where we hope you learn about the fascinating weather that affects us day to day. This is a great learning resource for parents and teachers.
We can feel if it is hot or cold, wet or dry, calm or windy. What we cannot feel is the pressure of the atmosphere. The earth’s atmosphere is the layer of gases which surrounds the earth. We usually just refer to this as air.
Pressure means how much the atmosphere presses on things. We cannot feel the air, or the atmosphere, pressing on us, but atmospheric pressure controls our weather.
We have areas of low and high pressure because the sun does not heat the whole earth equally. Much more heat is given to the atmosphere at the Equator than to the Poles.
When air is heated, it expands and, becoming lighter, rises by what is called convection. As this warm air rises, the pressure of the atmosphere closer to the ground becomes lower.
The cold air from the Poles then moves into these areas of low pressure while the warm equatorial air moves outwards at high level towards the Poles. Here it cools and sinks, taking the place of the air which is now at the Equator.
In this diagram we can see the red arrows on either side which show how the warm air rises from the equator.
Follow the air round the earth at the higher level. As it moves from the equator to the poles, it changes from warm (red) to blue (cold).
When the air reaches the poles it sinks, shown by the blue arrows at North and South Poles.
Now the whole circle starts again.
This air movement is what we call wind.
Of course it is not quite as simple as that.
As the warm equatorial air moves towards the poles, some of it cools and comes down towards the earth’s surface again around the line of latitude at 30 °. It then divides and goes in different directions. Some of it meets the cold air coming from the Poles and rises above it.
Again, around latitude 60 °some of the air above cools and sinks.
As you can see from our diagram the air continues to rise and sink as it heats and cools and meets other currents of air. All these red lines represent the currents of air, or winds circling the earth.
Sailors’ names for the winds
Until the second half of the nineteenth century almost all sea-going ships were driven by the wind. Sailors had an excellent knowledge of wind conditions around the world and names for the different movements of air.
- The Doldrums. Around the equator the air movement consists of up and down currents rather than sideways winds. As a result sailing ships could lie becalmed for weeks, without a breath of wind to move them. These wind conditions are known as the Doldrums.
- The Trade Winds are found on the north and south sides of the Doldrums. They blow in towards the Equator to take the place of the warm air that rises from there.
The Trade Winds are very reliable and carried Magellan on his voyage across the Pacific and Vasco da Gama to India after he had rounded the Cape of Good Hope.
Trade across the oceans relied on these winds until the coming of steamships.
- The Westerlies are the strong winds between latitudes 30 and 60, which bring so many gales to the west coast of Europe.
- The Roaring Forties are the strong gale-force winds south of the Equator which make rounding Cape Horn so difficult.
Power from the wind
The world no longer depends on the force of the wind to carry ships across the sea. Nor does the world depend on wind to turn the sails of the windmills that used to grind our wheat into flour. However, we have now discovered a new use for the power of the wind.
The wind can be used to turn vast wind turbines for producing electricity. These massive turbines can be placed on land or built at sea and produce electricity without polluting the air.
Power from the wind
The windiest place on the Earth is Port Martin in Antartica. The winds there average more than 40 miles per hour on at least 100 days each year.
The least windy place on the Earth is also in Antartica! At Dome A the winds hardly blow at all.