GS Paper I 2026 · Q4
10 marks · 150 words Geophysical phenomena & changing features X-Ray · Core #43What is the Fujiwhara effect? Explain its impact on the movement and intensity of tropical cyclones.
🧭 How to think about this question · directive: “what is / explain”
Decode the demand
- Two directives: 'what is' asks for a tight definition with the threshold distance and the sense of rotation; 'explain its impact' asks for cause-and-effect on two things UPSC names: movement and intensity.
- The phenomenon in one line: two cyclones close enough to feel each other's winds orbit a common centre; the stronger steers, the weaker pays.
- Three things to answer: the definition with its mechanism, what happens to the tracks, what happens to the strength (merger or decay).
- The trap: writing only 'they dance around each other'. The examiner wants the distance threshold, the hemisphere rule, the forecasting problem and one documented storm pair.
Think in a structure
- Open with the definition and the scientist's name and date, then a line that links track and strength to one rule.
- Three headings in the question's words, then a forecaster's heading so the answer ends on action, not uncertainty.
- Visuals: a flowchart of the interaction from approach to outcome, and a small table of the three intensity outcomes with their conditions, both drawable in thirty seconds.
- Anchor the close on an early-warning goal, because the whole value of knowing this effect lies in better forecasts.
Facts checked at source (7)· open
- Hong Kong Observatory: two tropical cyclones begin to affect each other at a separation of about 12 degrees of latitude (about 1,350 km); in the northern hemisphere they rotate anticlockwise about their common centre, and the stronger dominates the track of the weaker.What is Fujiwhara Effect? — Hong Kong Observatory (educational resource) ↗“two tropical cyclones may start to affect each other when the distance between them is around 12 latitude degrees (about 1350 kilometers)”
- The effect is named after Dr Fujiwhara of Japan, who experimented with water vortices from 1921 to 1923.What is Fujiwhara Effect? — Hong Kong Observatory (educational resource) ↗“Dr. Fujiwhara of Japan who performed a series of experiments and observations on water vortices from 1921 to 1923”
- In a Fujiwhara pair, the stronger cyclone dominates the track of the weaker.What is Fujiwhara Effect? — Hong Kong Observatory (educational resource) ↗“The stronger cyclone tends to have a dominant effect on the track of the weaker one.”
- Hong Kong Observatory research: merger is unlikely when two cyclones are 1,200 km or more apart; the interaction ends when an outside system intervenes, one weakens or they merge.What is Fujiwhara Effect? — Hong Kong Observatory (educational resource) ↗“Research by the Observatory shows that if the distance between the two tropical cyclones is 1200 kilometers or more, the chance of merging is slim.”
- NHC: Hurricane Hilary's outflow raised southerly shear over Irwin from 26 July 2017, rapidly weakening it, and affected Irwin's track and intensity for several days.Tropical Cyclone Report: Hurricane Irwin (EP102017) — US National Hurricane Center, NOAA ↗“Hurricane Hilary had strengthened, and its outflow began to negatively affect Irwin on 26 July. Southerly wind shear quickly increased, causing Irwin to rapidly weaken”
- The UN Early Warnings for All initiative aims to protect everyone on Earth through early warning systems by the end of 2027.Early Warnings for All — World Meteorological Organization ↗“everyone on Earth is protected from hazardous weather, water, or climate events through life-saving early warning systems by the end of 2027”
- The US National Weather Service defines the Fujiwhara effect as tropical cyclones within 300–750 nautical miles of each other, depending on their sizes, beginning to rotate about a common midpoint.NWS Glossary: Fujiwhara effect — US National Weather Service, NOAA ↗“of each other begin to rotate about a common midpoint”
Question: UPSC's CS (Main) 2026, GS Paper I — paper ↗. Guidance: Minimalist IAS, checked 8 Nov 2026 (how we verify). ·
Model answer· 374words · Minimalist IAS
The Fujiwhara effect is the binary interaction of two tropical cyclones close enough to rotate about a common centre; Japan's Dr Sakuhei Fujiwhara showed it in water-vortex experiments of 1921–23. Its mark on track and strength follows one rule: the stronger steers, the weaker pays.
What the Fujiwhara effect is
- Threshold: interaction begins at about 12 degrees of latitude (~1,350 km) apart (Hong Kong Observatory).
- US National Weather Service: 300–750 nautical miles, depending on the cyclones' sizes.
- Mechanism: each vortex is carried along by the other's wind field, so the two orbit a common centre.
- Sense of rotation: anticlockwise in the northern hemisphere, clockwise in the southern, mirroring the cyclones' own spin.
- Two cyclones within ~1,350 km
- Orbit common centre
- Stronger steers weaker
- Merge, decay or separate
Impact on movement
- Dominance: the stronger cyclone dominates the weaker one's track, swinging it around or flinging it away.
- Erratic tracks: loops, sudden turns and stalls replace the smooth parabolic path forecasters expect.
- Landfall uncertainty: a track shift of a few hundred kilometres changes the coastline at risk and the warning area.
Impact on intensity
| Outcome | What happens | Condition |
|---|---|---|
| Merger | One larger, longer-lived system | Closer than ~1,200 km |
| Decay | Weaker storm sheared by the stronger's outflow | Large intensity gap |
| Separation | Pair pulled apart, each continues | Outside steering system intervenes |
- Merger: closing in, the two may combine into a larger, longer-lived system; at 1,200 km or more apart, merger is unlikely.
- Weakening: the dominant cyclone's outflow raises wind shear over the weaker one and starves it of inflow.
- July 2017: Hurricane Hilary's outflow raised southerly shear over Irwin, which weakened rapidly (NHC).
- End of interaction: an outside large-scale system takes over, one cyclone weakens, or the two merge.
What forecasters and disaster managers can do
- Ensemble modelling: run many model members so the pair's orbit, not a single track, shapes the forecast.
- Wider warning cones: issue a broader coastal alert the moment binary interaction is detected.
- Joint basin watch: IMD tracks Bay of Bengal and Arabian Sea systems together as the regional centre for the North Indian Ocean.
Modelling such binary interactions sharpens track and intensity forecasts, which serves the UN's Early Warnings for All goal of protecting everyone through early warning systems by the end of 2027.
Prepared by Minimalist IAS to the answer standard of Shivam Singh (mentor, 7 years) · facts checked against official sources (how we verify) · updated 8 Nov 2026. UPSC publishes no model answers: compare your structure and coverage with this, then write your own.
