degrees from the equator. They reach their greatest intensity while located over warm tropical water. As soon as they move inland, they begin to weaken, but often not before they have caused great destruction.

           The Philippines is prone to tropical cyclones due to its geographical location which generally produce heavy rains and flooding of large areas and also strong winds which result in heavy casualties to human life and destruction of crops and properties. Thus, it is of utmost importance to have sufficient knowledge on such maritime phenomena for beneficial purposes.

         Tropical cyclones derive their energy from the latent heat of condensation which made them exist only over the oceans and die out rapidly on land. One of its distinguishing features is its having a central sea-level pressure of 900 mb or lower and surface winds often exceeding 100 knots. They reach their greatest intensity while located over warm tropical waters and they begin to weaken as they move inland. The intensity of tropical cyclones vary, thus, we can classify them based upon their degree of intensity. The classification of tropical cyclones according to the strength of the associated winds are as follows:

TROPICAL DISTURBANCE is a discrete weather system with an apparent circulation. It is characterized by a poorly developed wind circulation of weak velocities and with one or no closed isobars (isobars are lines of equal pressures). This is commonly observed throughout the wet tropics and sub-tropics.

TROPICAL DEPRESSION is a weak low pressure disturbance with a definite surface circulation having maximum wind speed of up to 63 kilometers per hour (kph) or approximately less than 25 mile per hour (mph). It has one or more closed isobars and is most common in the equatorial regions or inter-tropical convergence and less frequent in the trades.

TROPICAL STORM, a moderate tropical cyclone with maximum wind speed of 64 to 118 kph (25 to 75 mph) and with closed isobars.

TYPHOON, an intense tropical cyclone with maximum wind speed exceeding 118 kph.


A version of this article appeared in print on the second quarter 2012 issue on page 31 of the S&T Post. 

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