Science Puzzle
Why Does the Sea Breeze Turn Around at Night?
Spend a day at the coast and by mid-afternoon a pleasant breeze is blowing in off the sea onto your face. Stay until after dark and the breeze has quietly reversed, now blowing out from the land toward the water.
Why does the coastal breeze turn around at night?
The Answer
Because land and sea change temperature at very different speeds, and the breeze always blows toward whichever surface is warmer.
Sand and rock heat up quickly in sunshine, so by afternoon the land is much hotter than the sea. Hot land warms the air above it, that air becomes lighter and rises, and cooler air from over the water slides in underneath to replace it. That inflow is your sea breeze.
After sunset the land loses its heat just as quickly and, within a few hours, is colder than the sea. Water is stubborn: it takes a great deal of energy to warm it, and it releases that warmth slowly, so the sea is still holding the day's heat. Now the rising air is over the water, and cool air drains off the land to take its place. The breeze has reversed.
The same mechanism, scaled up to a continent and an ocean, drives the monsoon.
The principle: Differential heating and convection. Land warms and cools faster than water, and air flows toward the warmer surface where rising air leaves a gap beneath it.