Land and sea breezes

Hot sand, cool water

On a winter holiday in Kerala, Palden noticed something at the beach. During the day the sand felt hotter than the sea water, but at night the sand was cooler than the water. Different things heat up and cool down at different rates, and near the coast this makes the wind change direction every day.

Soil and water in the sun

Activity 7.4 checks this. Two identical bowls are filled halfway, one with soil and one with water, and a thermometer is placed in each, with its bulb inside the soil or water but not touching the bowl. The set-up is kept in sunlight, and the temperatures are noted every 5 minutes.

Time	What the thermometers show (sample)
0 minutes	soil 28 °C, water 28 °C
10 minutes	soil 34 °C, water 30 °C
20 minutes	soil 38 °C, water 32 °C

The soil’s temperature rises more than the water’s in the same time, so soil heats up faster than water. When both bowls are brought indoors, the soil also cools down faster. (Your readings will depend on the day; the pattern stays the same.)

The sea breeze by day

In the daytime, the land near the sea heats up faster than the water. The air above the hot land warms up, expands, becomes lighter and rises. Cooler air from over the sea moves in to take its place. This cool wind from the sea to the land is a sea breeze. It brings relief on hot days, which is why houses in coastal areas often have windows facing the sea.

The land breeze at night

After sunset, the process reverses. With no sunlight, the land cools down faster than the sea. Now the air above the sea is the warmer air, so it rises, and cooler air from the land moves out to take its place. This cool wind from the land to the sea is a land breeze. People living near the seashore notice that the wind blows one way by day and the other way at night.

A daily loop of air

Both breezes are convection at work: warm air rises, cooler air sinks and flows in sideways near the ground, and higher up the air flows back to close the loop. The only thing that changes between day and night is which side, land or sea, is warmer.

Time	What happens
Day	land is warmer; air rises over the land; sea breeze from sea to land
Night	sea is warmer; air rises over the sea; land breeze from land to sea

Why the coast has milder weather

Because water heats and cools slowly, large water bodies act like a store of warmth. During the day the sea stays cooler than the land and its breeze cools the coast; at night the sea stays warmer and keeps the air near it from getting very cold. So places near the sea, such as coastal Kerala, have milder temperatures than places far inland, where days can be much hotter and nights much colder.

People and the breezes (Extra)

Sea and land breezes have long shaped coastal life. Traditionally, fishermen with sail boats have set out early, helped by the land breeze blowing towards the sea, and come back later in the day with the sea breeze blowing towards the land.

Read about all three ways heat travels in conduction, convection and radiation. Answers to the textbook questions are in the NCERT solutions for Chapter 7; see all of Chapter 7 notes.