Why the Sea Breeze Reverses After Dark

Because land heats up and cools down faster than water. During the day the land becomes hotter than the sea, and at night it becomes colder — and the wind blows towards whichever side is warmer, because warm air rises and cooler air moves in to replace it.

This page covers everything in the CBSE Class 7 Science chapter's second half: sea and land breezes, how the Sun drives the water cycle, what happens to rain after it lands, and traditional practices that conserve water.

Both are convection currents on a very large scale.

A sea breeze blows during the day. The land heats faster than the sea, so the air above the land becomes hot, expands and rises. Cooler air from over the sea moves in to take its place, and that incoming wind is the sea breeze — blowing from sea to land.

A land breeze blows at night. Now the land cools faster than the sea, so the sea is warmer. Air above the sea rises, and cooler air from the land moves out to replace it — a wind blowing from land to sea.

For example, people in coastal cities such as Chennai or Mumbai feel a cool wind from the sea in the afternoon, and a breeze in the opposite direction late at night.

The name always describes where the wind comes from, not where it goes — which is why a sea breeze cools the land, and reversing that in an answer loses the mark.

The water cycle is powered entirely by heat from the Sun, and runs through four steps.

Evaporation: heat from the Sun turns water in seas, rivers, lakes and ponds into water vapour, which rises into the air.

Transpiration: plants release water vapour from their leaves, adding to the moisture in the air.

Condensation: as the vapour rises it reaches cooler air, and turns back into tiny droplets that gather as clouds.

Precipitation: the droplets join, grow heavy and fall as rain, and in cold regions as snow or hail.

The water then returns to rivers and seas, and the cycle repeats endlessly.

For example, wet clothes drying on a line is evaporation happening in front of you, and water droplets forming on the outside of a cold bottle is condensation.

Notice that both evaporation and transpiration feed the same step. Forests therefore contribute a great deal of moisture to the air, which is one reason cutting them affects local rainfall.

Rain water takes one of two paths, and the split matters enormously for water supply.

Run-off is water that flows over the surface into drains, streams and rivers, and eventually back to the sea. It leaves quickly and is largely lost to local use.

Infiltration is water that soaks down into the soil. Some is taken up by plant roots, and the rest keeps seeping down until it is stored underground as groundwater, held in water-bearing layers of rock called aquifers. This is the water that wells, borewells and handpumps draw on.

How much infiltrates depends on the ground. Water passes fastest through gravel, more slowly through sand, and slowest through clay, whose particles are packed so tightly that little can pass. Concrete and tar allow almost none at all.

So a city paved over with concrete sends far more water to run-off and far less to groundwater — which is exactly why urban areas both flood in heavy rain and run short of water later.

Both store water at a time when it is plentiful, for a time when it is not.

An ice stupa is used in Ladakh, a cold desert where streams freeze in winter and farmers need water in spring, before the glaciers melt. Stream water is piped downhill and sprayed into the freezing air, where it freezes into a tall cone of ice. The cone's shape is the clever part: a cone has a small surface area for its volume, and it shades its own base, so it melts slowly and steadily through spring exactly when fields need watering.

Rainwater harvesting collects rain from rooftops and open ground and either stores it in tanks or directs it into pits and recharge wells so it can infiltrate and refill the groundwater rather than running off.

For example, a house with a rooftop pipe leading into a recharge pit is putting monsoon water back into the aquifer that its own borewell later draws from.

Both practices work with heat transfer rather than against it — the stupa uses slow melting, harvesting uses infiltration before evaporation can take the water back.