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.
If you listen to the
weather forecasts, you will often hear the terms low pressure or high pressure. Wind is caused by air moving from areas of high pressure to areas of low
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
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
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
°some of the air above cools and
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.
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.
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
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
Trade across the
oceans relied on these winds until the coming of steamships.
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
Power from the
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 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
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
Useful Websites about
kids.esdb.bg - Nice
yahoo.com - Fun Facts for Kids About
kidwind.org - Kid Wind
dltk-kids.com - Weather