---
title: "Winds and the Global Circulation"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 24
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 24 — Winds and the Global Circulation

For centuries, sailing ships leaving the ports of Europe and Africa for the Americas relied on a steady wind from the east, which blows on both sides of the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and carried them across the Atlantic. Sailors all over the world knew it, and the early trade with the Americas depended on it: it is the *trade wind*. Even now, cargo ships use the [trade winds](#def-g8-winds-global-circulation-global-winds), and the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) currents they drive, to shorten their voyages.

![A sailing ship running before a steady wind on the open ocean (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/img-1e6636dbfd1a.jpg)

*A sailing ship running before a steady wind on the open [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) (AI-generated illustration of a generic scene).*

Why does the trade wind blow from the east, and always in the same places? Why are there [deserts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) near the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics) and [rainforests](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-rainforest) near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)? The answer starts with the Sun.

**You already know.**

Wind is air in motion (the physics books). Air presses on everything it touches, and the pressure falls with height ([Chapter 23](https://one-course.com/books/earth/1/en/chapter/23-the-atmosphere#ch-g8-the-atmosphere)). The [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) happens in the [troposphere](https://one-course.com/books/earth/1/en/chapter/23-the-atmosphere#def-g8-the-atmosphere-layers), the lowest layer of the atmosphere. The Earth has [climate zones](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate-zone): hot and wet near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), with [rainforests](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-rainforest); hot and dry [deserts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) near the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics); temperate, then cold zones towards the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) ([Chapter 11](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#ch-g5-weather-and-climate)).

## 24.1 The Sun heats the Earth unevenly

At noon on the day of an equinox, the Sun is straight overhead at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). Its rays arrive steeply there and fall on a small patch of ground. Further north or south, the same rays arrive at a slant and are spread over a larger patch: each square metre receives less of their energy. At [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) $60{}^{\circ}$, the same beam covers twice the ground it covers at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), so each square metre gets half as much.

![Two identical beams of sunlight at noon on an equinox. At the equator they fall straight down on a small patch (thick line); at 60 of latitude they slant and cover twice as much ground.](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/fig-5684ee8388c0.svg)

*Two identical beams of sunlight at noon on an equinox. At the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) they fall straight down on a small patch (thick line); at $60{}^{\circ}$ of [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) they slant and cover twice as much ground.*

| [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) | $0{}^{\circ}$ | $30{}^{\circ}$ | $45{}^{\circ}$ | $60{}^{\circ}$ | $75{}^{\circ}$ |
| --- | --- | --- | --- | --- | --- |
| ground covered by the same beam | 1 | 1.15 | 1.41 | 2 | 3.86 |
| energy per square metre (% of the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)’s) | 100 | 87 | 71 | 50 | 26 |

As the Earth goes round the Sun, its tilted axis makes the overhead Sun move between the two [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics), which gives the seasons (the physics books); but all year round, the regions near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) receive much more energy than those near the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). This uneven heating is the main driver of the winds: the atmosphere carries warm air towards the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) and cold air towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator).

## 24.2 Rising and sinking air

Warm air is lighter than cold air: it rises, and the cooler air around flows in to take its place. Where air rises, it presses less on the ground: the pressure there is low. Where air sinks, it presses more: the pressure is high. At the ground, the wind blows from high pressure towards low pressure. Rising, cooling, sinking and flowing back, the air turns in a loop.

**Definition 24.1 (Convection cell).**

A *convection cell* in the atmosphere is a loop of moving air: air warmed from below rises, spreads out at height, cools and sinks somewhere else, and flows back along the ground to where it rose. Under the rising air the pressure is low; under the sinking air it is high.

**In the lab — Warm air rises.**

Indoors, cut a spiral out of a sheet of thin paper, like the peel of an apple, and hang it by its centre on a thread. Hold it high above a warm radiator in winter (never above a flame or a candle): it slowly starts to turn. The air warmed by the radiator rises and pushes on the spiral. Hold it near a cold window: does it turn the other way, or not at all?

The smallest [convection cells](#def-g8-winds-global-circulation-convection-cell) can be felt on a sunny day at the seaside. The land and the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) receive the same sunshine, but the Sun’s rays do not go deep into the ground, while the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) spreads its heat through a thick layer of water. So the land warms up much more by day, and cools down much more at night.

![The sea breeze and the land breeze: two small convection cells that turn opposite ways. Red arrows: rising warm air; H and L: high and low pressure at the ground. By day the loop reaches 1000 to 1500\, m up.](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/fig-0a600493e97c.svg)

*The [sea breeze](#def-g8-winds-global-circulation-breezes) and the [land breeze](#def-g8-winds-global-circulation-breezes): two small [convection cells](#def-g8-winds-global-circulation-convection-cell) that turn opposite ways. Red arrows: rising warm air; H and L: high and low pressure at the ground. By day the loop reaches 1000 to $1500\,\mathrm{m}$ up.*

**Definition 24.2 (Sea breeze and land breeze).**

A *sea breeze* is a wind that blows from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) towards the land by day, when the land is warmer than the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). A *land breeze* blows from the land towards the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) at night, when the land has become colder than the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

![Kites flying in the afternoon wind on a beach (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/img-697bdfc18e7a.jpg)

*Kites flying in the afternoon wind on a beach (AI-generated illustration of a generic scene).*

## 24.3 The global winds

The same thing happens on the scale of the whole Earth. Near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the strongly heated air rises; it spreads towards the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) at height and sinks again near $30{}^{\circ}$ of [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). Averaged over many years, the [troposphere](https://one-course.com/books/earth/1/en/chapter/23-the-atmosphere#def-g8-the-atmosphere-layers) of each [hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) turns in three great [convection cells](#def-g8-winds-global-circulation-convection-cell): one from the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) to about $30{}^{\circ}$, one from about $30{}^{\circ}$ to about $60{}^{\circ}$, and one from about $60{}^{\circ}$ to the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). Meteorologists call them the Hadley, Ferrel and polar cells. Their limits move with the seasons, and the middle cell is a rough average of many passing [storms](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm).

The winds that blow at the ground would go straight from high to low pressure if the Earth did not turn. Because the Earth rotates, moving air is turned aside: to the right in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), to the left in the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). So the air flowing back to the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) does not come from the north, but from the north-east in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and from the south-east in the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator).

**Definition 24.3 (The global winds).**

The surface winds that blow most often in each belt of [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) are:

- the *trade winds* , between the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and about $30{}^{\circ}$ , blowing from the north-east in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and from the south-east in the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) ;
- the *westerlies* , between about $30{}^{\circ}$ and $60{}^{\circ}$ , blowing from the west (from the south-west in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) , from the north-west in the southern);
- the *polar easterlies* , between about $60{}^{\circ}$ and the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) , blowing from the east.

![The global circulation, simplified. Left: the belts of high (H) and low (L) pressure at the ground. On the globe: the surface winds, turned aside by the Earth’s rotation. Right: the three convection cells of each hemisphere seen from the side, at the same latitudes (the polar cells are drawn very thin because the polar band is narrow on the globe).](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/fig-7163c360b7d3.svg)

*The [global circulation](#def-g8-winds-global-circulation-circulation), simplified. Left: the belts of high (H) and low (L) pressure at the ground. On the [globe](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-globe): the surface winds, turned aside by the Earth’s rotation. Right: the three [convection cells](#def-g8-winds-global-circulation-convection-cell) of each [hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) seen from the side, at the same [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) (the polar cells are drawn very thin because the polar band is narrow on the [globe](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-globe)).*

**Method 24.4 (Finding the global wind at a latitude).**

1. Find the belt: from the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) to about $30{}^{\circ}$ , from $30{}^{\circ}$ to $60{}^{\circ}$ , or from $60{}^{\circ}$ to the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) .
2. Find which way the air flows at the ground in that belt: towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) in the first and the last belt, towards the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) in the middle one.
3. Turn this flow aside: to the right in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) , to the left in the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) .
4. Say where the wind comes from: winds are named after the direction they blow *from* .

**Example 24.5 (The wind at 40∘40{}^{\circ}40∘ south).**

At $40{}^{\circ}$ S we are in the middle belt, between $30{}^{\circ}$ and $60{}^{\circ}$. There the air at the ground flows towards the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles), that is towards the south. In the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) it is turned to the left: for air going south, the left is the east, so the air ends up moving towards the south-east. It therefore comes from the north-west: it is one of the [westerlies](#def-g8-winds-global-circulation-global-winds), blowing from the north-west.

Near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), where the [trade winds](#def-g8-winds-global-circulation-global-winds) of the two [hemispheres](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) meet, the winds die down: sailors called this belt of calm the *doldrums*. The warm, damp air rises there, cools and gives persistent showers and thunderstorms. Near $30{}^{\circ}$, under the sinking air, the winds are light and [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is rare.

**Definition 24.6 (Global circulation of the atmosphere).**

The *global circulation* of the atmosphere is the large-scale movement of air around the planet, seen when the winds are averaged over many years. It carries heat from the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) towards the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles), and water vapour from one belt of [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) to another.

## 24.4 The global circulation and the climate zones

The [global circulation](#def-g8-winds-global-circulation-circulation) explains much of the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of [climates](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate). Where air rises, near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), it cools and its water vapour turns into [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud): [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is frequent, and [rainforests](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-rainforest) grow. Where the air sinks, near $30{}^{\circ}$ north and south, it warms as it comes down, its [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) dry up, and [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is rare: the great hot [deserts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert), such as the Sahara, lie in these belts. In the middle [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude), the [westerlies](#def-g8-winds-global-circulation-global-winds) bring one [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) after another, with [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) at all seasons.

![The Americas seen by the GOES-16 weather satellite, high above the equator, on 26 July 2022. Many clouds have formed over the tropics and the Amazon rainforest, where warm, damp air rises; far to the south, in the southern winter, the storms of the westerlies swirl over the ocean. NOAA (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/img-7cae9a2a0ecd.jpg)

*The Americas seen by the GOES-16 [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) satellite, high above the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), on 26 July 2022. Many [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) have formed over the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics) and the Amazon [rainforest](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-rainforest), where warm, damp air rises; far to the south, in the southern winter, the [storms](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) of the [westerlies](#def-g8-winds-global-circulation-global-winds) swirl over the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). NOAA (public domain, via Wikimedia Commons).*

**Example 24.7 (Four towns, four belts).**

The four towns of the [climate charts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate-chart) of [Chapter 11](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#ch-g5-weather-and-climate) all lie in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), at very different [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). Their yearly [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) follow the belts of the [global circulation](#def-g8-winds-global-circulation-circulation) (chart below). Hilo, at $20{}^{\circ}$ N, is on an island in the trade-wind belt, in the middle of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), where the [trade winds](#def-g8-winds-global-circulation-global-winds) pick up water vapour: about $3000\,\mathrm{mm}$ fall there in a year. Phoenix, at $33{}^{\circ}$ N, lies close to $30{}^{\circ}$, under the sinking air: less than $200\,\mathrm{mm}$. Seattle, at $47{}^{\circ}$ N, is in the [westerlies](#def-g8-winds-global-circulation-global-winds), whose [storms](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) bring about $1000\,\mathrm{mm}$ a year, most of it from autumn to spring. Utqiaġvik, at $71{}^{\circ}$ N, lies under the cold air that sinks near the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles): less than $150\,\mathrm{mm}$, a cold [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert).

![Yearly rain and snow (averages 1991–2020) of the four towns of the climate charts, placed at their latitudes. The wet towns lie where air rises, near the equator and near 60; the dry ones where it sinks, near 30 and near the pole. NOAA climate normals.](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/fig-96504fa8a85b.svg)

*Yearly [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) (averages 1991–2020) of the four towns of the [climate charts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate-chart), placed at their [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). The wet towns lie where air rises, near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and near $60{}^{\circ}$; the dry ones where it sinks, near $30{}^{\circ}$ and near the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). NOAA [climate](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) normals.*

Four towns are only four points: [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and cold or warm [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) also change how much [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) a place receives. The belts give the broad pattern of the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of [climates](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate), not the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) of every town.

## 24.5 Exercises

**Exercise 24.1 ★.**

Name the global wind that blows most often: (a) at $15{}^{\circ}$ north; (b) at $45{}^{\circ}$ south; (c) at $75{}^{\circ}$ north; (d) at $20{}^{\circ}$ south. Give its direction each time.

**Solution of Exercise 24.1.**

(a) The north-east trade wind, from the north-east. (b) The [westerlies](#def-g8-winds-global-circulation-global-winds), from the north-west. (c) The [polar easterlies](#def-g8-winds-global-circulation-global-winds), from the east (from the north-east). (d) The south-east trade wind, from the south-east.

**Exercise 24.2 ★.**

Complete each sentence. (a) Warm air … and cold air …. (b) Under rising air, the pressure at the ground is …. (c) At the ground, the wind blows from … pressure towards … pressure. (d) In the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the Earth’s rotation turns moving air to the ….

**Solution of Exercise 24.2.**

(a) Warm air rises and cold air sinks. (b) Low. (c) From high pressure towards low pressure. (d) To the right.

**Exercise 24.3 ★.**

True or false? Correct the false sentences. (a) A [sea breeze](#def-g8-winds-global-circulation-breezes) blows from the land to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (b) The land warms up faster than the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) by day. (c) The [trade winds](#def-g8-winds-global-circulation-global-winds) blow from the west. (d) Many great [deserts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) lie near $30{}^{\circ}$ of [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude).

**Solution of Exercise 24.3.**

(a) False: a [sea breeze](#def-g8-winds-global-circulation-breezes) blows from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) to the land (by day). (b) True. (c) False: they blow from the east (from the north-east or the south-east). (d) True.

**Exercise 24.4 ★.**

Draw a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) with the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) on the left and the land on the right. Add the Sun, then draw with arrows the loop of air of the [sea breeze](#def-g8-winds-global-circulation-breezes). Mark H and L where the pressure at the ground is high and low.

**Solution of Exercise 24.4.**

The drawing shows a loop: along the ground, an arrow from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) to the land; over the land, an arrow going up; at height, an arrow from the land back over the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); over the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), an arrow going down. H is marked on the ground under the sinking air (over the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean)), L under the rising air (over the land).

**Exercise 24.5 ★★.**

Use the table of beams in this chapter. (a) At $45{}^{\circ}$, how much more ground does a beam cover than at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)? (b) What percentage of the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)’s energy does each square metre receive at $60{}^{\circ}$? (c) At $75{}^{\circ}$? (d) Explain in one sentence why the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) are cold.

**Solution of Exercise 24.5.**

(a) 1.41 times as much ground. (b) 50 %. (c) 26 %. (d) Near the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles), sunlight arrives at a slant and is spread over a large area, so each square metre receives little energy.

**Exercise 24.6 ★★.**

A boat sails due south through the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and then the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). (a) Which winds does it meet, in order, from $70{}^{\circ}$ N to $70{}^{\circ}$ S? (b) Where is it likely to find calm air and frequent showers? (c) Where are winds light and [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) rare?

**Solution of Exercise 24.6.**

(a) [Polar easterlies](#def-g8-winds-global-circulation-global-winds) ($70{}^{\circ}$ to $60{}^{\circ}$ N), [westerlies](#def-g8-winds-global-circulation-global-winds) ($60{}^{\circ}$ to $30{}^{\circ}$ N), north-east [trade winds](#def-g8-winds-global-circulation-global-winds) ($30{}^{\circ}$ N to the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)), south-east [trade winds](#def-g8-winds-global-circulation-global-winds) ([equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) to $30{}^{\circ}$ S), [westerlies](#def-g8-winds-global-circulation-global-winds) ($30{}^{\circ}$ to $60{}^{\circ}$ S), [polar easterlies](#def-g8-winds-global-circulation-global-winds) ($60{}^{\circ}$ to $70{}^{\circ}$ S). (b) Near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), in the doldrums. (c) Near $30{}^{\circ}$ N and $30{}^{\circ}$ S, under the sinking air.

**Exercise 24.7 ★★.**

At a beach, it is $31{}^{\circ}\mathrm{C}$ on the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) at 3 p.m. and $22{}^{\circ}\mathrm{C}$ over the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). At 3 a.m. it is $17{}^{\circ}\mathrm{C}$ on the land and $21{}^{\circ}\mathrm{C}$ over the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (a) Which is warmer at 3 p.m.? Which way does the wind blow? (b) And at 3 a.m.? (c) By how many degrees does the land temperature change between the two times? And the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) temperature? (d) Explain the difference.

**Solution of Exercise 24.7.**

(a) The land ($31 > 22$): the wind blows from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) to the land, a [sea breeze](#def-g8-winds-global-circulation-breezes). (b) The [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ($21 > 17$): the wind blows from the land to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), a [land breeze](#def-g8-winds-global-circulation-breezes). (c) The land: $31 - 17 = 14$ degrees; the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): $22 -
21 = 1$ degree. (d) The Sun’s rays heat only a thin layer of ground, which warms and cools quickly; the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) spreads its heat through a thick layer of water, so its temperature hardly changes between day and night.

**Exercise 24.8 ★★.**

Air flows from the high-pressure belt near $30{}^{\circ}$ N towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). (a) Without the Earth’s rotation, from which direction would it blow? (b) Which way does the rotation turn it in this [hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)? (c) From which direction does it blow then? (d) Answer the same three questions for the air flowing towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) from $30{}^{\circ}$ S.

**Solution of Exercise 24.8.**

(a) From the north. (b) To the right. (c) From the north-east (it is the north-east trade wind). (d) Without rotation it would blow from the south; the rotation turns it to the left; it blows from the south-east.

**Exercise 24.9 ★★.**

Explain, step by step, why it [rains](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) so often near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator): use the words *heated*, *rises*, *cools*, *[clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud)*. Then explain why there is a [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) near $25{}^{\circ}$ N in North Africa: use the words *sinks*, *high pressure*, *dry*.

**Solution of Exercise 24.9.**

Near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the ground and the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) are strongly *heated* by the Sun; the warm, damp air *rises*; as it rises it *cools*, and its water vapour turns into *[clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud)*, which give showers and thunderstorms. Near $25{}^{\circ}$ N, the air that rose at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) *sinks*; under it the pressure at the ground is *high*; the sinking air warms, its [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) dry up and the region stays *dry*: a [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert).

**Exercise 24.10 ★★.**

A sailing ship leaves a port at $40{}^{\circ}$ N on the west [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Europe for the Caribbean, at $15{}^{\circ}$ N. (a) Which wind would it meet at $40{}^{\circ}$ N? Would it help the ship sail west? (b) Why should the ship first sail south? (c) Which wind carries them west, and between which [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude)? (d) For the return voyage to Europe, which wind should the ship look for, and at what [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude)?

**Solution of Exercise 24.10.**

(a) The [westerlies](#def-g8-winds-global-circulation-global-winds). No: they blow from the west, against a ship sailing west. (b) To reach the belt of the [trade winds](#def-g8-winds-global-circulation-global-winds), south of about $30{}^{\circ}$ N. (c) The north-east trade wind, between the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and about $30{}^{\circ}$ N. (d) The [westerlies](#def-g8-winds-global-circulation-global-winds), north of about $30{}^{\circ}$ N (near $40{}^{\circ}$ N, the [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) of the home port).

**Exercise 24.11 ★★★.**

At [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) $L$, a beam of sunlight at noon on an equinox covers $1 \div
\cos L$ times the ground it covers at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) (on a calculator, $\cos 60^\circ = 0.5$). (a) Check the value for $60{}^{\circ}$. (b) Using $\cos 30^\circ \approx 0.866$, check the value for $30{}^{\circ}$, rounded to two decimals. (c) Suppose a solar panel lying flat receives $1000\,\mathrm{W}$ on each square metre at noon at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) on that day. About how much would the same panel receive at $30{}^{\circ}$? At $60{}^{\circ}$?

**Solution of Exercise 24.11.**

(a) $1 \div 0.5 = 2$: the table gives 2. (b) $1 \div 0.866 \approx
1.1547$, that is **1.15**. (c) At $30{}^{\circ}$: $1000 \div 1.15 \approx
870\,\mathrm{W}$ on each square metre (exactly $1000 \times 0.866 =
866\,\mathrm{W}$). At $60{}^{\circ}$: $1000 \div 2 = 500\,\mathrm{W}$.

**Exercise 24.12 ★★★.**

In each [hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the [global circulation](#def-g8-winds-global-circulation-circulation) has three cells. (a) Draw a side view of the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), with the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) at the bottom and the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) at the top, and the three cells as loops; show where the air rises and where it sinks. (b) Mark the high and low pressure belts at the ground. (c) Add the surface winds of each cell, turned to the right. (d) Use your drawing to explain why the [westerlies](#def-g8-winds-global-circulation-global-winds) blow towards the north-east.

**Solution of Exercise 24.12.**

(a) Three loops: air rises at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and sinks near $30{}^{\circ}$; rises near $60{}^{\circ}$ and sinks near $30{}^{\circ}$; rises near $60{}^{\circ}$ and sinks at the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). (b) L at the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and near $60{}^{\circ}$; H near $30{}^{\circ}$ and at the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). (c) From $30{}^{\circ}$ to the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the surface air goes south and is turned to the right: north-east trade wind. From $30{}^{\circ}$ to $60{}^{\circ}$, it goes north and is turned to the right: it comes from the south-west. From the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) to $60{}^{\circ}$, it goes south, turned to the right: polar easterly. (d) In the middle cell the surface air flows towards the [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles), that is north; turned to the right, it heads towards the north-east: these are the [westerlies](#def-g8-winds-global-circulation-global-winds).

## 24.6 Problem: Sailing with the trade winds

**Problem 24.1.**

Weekend problem — a sailing ship crosses the Atlantic from Cape Verde to Barbados with the north-east trade wind; the route is measured on the map, the wind is read, and the length of the crossing is worked out

A sailing ship leaves Praia, in the Cape Verde Islands, for Bridgetown, in Barbados. The [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) shows the route and the trade wind.

![](https://one-course.com/images/onecourse/chapters/earth-1/g8-winds-global-circulation/fig-d6c3ea7b9a17.svg)

**Part I — The route.**

1. Praia lies at $14.9{}^{\circ}$ N, $23.5{}^{\circ}$ W; Bridgetown at $13.1{}^{\circ}$ N, $59.6{}^{\circ}$ W. How many degrees of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) separate them?
2. Both ports lie close to $14{}^{\circ}$ N. Along this [parallel](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) , one degree of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) measures about $108\,\mathrm{km}$ . About how many kilometres is the crossing? Round to the nearest hundred.
3. Measured exactly on the round Earth, the shortest route is $3896\,\mathrm{km}$ . Is the answer of question 2 a good estimate?
4. Do the ports lie in the belt of the [trade winds](#def-g8-winds-global-circulation-global-winds) ? Explain.

**Part II — The wind.**

5. From which direction does the trade wind blow here?
6. The ship sails towards the west. Does the wind push it forwards, backwards or sideways? Explain with the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) .
7. Why does the trade wind blow in the same direction almost every day of the crossing?
8. On the way back to Europe, why should the captain not try to sail east along $14{}^{\circ}$ N?

**Part III — The crossing.**

9. Suppose the ship averages $7\,\mathrm{km}$ per hour. How many kilometres does it cover in a day of 24 hours?
10. If the wind died down for 3 days on the way, how many kilometres of progress would the ship lose?
11. With a steady trade wind, how many days does the ship need for $3896\,\mathrm{km}$ ? Round to one decimal.
12. To the nearest whole day, how long does the crossing last? Is it more or less than a month?

**Solution of Problem 24.1.**

**1.** $59.6 - 23.5 = 36.1$ degrees of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude).

**2.** $36.1 \times 108 \approx 3899$: about $3900\,\mathrm{km}$.

**3.** Yes: $3900 - 3896 = 4$ km, a tiny difference.

**4.** Yes: both lie near $14{}^{\circ}$ N, between the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) and about $30{}^{\circ}$ N, the belt of the north-east trade wind.

**5.** From the north-east (the arrows point towards the south-west).

**6.** Forwards, and a little sideways: the wind comes from behind the ship, slightly from its right, and pushes it towards the west.

**7.** It is part of the [global circulation](#def-g8-winds-global-circulation-circulation): air keeps sinking near $30{}^{\circ}$ N and rising near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), so the surface air keeps flowing towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), and the Earth’s rotation always turns it to the right.

**8.** The trade wind would blow against the ship. The captain should sail north, beyond about $30{}^{\circ}$ N, to find the [westerlies](#def-g8-winds-global-circulation-global-winds).

**9.** $7 \times 24 = 168\,\mathrm{km}$ a day.

**10.** $3 \times 168 = 504\,\mathrm{km}$.

**11.** $3896 \div 168 \approx 23.2$ days.

**12.** About **23 days**: less than a month.