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Focus: GS-I Geography, GS-III Disaster Management

Why in news?

  • Philippine officials ordered evacuation of thousands of residents in the southern part of the main Luzon island as a category 5 storm – Typhoon Goni – that is the world’s strongest in 2020 approaches the Southeast Asian nation.
  • Recently Typhoon Molave in October 2020 killed 22 people, mostly through drowning in provinces south of the capital Manila, which is also in the projected path of Goni, the 18th tropical storm in the country.


  • Typhoon Goni, with 215 kph sustained winds and gusts of up to 265 kph will make landfall as the strongest typhoon to hit the Philippines since Haiyan that killed more than 6,300 people in November 2013.
  • Pre-emptive evacuations have started in coastal and landslide-prone communities.
  • Authorities are facing another hurdle as social distancing needs to be imposed in evacuation centres to prevent the spread of coronavirus. Typhoon Goni, moving westward from the Pacific Ocean, will bring intense rains over the capital of Philippines (Manila) and 14 provinces nearby, threatening of floods and landslides.
  • Another typhoon, Atsani, is gaining strength just outside the Philippines.
  • Around 20 typhoons hit the Philippines every year.

Tropical cyclone

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls.

Depending on its location and strength, a tropical cyclone is referred to by different names:

  1. Cyclones in the Indian Ocean
  2. Hurricanes in the Atlantic
  3. Typhoons in the Western Pacific and the South China Sea
  4. Willy-willies in Western Australia

“Tropical” refers to the geographical origin of these systems, which form almost exclusively over tropical seas.

  • “Cyclone” refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
  • The opposite direction of circulation is due to the Coriolis effect.

How are Cyclones Formed?

  1. Tropical cyclones typically form over large bodies of relatively warm water. Warm water > Evaporation > Rising up of air > Low Pressure area.
  2. They derive their energy through the evaporation of water from the ocean surface, which ultimately re-condenses into clouds and rain when moist air rises and cools to saturation.
  3. Water takes up heat from the atmosphere to change into vapour.
  4. When water vapour changes back to liquid form as raindrops, this heat is released to the atmosphere.
  5. The heat released to the atmosphere warms the air around.
  6. The air tends to rise and causes a drop in the pressure.
  7. More air rushes to the centre of the storm.
  8. This cycle is repeated.

Why are Cyclones formed in a spiralling pattern?

  • Simple answer is: Coriolis Effect.
  • The Coriolis Effect causes any moving body on the rotating Earth to turn to the Right (clockwise) in the Northern Hemisphere and to the Left (counter clockwise) in the Southern Hemisphere.
  • In Low-Pressure zones where air ascends, like in case of a cyclone, winds blowing from areas of high pressure inside towards areas of low pressure, are Deflected to the Right in Northern Hemisphere and to the Left in the Southern Hemisphere.
  • Since they are deflected, they don’t arrive straight into the low-pressure zone, but instead end up circling around the low-pressure zone.

What is a Low-pressure area?

  • A low-pressure area, low area or low is a region on the topographic map where the air pressure is lower than that of surrounding locations.
  • Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere.
  • The formation process of a low-pressure area is known as cyclogenesis.
  • Within the field of meteorology, atmospheric divergence aloft occurs in two areas.
  • The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere).
  • A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength.
  • Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

How are Low-Pressure Areas formed?

  • Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses.
  • Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth’s surface.
  • Large-scale thermal lows over continents help drive monsoon circulations.
  • Low-pressure areas can also form due to organized thunderstorm activity over warm water.

Structure of a Cyclone

  • A cyclone’s center, known in a mature tropical cyclone as the eye, is the area of lowest atmospheric pressure in the region.
  • Near the center, the pressure gradient force and the force from the Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure.

The eye

  • The eye of the storm is the centre. It’s a relatively calm space.
  • When the eye passes over an area, winds slow down and everything feels like it has cleared up.
  • But this is the proverbial calm before the storm, as the part that comes after the eye usually inflicts the most damage.

The eyewall

  • This is where the most effective part of a cyclone rests.
  • The eyewall houses extremely high wind speeds, causing damage to both lives and property.
  • It is a ring of thunderstorms, and changes in the eye or the eyewall affects the storm’s intensity.


  • These are the outer parts of a cyclone where sudden bursts of rain happen.
  • There can also be gaps betwen rainbands where no rain or wind occurs.

Landfall, what happens when a Cyclone reaches land from the ocean?

  • Tropical cyclones dissipate when they can no longer extract sufficient energy from warm ocean water.
  • A storm that moves over land will abruptly lose its fuel source and quickly lose intensity.
  • A tropical cyclone can contribute to its own demise by stirring up deeper, cooler ocean waters. tropical cyclone can contribute to its own demise by stirring up deeper, cooler ocean waters.

-Source: The Hindu

December 2023