Revision summary
Cyclones need SST near 26.5 °C and deep moisture to release latent heat. Coriolis and low shear organise a warm-core vortex over tropical seas. Land cuts the vapour supply and adds friction that fills the eye. Dry wrap-around air and hills speed decay of the warm core. Remnants can still flood inland without remaining a cyclone.
Model answer
Introduction
A tropical cyclone is a warm-core storm that lives on latent heat released when moist air rises over a hot sea. The statement in the question is the energy budget of that storm: the ocean is the furnace; the land is where the furnace is unplugged.
Body
Why they start at sea
- Sea-surface temperatures of about 26.5 °C or more, through a sufficient depth, supply water vapour; condensation in towering cumulonimbus towers releases latent heat that lowers surface pressure and organises a vortex.
- The Coriolis force, negligible within about 5° of the equator, is needed to spin the inflow; hence Bay of Bengal and Arabian Sea cyclones form over tropical ocean, not on the equator itself and not over a dry continent.
- Low vertical wind shear lets the warm core stay stacked; a pre-existing low, the monsoon trough, or an easterly wave can provide the seed, but the heat engine remains maritime.
- High ocean heat content in the post-monsoon Bay of Bengal explains why Odisha–Andhra–West Bengal coasts see intense cyclones (Phailin 2013, Fani 2019) while a desert interior cannot start one.
Why they die inland
- Over land the storm loses its vapour supply; rainfall may continue, but the latent-heat furnace weakens as soil and vegetation cannot match a 26.5 °C mixed layer of sea.
- Surface friction over rough land fills the eye, disrupts the eyewall, and cuts inflow organisation; the pressure gradient collapses from the outside in.
- Cooler land, orographic blocking, and dry continental air wrap in and erode the warm core; remnants may still flood the Ganga plain as a low, which is rain without cyclone structure.
- Exception language: a cyclone that stays over the sea or skirts a wet delta (Sundarbans) decays slower than one that hits the Eastern Ghats or the dry Deccan; the rule in the question still holds as the first-order energy story.
Flow diagram
flowchart TD O[Warm SST vapour] --> L[Latent heat low pressure] C[Coriolis low shear] --> V[Marine vortex] L --> V Land[Land friction dry air] --> D[Eyewall collapse] V --> D
Conclusion
Tropical cyclones are ocean heat engines organised by Coriolis and low shear. On land they lose moisture and gain friction, so the vortex fills and dies, even if leftover rain still floods far inland.
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