Describe the causes of origin, structure and weather associated with tropical cyclones.

Causes of origin

• Sea-surface temperature of about 26.5°C or more through a deep warm layer supplies latent heat when water evaporates and condenses in towering cumulonimbus.
• Sufficient Coriolis force, hence origin usually beyond about 5° latitude, allows rotation; storms rarely form on the equator.
• A pre-existing low, often along the Inter-Tropical Convergence Zone, and weak vertical wind shear let the column organise instead of being torn apart.
• High mid-tropospheric humidity and upper-level outflow complete the heat engine; dry air or strong shear over parts of the Arabian Sea in some seasons suppresses storms.
• In the North Indian Ocean the season is mainly May–June and October–November, when the monsoon trough and post-monsoon warmth coincide; the Bay of Bengal breeds more storms than the Arabian Sea because it is warmer, more humid, and more enclosed.

Structure

• At the centre is the eye: relatively calm, often clear, with slow subsidence and the lowest pressure.
• The eyewall is a ring of the strongest winds and heaviest rain; the storm’s violence is here, not in the geometric centre.
• Spiral rainbands feed inward; low-level inflow is cyclonic, upper outflow is anticyclonic in a mature system.
• The whole system is a warm core: temperatures in the centre are higher than the surroundings at the same height, which distinguishes it from a cold-core extra-tropical cyclone.
• Size varies from a compact severe cyclone to a giant; IMD classifies by sustained wind from depression through super cyclone.

Associated weather

• Destructive winds flatten trees and kutcha houses; the right-forward sector relative to motion is often the worst in the Northern Hemisphere.
• Torrential rain causes riverine and urban floods far inland, as in 2018 Kerala rains after a storm remnant or in Odisha landfalls.
• Storm surge—a wind-driven pile of sea—kills most on low coasts such as the Sundarbands, Odisha, and Andhra; this is not a tsunami.
• After landfall the heat source is cut, the system fills, and weather becomes a rain-depression; mountains can still wring extreme rain.
• India names storms and uses IMD cone-of-uncertainty warnings; coastal Uttar Pradesh is not a prime landfall belt, but Bay storms still modulate monsoon moisture for the state.