Rapid intensification

Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Beyond NCERT

Meaning

Rapid intensification (RI) is when a tropical cyclone's maximum sustained winds rise by at least 30 knots (about 55 km/h) within 24 hours. This is the definition used by NOAA's National Hurricane Center [1][2].

It matters because a weak storm can become a severe one in a single day. People then get much less time to be warned and to leave their homes. RI is also still one of the hardest things to forecast.

Explanation

What RI measures

• Maximum sustained winds means the highest steady wind speed in the storm, not short gusts.
• RI measures how fast the storm grows. It is not a storm category.
• A storm can go through RI and still be below the top category.

• The test is only this: did the wind rise by ≥30 knots in 24 hours? [1]

• How it compares with the IMD scale (for the North Indian Ocean) [7][8]:

• Cyclonic Storm: 34–47 knots
• Severe Cyclonic Storm: 48–63 knots
• Very Severe Cyclonic Storm: 64–89 knots
• A jump of 30 knots in one day can push a storm up one or two categories. For example, it could go from Cyclonic Storm to Very Severe Cyclonic Storm.

How it works: the cyclone as a heat engine

• Fuel: warm sea water evaporates. The moist air rises and cools. Its water vapour condenses into cloud.
• Energy release: condensation gives out latent heat (heat stored in water vapour and released when it turns back into liquid water).
• The engine speeds up:
• Latent heat warms the storm's warm core (the warm central column of air).
• Warm air rises faster, so more moist air is pulled in from the sea.

• Surface pressure falls further, and the winds get stronger.

• RI happens when every part of this engine works at full power together. The storm then strengthens much faster than usual.

Conditions that favour RI

• Very warm and deep warm water
• A cyclone needs a sea warmer than about 26.5°C down to about 50 m [3].
• A storm churns up the sea. If the warm layer is thin, cold water from below comes up and cuts off the fuel.

• A deep warm layer keeps the fuel coming, even under a strong storm.

• Weak vertical wind shear

• Vertical wind shear is the change in wind speed or direction with height.
• When shear is weak, the warm core builds straight up, and the storm grows as one upright column [4].

• When shear is strong, the storm tilts and is torn apart [4].

• A moist middle troposphere

• The troposphere is the lowest layer of the atmosphere, where weather happens.
• Dry air at middle levels weakens rising clouds. Moist air lets them grow tall.

Early signs seen from space

• IMD scientists studied Amphan's RI using satellite images [5].
• In 37 GHz microwave satellite images, rings of convection showed up before the RI began. Convection means warm, moist air rising strongly to form thunderclouds.
• Signs like these can help forecasters predict RI earlier [5].

Where it occurs

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

• Because of these storms, the Arabian Sea is a growing concern.

• Bay of Bengal (India's east coast):

• Amphan (2020): the first super cyclone of this century over the Bay of Bengal [5].

• Its life ran from 16–21 May 2020. It crossed the West Bengal coast across the Sundarbans as a Very Severe Cyclonic Storm [6].

• Eastern Pacific (world):

• Hurricane Otis (2023) struck Acapulco, Mexico, after an RI that forecasters did not expect.

• When it happens in India: RI needs weak shear, so it fits the two weak-shear cyclone seasons:

• April–May, for example Amphan in May 2020 [6]
• October–December
• During the south-west monsoon (June–September), strong shear between the low-level monsoon westerlies and the high Tropical Easterly Jet works against strong cyclones.

Don't confuse with

• Cyclogenesis (cyclone formation): this is the birth of a cyclone from a weak low. RI happens to a storm that already exists, when it strengthens very quickly.
• Super Cyclonic Storm: this is an IMD category fixed by wind speed (≥120 knots, ≥222 km/h) [7]. RI is a rate of strengthening (≥30 knots in 24 hours) [1]. A storm can go through RI without ever becoming a super cyclone.
• Naming of a cyclone: a storm gets a name when it reaches Cyclonic Storm strength (≥34 knots) [7]. This is a fixed limit, not a measure of speed of change, so do not mix up 34 knots (naming) with 30 knots (RI).
• Weakening after landfall: this is the opposite process. Over land, the supply of moist air and latent heat is cut off, so winds drop.

Prelims Hooks

• Rapid intensification = a rise in maximum sustained winds of ≥30 knots (≈55 km/h) in 24 hours (NOAA National Hurricane Center definition) [1].
• Favourable conditions: warm, deep warm water + weak vertical wind shear + moist middle troposphere. Strong shear stops RI.
• Ockhi (2017): Lakshadweep Sea, near the Kerala and Tamil Nadu coasts. Tauktae (2021): Arabian Sea. Amphan (2020): Bay of Bengal. Otis (2023): eastern Pacific, struck Acapulco, Mexico.
• Amphan (May 2020): first super cyclone of the 21st century in the Bay of Bengal. It crossed the West Bengal coast across the Sundarbans [5][6].
• Trap: the numbers are close. 30 knots/24 h = RI [1]. 34 knots = named Cyclonic Storm [7]. ≥120 knots = Super Cyclonic Storm [7].
• Before Amphan's RI, rings of convection appeared in 37 GHz microwave satellite images [5].

Mains Points

• Climate change and RI (GS-III, disasters):
• As oceans warm and the warm layer gets deeper, RI is expected to become more frequent.

• The Arabian Sea, once seen as calmer than the Bay of Bengal, is now a growing concern (Ockhi 2017, Tauktae 2021).

• The warning-time problem:

• RI shrinks the time between a warning and landfall. A storm forecast as weak can hit as a severe one (Ockhi, Otis).

• Responses: better ocean observations (buoys, Argo floats) to measure how deep the warm water goes, satellite microwave monitoring to spot early signs [5], and impact-based warnings (warnings that say what damage to expect, not just the wind speed).

• Preparedness on vulnerable coasts:

• IMD's graded classification sets off graded warnings to coastal states [7].
• On low, flat deltas such as the Sundarbans (West Bengal) and coastal Odisha, storm surge makes quick evacuation and cyclone shelters essential, especially when RI cuts lead time.

Related concepts

• Tropical cyclone
• Cyclone
• Conditions for tropical cyclone formation
• Vertical wind shear

Read more

• Tropical cyclones: what they are and how they form