9.1.5.7. Tropical Cyclones

Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large-scale destruction caused by violent winds, very heavy rainfall and storm surges. This is one of the most devastating natural calamities. They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia.

Tropical cyclones are intense weather systems that form over warm ocean waters in the tropics. They bring severe weather conditions that can lead to significant destruction, including strong winds that can cause damage to infrastructure, torrential rainfall that can lead to flooding, and storm surges that can inundate coastal areas. They are known by different names in various parts of the world, depending on where they occur.

Think of a tropical cyclone like a giant whirlpool of wind and rain, similar to how a washing machine's spin cycle swirls clothes around. Just as clothes can get tossed violently in the washing machine, so can the winds in a cyclone, leading to danger and destruction for anything in its path.

Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are: (i) Large sea surface with temperature higher than 27° C; (ii) Presence of the Coriolis force; (iii) Small variations in the vertical wind speed; (iv) A pre-existing weak-low-pressure area or low-level-cyclonic circulation; (v) Upper divergence above the sea level system.

For tropical cyclones to form, several key conditions must be met. First, the ocean water must be warm enough (at least 27°C) to provide the energy needed for the storm. The Coriolis force, caused by the Earth's rotation, is necessary to initiate the cyclone's spinning motion. Additionally, there needs to be minimal vertical wind shear, meaning that the wind speed should not change too drastically with height. A weak low-pressure area helps to kickstart the cyclone, and the divergence of air at higher altitudes allows the storm to grow.

Imagine a tornado forming in a field. It needs just the right mix of warm and cold air along with the right wind conditions. Similarly, a tropical cyclone requires specific 'ingredients'—like warm ocean water—as a chef needs key ingredients to bake a cake.

The energy that intensifies the storm comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of moisture from the sea, the storm is further strengthened. On reaching the land the moisture supply is cut off and the storm dissipates. The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.

Tropical cyclones derive their energy from warm, moist air that rises in the form of cumulonimbus clouds, leading to condensation. This process releases heat, which strengthens the storm. As long as the cyclone remains over warm ocean waters, it continues to grow. However, when it makes landfall, it loses its moisture supply and starts to weaken and eventually dissipate.

Think of the storm like a car running on fuel. When the car (cyclone) is on the highway (ocean), it runs fast and strong. But as soon as it reaches a dry road (land), it runs out of fuel and starts to slow down.

A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air. Around the eye is the eye wall, where there is a strong spiralling ascent of air to greater height reaching the tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km per hour. Torrential rain occurs here.

In a fully developed tropical cyclone, the structure includes an 'eye'—a calm center surrounded by the 'eye wall,' which contains the most intense winds and heavy rainfall. The eye can span a significant distance, and the winds can be incredibly powerful, often leading to severe destruction when the cyclone makes landfall.

Picture standing in the eye of a hurricane. Inside, it's oddly peaceful, like standing in a quiet room, but just outside the walls, it’s stormy and chaotic—like being in the middle of a spinning tornado, dramatically different both inside and out.

The cyclone creates storm surges and they inundate the coastal lowlands. The storm peters out on the land.

Tropical cyclones often generate storm surges—large waves caused by the winds pushing water toward the shore. This can lead to flooding of coastal areas. Once the cyclone reaches land, it begins to weaken due to a lack of warm ocean water to fuel it, ultimately dissipating over the land.

Imagine trying to blow up a balloon; as long as you keep blowing into it, it stays inflated (like the cyclone over warm water). But if you stop supplying air (like when it moves onto land), it begins to deflate quickly.

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