S4.14.3 Chapter 3

Winds, monsoons and storms

Hana lives on a coast. In the afternoon a cool wind blows in from the sea, and at night the wind blows out to sea. Why?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The sea breathes in and out with the tides, and pushes the air with it.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The wind always blows from the land to the sea, and in the afternoon it only feels stronger.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The Moon pulls the air towards the sea at night and releases it by day.
You picked C. Read on. At the end, the chapter comes back to your guess.
D By day the land warms more than the sea, so air rises over the land and cool air flows in. At night the pattern turns round.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain land and sea breezes using convection
Explain how monsoons and tropical storms form

Last chapter you saw that wind blows from high pressure to low pressure. Why does the pressure differ from place to place? A large part of the answer is that the Sun warms the Earth unevenly. Warm air and cool air behave in different ways.

Warm air expands, so it is less dense than the cool air round it. It floats upward. Cool air is denser and sinks. This movement is convection. Where air rises, the pressure at the ground is lower. Cool air flows in along the ground to take its place.

Land and sea warm at very different speeds. Water has a specific heat capacity of 4 200 J/kg °C. Dry soil has about 800 J/kg °C. The same sunshine warms the land about five times as much as the same mass of sea water. Water also mixes, so the heat is spread down into the sea, while on land it stays near the surface.

Worked example

The same 84 000 J of heat goes into 10 kg of water and into 10 kg of dry soil. By how many °C does each warm up? Use 4 200 J/kg °C for water and 800 J/kg °C for soil.

Use the rise in temperature = energy ÷ (mass × specific heat capacity).
Water: 84 000 ÷ (10 × 4 200) = 84 000 ÷ 42 000 = 2 °C.
Soil: 84 000 ÷ (10 × 800) = 84 000 ÷ 8 000 = 10.5 °C.

Answer: water 2 °C, soil 10.5 °C

The same heat warms the soil about five times as much as the water.

Now put land beside the sea on a sunny day. The land warms up faster than the sea. The air over the land warms and rises. The pressure near the ground falls over the land. The cooler, heavier air over the sea flows in to fill the gap. Higher up, the air that rose spreads out and flows back towards the sea, where it sinks. The air goes round a loop.

Draw it first

A sea breeze, step by step

1 In the sun, the land warms faster than the sea. Here it is 30 °C against 26 °C.
2 Air over the warm land warms, expands and rises.
3 Cooler air from over the sea flows in along the ground to take its place. This wind is named for where it comes from: a sea breeze.
4 The rising air flows back out over the sea at height, and sinks there. The air goes round a loop.

The sea breeze starts in the morning, as the land warms. It is often strongest in the afternoon, and it can carry cool air some tens of kilometres inland. It is why many coasts are cooler than the land behind them on hot days.

At night the Sun is gone and the land loses heat quickly. The sea cools slowly, so the sea becomes the warmer side. The loop turns round. Air rises over the sea, and cool air flows out from the land along the ground. This is the land breeze. It is usually gentler than the sea breeze.

The land breeze at night. The sea is warmer, so air rises over it and the wind blows out from the land.

See it three ways

As a picture

In words

The wind along the ground blows from the cooler side to the warmer side. A breeze is named for the place it comes from.

In symbols

Day: land warmer → sea breeze. Night: sea warmer → land breeze.

Now think much bigger. A monsoon is a wind that changes direction with the season. The word comes from an Arabic word for season. Monsoons are found in several parts of the world, such as South Asia, East Asia, West Africa and northern Australia.

A monsoon is a sea breeze and a land breeze on a huge scale, and over months, not hours. In the warm season, the Sun is high over a large land mass. The land heats and a low forms over it. Moist air flows in from the warm ocean, rises, cools and drops heavy rain. This is the wet season.

The wet season. The land is hot, with low pressure. Moist air flows in from the sea and rain falls on the land.

In the cool season the land loses heat and becomes cooler than the sea. A high forms over the land, and a low over the sea. The wind blows out from the land to the sea. It is dry, because it comes from the land. This is the dry season.

The dry season. The land is cooler than the sea, with high pressure. Dry air flows out to the sea.

The monsoon is helped by the tilt of the Earth. As the seasons change, the place where the Sun is overhead moves north or south of the equator. The belt of heaviest rain moves with it. Many farmers plant when the rains come. A late or weak monsoon can bring drought. A very strong one can bring floods.

Last, the most violent winds on Earth. A tropical storm is a huge spinning storm that forms over warm sea in the tropics. Its strongest kind is called a hurricane, a typhoon or a cyclone, according to the sea where it forms. They are the same kind of storm.

One kind of storm, three names. The south Pacific also has cyclones, and the east Pacific has hurricanes.

A tropical storm needs three things. First, a sea warmer than about 26 to 27 °C, down to about 50 m. Second, a place at least about 5° of latitude from the equator, so that the Earth’s spin can turn the winds. Third, winds that stay much the same from low to high, so the storm can build a tall tower.

Here is how it grows. Warm sea water evaporates, and warm, moist air rises. As it rises it cools, and the vapour condenses into cloud. Condensing gives out heat, which warms the air and makes it rise even faster. The pressure at the sea falls, more air flows in, and the whole thing spins.

A tropical storm from above, in the northern hemisphere. In the southern hemisphere it spins clockwise.

The middle of the storm is the eye. It is often 30 to 60 km wide, and it is calm and nearly clear, because air sinks there. Round it is the eyewall, a tower of cloud where the wind is strongest and the rain is heaviest.

A storm cut through the middle. Air spirals in over the warm sea, rises in the eyewall, spreads out at the top, and sinks in the eye.

The storm feeds on warm, moist air. Over land or cold water, it loses its fuel and weakens. A storm is called a hurricane, typhoon or cyclone when its steady wind passes about 120 km/h. It can be several hundred km across.

The greatest danger is often the storm surge. The wind and the low pressure raise the sea level, sometimes by several metres, and the sea floods the coast. Heavy rain also causes floods and landslides. When a warning is given, move to the shelter that the authorities name.

Try it

Watch the wind change through the day

You need: the wind vane from the last chapter, or a strip of light cloth tied to a stick, paper and a pencil, a place in the open.

Be safe: Do not go out in a storm to read the vane. Stay away from deep water and cliffs, and do not stand under a tree when there is lightning.

Put the vane or the cloth in the open, away from walls. Mark north.
On a calm sunny day, note the wind direction in the morning, at midday, in the late afternoon and after dark. Write the direction the wind comes from.
Repeat for several days. If you live by the sea or a large lake, note the direction against the water.
Look for a pattern: does the wind come from the water in the afternoon and from the land at night?

What you will see: Near a coast or a large lake, the wind often comes from the water in the afternoon. At night it often comes from the land. Inland, far from water, you may see little pattern, because the breeze does not reach so far.

If you cannot try it, this is what happens.

A wrong idea many people have

By day the sea is warmer than the land, so the air rises over the sea.

Why it feels right: Water holds a great deal of heat, and it can feel warm to swim in, so it seems warmer than the ground.

The test that breaks it: Dry soil has a specific heat capacity of 800 J/kg °C and water has 4 200 J/kg °C. In the worked example the same heat raised the soil by 10.5 °C and the water by 2 °C. In the sun the land gets warmer than the water.

What is true: By day the land is the warmer side, so air rises over the land and a sea breeze blows in. At night it turns round, because the sea holds its heat longer.

A wrong idea many people have

A monsoon is a heavy rainstorm.

Why it feels right: On the news, the word monsoon is used when there is heavy rain, so it seems to mean the rain.

The test that breaks it: The word means season. A monsoon is a seasonal wind that changes its direction. It brings a wet season, when the wind blows from the sea, and a dry season, when it blows from the land.

What is true: A monsoon is a seasonal change in wind. The heavy rain belongs to the wet part of the cycle, and the dry season belongs to it too.

A wrong idea many people have

The eye is the stormiest part of a tropical storm.

Why it feels right: It is the centre, so it seems it must hold the most violent weather.

The test that breaks it: Look at the cross section. In the eye the air sinks, and sinking air clears cloud. The air rises in the eyewall, round the eye, where the cloud is tallest and the wind is strongest.

What is true: The eye is calm and often nearly clear. The eyewall round it has the strongest winds and the heaviest rain. As the eye passes, the wind drops, and then it returns from the other direction.

Key words

convection: the movement of heat by a fluid: warm air rises and cool air sinks.
land breeze: a night wind that blows from the land out to the sea.
monsoon: a wind that changes direction with the season, with a wet season and a dry one.
tropical storm: a huge spinning storm that forms over warm sea in the tropics.
eye: the calm centre of a tropical storm.
eyewall: the ring of tall cloud round the eye, with the strongest wind and rain.
storm surge: a rise in sea level, driven by a storm, that floods the coast.

Back to your guess

The answer is D: By day the land warms more than the sea, so air rises over the land and cool air flows in. At night the pattern turns round.

The land warms and cools faster than the sea. By day air rises over the warm land and cooler air flows in from the sea. At night the land is cooler, so the air rises over the warmer sea and the wind blows out. Warm air rising and cool air sinking is convection.

Keep it

Copy this into your exercise book.

Day: land warmer, air rises over land, sea breeze. Night: sea warmer, land breeze. Monsoon: wind turns with the season. Storm: warm sea, spirals, eye calm, eyewall fierce.

Draw: Draw a coast with a loop of arrows. They go up over the warm side, back at height, down over the cool side, and in along the ground.