Lesson 10 of 13 · 6 min
Convection
NCERT §10.9.2
Back in Chennai before the trip, Tara noticed the afternoon wind always came off the sea, cool and salty. Late at night, on the terrace, the air drifted the other way.
The lesson in notes
In short
Convection carries heat by the actual movement of matter. It can happen only in fluids.
Natural convection is driven by gravity: fluid heated from below expands, becomes less dense, and is pushed up by buoyancy; colder, denser fluid sinks to replace it. The circulation repeats and carries heat upward.
Forced convection moves the fluid with a pump or fan. Examples: forced-air heating in homes, the human circulatory system with the heart as the pump, and the cooling system of a car engine.
The body's blood circulation is forced convection that helps keep body temperature steady.
Sea breeze by day: land heats faster than water, the air above land warms and rises, and cooler air from the sea moves in to take its place; the loop closes aloft.
Land breeze at night: land loses heat faster, the water surface is now warmer, and the whole cycle reverses.
Trade wind: steady surface wind blowing from the north-east towards the equator. Near the equator surface air is hot and rises, while polar air aloft is cool.
Earth's rotation changes the simple loop. Air near the equator moves eastward at about 1600 km/h while air near the poles has almost no such speed, so the air descends at about 30° N rather than at the poles and returns to the equator.