Definition of a Tropical Cyclone
A tropical cyclone is a violent rotating system of low atmospheric pressure. The rate of pressure drop in its center can reach about 15.5 hectopascals from the surrounding environment, while the maximum wind speed in it can reach 64 knots or more.
Tropical cyclones form in specific regions of equatorial seas and oceans where sea surface temperatures exceed 26°C, typically located between latitudes 5° and 20° north and south of the equator.
It is worth noting that once these types of cyclones reach land or pass over cooler waters, they gradually weaken due to the loss of the warm water vapor that fuels them.
Stages of Cyclone Formation
Cyclones do not suddenly appear from nothing; rather, they go through several life stages, depending on the availability of favorable atmospheric conditions that allow them to develop. These stages can be tracked and observed over a sufficient period of time.
They begin as clusters of cumulonimbus clouds over warm ocean waters where the sea surface temperature exceeds 26°C due to direct heating from intense solar radiation.
Consequently, the layer of air in contact with the sea surface warms up, causing the air pressure to drop. The warm air expands and rises (creating a low-pressure system), attracting winds toward the center of low pressure.
This leads to increased evaporation, with water vapor rising until it condenses—releasing latent heat in the process.
This further lowers the air pressure and results in the rotation of air masses laden with thunderstorms and heavy rains.
As wind speeds around the center increase, the low-pressure system deepens and evolves into a tropical storm, and then into a full-fledged tropical cyclone. It is then classified on the cyclone intensity scale.
Cyclone Classification
Cyclone intensity is measured using the Saffir-Simpson Scale, which consists of five categories:
- Category 1: Wind speeds between 119–153 km/h; wave height between 1.2–1.5 meters.
- Category 2: Wind speeds between 154–177 km/h; wave height between 1.8–2.4 meters.
- Category 3: Wind speeds between 178–208 km/h; wave height between 3.7–2.7 meters.
- Category 4: Wind speeds between 209–249 km/h; wave height between 4.5–5.5 meters.
- Category 5: Wind speeds of 250 km/h or higher; wave height may exceed 5.5 meters.
Names of Cyclones
The names of cyclones vary by region:
- In the North Atlantic, Northeastern Pacific, and Caribbean Sea, they are called “hurricanes.”
- In the Northwestern Pacific, they are called “typhoons.”
- In the Indian Ocean and Southwestern Pacific, they are simply referred to as “tropical cyclones.”
Important Facts About Tropical Cyclones
There are two cyclone seasons in the North Indian Ocean (which includes the Arabian Sea and the Bay of Bengal):
- The first season occurs during May and June.
- The second season occurs during October and November.
The annual average of tropical cyclones in the North Indian Ocean represents about 5–6% of the global annual average. Cyclone formation in the Bay of Bengal is four times more frequent than in the Arabian Sea. One reason for this is that the Arabian Sea’s waters are relatively cooler than those of the Bay of Bengal.
Most cyclones that form over the Bay of Bengal weaken as soon as they make landfall. Therefore, the chances of the Arabian Sea and Oman being directly affected by cyclones are low.