Use this Global Atmospheric Circulation practice exercise.
Changes to the Global Atmospheric Circulation as the climate changes.
Other Useful Links
Pathway: Basic weather
Atmospheric and Oceanic Circulation – Climate Zones – Air Masses – Pressure and Wind
Lesson overview: In this lesson we introduce air pressure, how differences in pressure can lead to air motion (wind) and how rising and sinking air can lead to low and high pressure respectively. We also introduce the Coriolis Effect and demonstrate how it can lead to rotating weather systems.
The action of the global atmospheric circulation cells, incoming and outgoing heat energy and the influence of upper-atmosphere winds creates areas of ‘high’ and ‘low’ pressure across the world – places where there is more or less atmosphere above the surface of the Earth. Air is constantly pulled from areas of high pressure towards areas of low pressure, being deflected by the Coriolis effect as it does so, to create winds that circle around High/ Low pressure systems. Pressure is shown on synoptic weather maps using isobars – lines of equal pressure – and winds blow approximately along these lines.
Learning objectives:
To understand that air has a mass and exerts a pressure.
To contrast high and low pressure.
To be able to explain why winds are created and the factors that affect the wind.
To be able to interpret weather charts
Key Teaching Resources
Pressure and Wind PowerPoint
Pressure and Wind PowerPoint (easier)
Pressure and Wind Worksheet
Pressure and Wind Worksheet (easier)
Teacher CPD/ Extended Reading
Read
Pressure and Wind – More for Teachers
Or watch
Alternative or Extension Resources
As air blows from high to low pressure in the atmosphere, the Coriolis force diverts the air so that it follows the pressure contours. In the Northern Hemisphere, this means that air is blown around low pressure in an anticlockwise direction and around high pressure in a clockwise direction.
Think about a person standing at the Equator. In the course of a day, the planet rotates once, meaning that you travel a colossal 2π x R (the radius of the Earth – 6370km) = 40,000km through space – a speed of about 1700km/ hr. You don’t notice that you are travelling so fast, because the air around you is travelling at the same speed, so there is no wind. On the other hand, if you are standing at a Pole, all you do in the course of a day is turn around on the spot, you have no speed through space and similarly the air around you is stationary.
Now, think about really fast moving, Tropical air which is being pulled towards the poles by a pressure gradient. As it travels polewards, it moves over ground which is rotating more slowly, and so it overtakes the ground, and looks like it is moving from west to east. Similarly, slow moving polar air will be left behind by the rotating Earth and look like it is moving from east to west if it is pulled equatorward by a pressure difference.
In general, moving air in the Northern hemisphere is deflected to the right by the Coriolis Effect.
As the air blows from high to low pressure the Coriolis force acts on it, diverting it, and we end up with air following the pressure contours and blowing around low pressure in an anticlockwise direction and around high pressure in a clockwise direction (both true only for the Northern Hemisphere).
In this diagram, the black arrows show the direction the air is moving in. The Coriolis force pulls the air to the right (red arrows). As the air is being pulled in to the depression by the pressure gradient (blue arrows), it is continuously deflected by the Coriolis Force. When the air moves in a circle around the depression, the Coriolis force (red arrows) is balanced by the pressure gradient force (blue arrows).
In summary, for the Northern Hemisphere:
Figure 2: Air blows around a low pressure in an anticlockwise direction and around a high pressure in a clockwise direction in the Northern Hemisphere © RMetS
What about the Southern Hemisphere?
In the Southern Hemisphere, winds blow around a high pressure in an anticlockwise direction and around a low pressure in a clockwise direction.
The simplest way of visualising why this is the case is to take a ball (or an apple or orange, or anything spherical!). Mark on the poles and the equator, and then mark a spot in the ‘northern hemisphere’ and the ‘southern hemisphere’ of your sphere. Rotate your sphere. Keeping it rotating, tilt your sphere so that you are looking at it from the North Pole – your Northern Hemisphere spot should be going round in an anticlockwise direction. Now, making sure you keep rotating your sphere in the same direction, tilt it so that you are looking at the ‘south pole’. Your southern hemisphere spot should be rotating in a clockwise direction. This demonstration doesn’t explain the Coriolis effect, but it does show how things can be seen differently in the two hemispheres of the same planet.
Data and Image Sources
For an example of wind blowing along pressure contours, see the BBC website.
Useful Links:
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