Cyclone

Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 6, Ch 2 "Oceans and Continents"; Class 7, Ch 3 "Climates of India"

Meaning

A cyclone is an intense low (a small area of very low air pressure) where winds rush in towards the centre and spiral inward. The winds turn anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. A tropical cyclone is the violent kind. It forms over warm tropical oceans and brings destructive winds, very heavy rain and storm surges to the coast.

Cyclones are among India's most deadly natural hazards, especially on the low, flat deltas of Odisha and West Bengal. Recent storms such as Amphan (2020) show that cyclones are becoming harder to forecast as the oceans warm.

Explanation

How a tropical cyclone works: a heat engine

• Fuel: warm sea water
• Warm sea water evaporates, and the moist air rises.

• Rising air cools, so its water vapour condenses into cloud.

• Engine: latent heat

• Condensation gives out latent heat (heat stored in water vapour and released when the vapour turns back into liquid water).
• This heat warms the core of the storm. Warm air rises faster, so more moist air is pulled in from the sea.

• The cycle repeats and keeps the storm going.

• Result: it needs the sea to survive

• The storm grows only while it stays over warm sea.

• After landfall (when the storm reaches the coast), or over cold water, its fuel is cut off and it weakens.

• Size: about 500–1,000 km across and 12–14 km high. That is roughly the full height of the troposphere in the tropics.

• Spin: the Coriolis force (the turning effect of the earth's rotation on moving air) bends winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This sets up the inward spiral.

Conditions needed for formation

All of these must be present at the same time:

• (a) A large, warm sea surface
• The sea must be warmer than about 26.5°C, and this warm water must reach down about 50 m [4]. (NCERT gives above 27°C.)
• Erik Palmén first noted the 26.5°C threshold in 1948 [4].
• Above this temperature, warm moist air rises strongly and builds tall thunderclouds (deep convection). Below it, few thunderstorms form [4].

• The warm layer must be deep. A storm churns up the sea, and if the warm layer is thin, cold water comes up from below and cuts off the energy.

• (b) Enough Coriolis force

• Without the turning, air would flow straight into the low and "fill it up". No spinning storm would form.

• Coriolis force is zero at the equator, so no tropical cyclone forms within about 0–5° of the equator.

• (c) Weak vertical wind shear

• Vertical wind shear is the change in wind speed or direction with height.

• Cyclones fail to form when the shear between the lower and upper troposphere (850–200 hPa) is more than about 20–25 knots [5].

• (d) A "seed": a pre-existing weak low or low-level cyclonic circulation

• Examples are easterly waves, or lows along the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet).

• (e) Upper-level divergence

• Divergence means air spreading out high in the troposphere.

• It carries away the rising air, so surface pressure keeps falling and more air flows in at the bottom.

• (f) An unstable troposphere

• Rising air stays warmer than the air around it, so it keeps rising and forms tall cumulonimbus clouds.

• Beyond NCERT: Gray's (1968) genesis parameters add moist air in the middle troposphere, because dry mid-level air weakens rising clouds. Formation peaks in late summer and autumn, when the seas are warmest.

Wind shear and India's two-peak cyclone season

• Weak shear: winds at all levels move the storm together. Its warm core (the warm central column of air) builds straight up into a deep, upright vortex [5].
• Strong shear: upper winds blow the top of the storm away from its base. The storm tilts and is torn apart, and the heat from condensation no longer sits over the low-level centre [5].
• Why the south-west monsoon (June–September) has few cyclones:
• Near the surface, strong monsoon westerlies blow.
• High up, the Tropical Easterly Jet blows from the east. (A jet stream is a narrow belt of very fast winds high in the troposphere.)
• The opposite directions create strong shear, so most monsoon systems stay as depressions.

Season	Months	Shear	Cyclone activity
Pre-monsoon	April–May	Weak	Peak 1 (e.g. Amphan, May 2020) [7]
South-west monsoon	June–September	Strong	Mostly depressions
Post-monsoon	October–December	Weak	Peak 2 (the main peak in the Bay of Bengal)

Classification and rapid intensification

• IMD scale for the North Indian Ocean (by maximum sustained surface wind speed) [8][9]:

IMD category	Wind (knots)	Wind (km/h)
Depression	17–27	31–49
Deep Depression	28–33	50–61
Cyclonic Storm	34–47	62–88
Severe Cyclonic Storm	48–63	89–117
Very Severe Cyclonic Storm	64–89	118–166
Extremely Severe Cyclonic Storm	90–119	167–221
Super Cyclonic Storm	≥120	≥222

• A disturbance gets a name only when it reaches Cyclonic Storm strength (34 knots or more) [8][9].
• Rapid intensification (RI): maximum sustained winds rise by at least 30 knots (about 55 km/h) in 24 hours (definition of NOAA's National Hurricane Center) [2][3].
• Causes: very warm and deep warm water, weak shear, and a moist middle troposphere.
• Danger: a weak storm can become severe in a single day. This leaves less time for warning and evacuation, and RI is still among the hardest things to forecast.

Where it occurs

• Belt: only between 30°N and 30°S, but not within about 0–5° of the equator.
• Regional names for the same storm:

Name	Region
Cyclones	Indian Ocean (Bay of Bengal, Arabian Sea)
Hurricanes	Atlantic Ocean and eastern Pacific
Typhoons	Western Pacific and South China Sea
Willy-willies	North-western Australia

• India:
• Bay of Bengal: gets more cyclones than the Arabian Sea. Its main peak is October–December.
  • Amphan (16–21 May 2020): the first super cyclone of this century over the Bay of Bengal. It crossed the West Bengal coast across the Sundarbans as a Very Severe Cyclonic Storm [6][7].
  • Storm surges are most dangerous on the low, flat deltas of Odisha and West Bengal.

• Arabian Sea: a growing concern for rapid intensification.

  • Ockhi (2017): in the Lakshadweep Sea. It strengthened quickly close to the Kerala and Tamil Nadu coasts.
  • Tauktae (2021): off India's west coast.

• World: Hurricane Otis (2023) in the eastern Pacific struck Acapulco, Mexico, after rapid intensification that nobody expected.

Don't confuse with

• Hurricane / Typhoon / Willy-willy: these are the same weather system as a tropical cyclone. Only the region is different: Atlantic and eastern Pacific / western Pacific and South China Sea / north-western Australia.
• Anticyclone: a high-pressure system. Its winds spiral outward (clockwise in the Northern Hemisphere) and it brings calm, dry weather. A cyclone is a low that pulls winds inward.
• Depression (IMD): a weaker stage, at 17–27 knots. It is not named. Naming starts only at Cyclonic Storm stage (≥34 knots) [8].
• Temperate (extra-tropical) cyclone: forms in the mid-latitudes along fronts (boundaries where warm and cold air masses meet), and it can form over land. A tropical cyclone forms only over warm tropical seas and runs on latent heat.

Prelims Hooks

• Tropical cyclones form between 30°N and 30°S but not within 0–5° of the equator, because Coriolis force there is almost zero.
• Trap: cyclones spin anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• Match the names: Typhoon = western Pacific and South China Sea; Hurricane = Atlantic and eastern Pacific; Willy-willy = north-western Australia; Cyclone = Indian Ocean.
• Energy comes from the latent heat of condensation, which is why a cyclone weakens after landfall. It needs sea water above 26.5°C down to about 50 m (NCERT: 27°C) [4].
• Strong vertical wind shear blocks cyclones during the south-west monsoon. This gives the North Indian Ocean a two-peak season: April–May and October–December.
• Rapid intensification = a wind rise of ≥30 knots (≈55 km/h) in 24 hours [2]. IMD Super Cyclonic Storm = ≥120 knots (≥222 km/h) [8]. Amphan (May 2020) was the first super cyclone of the 21st century in the Bay of Bengal [6][7].

Mains Points

• Several conditions must come together (warm deep sea, Coriolis force, weak shear, a pre-existing low, upper-level divergence, an unstable atmosphere). This explains:
• where cyclones form: over warm tropical oceans, never at the equator
• when they form: late summer and autumn worldwide; pre- and post-monsoon in India

• why the Bay of Bengal gets more cyclones than the Arabian Sea

• Rapid intensification and climate change (GS-III, disasters):

• Warmer seas with a deeper warm layer make RI more likely (Ockhi 2017, Tauktae 2021, Amphan 2020).

• RI cuts warning time. India needs better ocean observation (buoys, Argo floats), satellite microwave monitoring [6] and impact-based warnings.

• Disaster management:

• The IMD's graded classification [8] sets off graded warnings to coastal states.
• Storm surges flood the low, flat deltas of Odisha and West Bengal (Sundarbans), so timely evacuation and cyclone shelters save the most lives.

Related concepts

• Tropical cyclone
• Conditions for tropical cyclone formation
• Vertical wind shear
• Rapid intensification

Read more

• Tropical cyclones: what they are and how they form