Education and Careers

What is wind?

Wind is the movement of air over the surface of the Earth, from areas of high pressure to low pressure. But what causes the changes in pressure? There are a few concepts that we will have to explore to find exactly how this works, but ultimately all the energy on our planet comes from the Sun.

The Sun gives out all sorts of radiation, including heat and light energy, and is so powerful that it radiates 170,000,000 GW of energy to the Earth! Thats more energy in a second than all the electricity used in the UK in a month! When this energy reaches the Earth, the ground and other surfaces absorb it, and heat the surrounding air. It's these differences in temperature, together with the rotation of our planet, that create the wind.

About 1 to 2 per cent of the energy coming from the sun is converted into wind energy, which is enough to meet the electricity needs of the world three times over, and is a source of power that will never run out.

The density of air

Air, like all substances around us, has a certain density.

Density (kg/m3)= mass (kilograms) /volume (meters cubed)

The density of air is small but not zero. If air didn't weigh anything, the atmosphere would float off into space, which would be bad!

There is only a thin layer of air surrounding the earth, what we know as our atmosphere. This extends upwards more than 50 kilometres above ground level. At this height the density is less than 1% of the ground level value. If the earth were the size of a football, the atmosphere would be equivalent to a 1mm thick layer on the surface of the football.

Air pressure

Because there are miles of air above us and it is all pushing down, the air at the bottom gets squeezed creating a pressure, like the pressure you feel at the bottom of a swimming pool. The size of this pushing force over each unit of area is called the air pressure, or atmospheric pressure.

Pressure (Pascals)= force (Newtons) /area (m2)

(The unit of pressure is called the Pascal or Pa for short, 1 Pascal = 1 Newton per m2)

Atmospheric pressure

The pressure on the earth's surface due to the air above us is about 100000 Pa - 101,325 Pa on average. That's 1Kg pushing on every square cm! 101,325 Pa is also commonly referred to as 'one atmosphere'. The weight of a column of water 10 meters high would be needed to increase the air pressure at the base of the column by 1 atmosphere.

A barometer measures air pressure. If you took a barometer up in a hot air balloon you would see the pressure reading fall the higher the balloon goes. This happens because there is less air above the balloon the higher up into the atmosphere it goes. If you went too high the air pressure would become so low that you would not be able to breathe properly. This is why modern passenger jets have 'pressurised cabins' to keep the conditions similar to that at the earth's surface so the passengers are more comfortable.

There is another unit of pressure called the "milli-bar" or mbar for short. There are exactly 100 Pascals per milli-bar, so 1000 Mb is about one atmosphere.

If you watch the weather forecast on TV you may see a map showing atmospheric pressure. This is referred to as an isobar chart.

Isobars are similar to contour lines. Instead of the lines showing areas where the ground is the same height above sea level, the lines show areas where the atmospheric pressure is the same. The closer the lines are together the more rapidly the pressure changes from one place to another. This is similar to contour lines on a map, the closer they are together the more steep the slope.

Why does the pressure vary from place to place and from day to day?

There are two causes:

1) the rotation of the earth
As the earth spins on its axis it drags the atmosphere round with it. However, the air higher up in the atmosphere is less affected by this dragging/stirring effect. The difference in the air speed at different levels in the atmosphere causes the air to mix, forming turbulence, which causes wind at the earth's surface.

The rotation of the earth causes another related phenomenon, the Coriolis force.This is best demonstrated by example. Take a piece of paper and pin it onto something which will not get damaged, e.g. a carpet. Rotate the paper anti clockwise (to represent the movement of the earth), whilst at the same time trying to draw a straight line. The line you draw will appear curved.

A similar effect occurs when air is moving over the surface of the earth as it rotates. Instead of travelling in a straight line, the path of the moving air veers to the right. As a result instead of the air (or wind) moving in a straight line from areas of higher pressure to areas of lower pressure, it moves almost parallel to the isobars. The result is that the wind circles in a clockwise direction towards the area of low pressure. In the Southern hemisphere, the wind will circle in an anti-clockwise direction and clockwise in the Northern hemisphere.

2) the heating effect of the sun
The warming effect of the sun varies with latitude and with the time of day. Warmer air is less dense than cooler air, and rises above it, so the pressure above the equator is lower than the pressure above the poles.

The warming effect is greater over the equator as the sun is directly overhead. Nearer the earth's poles the angle at which the suns rays hit the earth is more acute, so the same amount of energy is spread over a greater area.

There are also local effects. This diagram illustrates the wind characteristics of coastal regions.

Land heats up and cools down more quickly than the sea.

  1. During the day the air above the land heats up, expands and therefore becomes less dense and rises.
  2. The atmospheric pressure above the land drops and air moves in from above the sea, where the air pressure is higher. This causes a sea breeze.
  3. During the evening, the temperature of the land drops much faster than the sea,
  4. The air above the sea becomes hotter than the air above the land, so it rises and a breeze flows from the coast out to sea, reversing the effect.

Summary

  • Atmospheric pressure is caused by the mass of the air above us
  • Pressure gradients are caused by the variable heating of the Earth by the sun, along with the rotation of the Earth.
  • Air movement or wind is due to pressure gradients from place to place balancing out the pressure