What is the Fujiwhara effect? Explain its impact on the movement and intensity of tropical cyclones. (Answer in 150 words)

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फुजिह्वारा प्रभाव क्या है? उष्णकटिबन्धीय चक्रवातों के संचलन तथा तीव्रता पर पड़ने वाले इसके प्रभाव को समझाइए। (उत्तर 150 शब्दों में दीजिए)

The question as printed in the 2026 General Studies paper

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Model answer

Drafted with AI by UPSC Answer Check against this question's marking rubric, to the 150-word length. UPSC does not publish answers for Mains — this is one way to score well, not an official key. How we build rubrics and answers.

The Fujiwhara effect, described by Japanese meteorologist Sakuhei Fujiwhara in 1921, occurs when two nearby tropical cyclones (typically within ~1,400 km) begin to orbit a common centre between them. Each storm's circulation advects the other, producing counterclockwise co-rotation in the Northern Hemisphere.

Impact on Movement: Mutual steering causes erratic deflections from normal tracks. Depending on relative size and distance, the pair may orbit each other, be flung apart, see the smaller storm absorbed, or merge into a single larger vortex.

Impact on Intensity: Proximity disrupts symmetric low-level inflow and upper-level outflow and sets up competition for moisture and ocean heat. The resulting vertical wind shear generally weakens one or both cyclones, though a merger can occasionally produce a larger, stronger system.

Commonly observed in the Pacific (e.g., Hurricanes Hilary and Irwin, 2017), the effect severely complicates track, intensity and landfall forecasting.

How this answer will be evaluated

Framework: Geography: explain the mechanism, then location follows

What earns full marks: Clear definition, accurate mechanism, distinct points on movement and intensity.

Evaluation rubric

Each sub-part is marked on its own, against the marks and word limit printed on the paper.

The answer Define the Fujiwhara effect and explain its impact on both the movement (track) and the intensity of tropical cyclones. 10 marks · 150 words

explain— definition/context → points in order → small example → short close
Must cover
- Definition: interaction of two cyclonic vortices
- Mechanism: mutual rotation around common center
- Impact on movement: altered track/deflection
- Impact on intensity: intensification or weakening
Loses marks
- Describing only the visual appearance without physics
- Confusing with general Coriolis effect
- Ignoring the 'intensity' aspect of the question
Earns more
- Outcomes of binary interaction: mutual orbiting, merger, absorption of the smaller storm, or separation
- Role of relative size and distance: the larger storm dominates the pair
- Consequences for track, intensity and landfall forecasting
Extra mark
- 30-second sketch of two spirals merging
- Specific example (e.g., Hurricanes Hilary and Irwin, 2017)

What "Explain" is asking you to do

Make the working of something clear — what sets it off, what follows from what, and what it produces. Explain is the Commission's mechanism word: it dominates the technical papers and the “explain why” stems, where the marks sit in the causal chain and not in the label.

Structure that answers it

State what it is → the initiating condition → the chain of cause, step by step → an instance where it plays out → what the chain produces

Where marks are lost

Describing what something looks like instead of why it works that way. Naming the stages without linking them reads as description too.

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