Global Atmospheric Circulation

Use this Global Atmospheric Circulation practice exercise.

Changes to the Global Atmospheric Circulation as the climate changes.

Weather and Climate: a Teachers’ Guide

Pathway: Basic weather

Atmospheric and Oceanic Circulation – Climate ZonesAir 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 – More for Teachers

Alternative or Extension Resources

Skills exercise – UK synoptic chart

Investigating the link between pressure and rain

Coriolis Effect

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:

• Low pressure is called a cyclone and has anticlockwise winds blowing around it.
• High pressure is called an anticyclone and has clockwise winds blowing around it.
• The wind tends to blow along the pressure contours.
• We name winds by the direction they are blowing from.
• Buys Ballot’s Law states that “In the Northern Hemisphere, if you stand with your back to the wind then the lower pressure will be on your left”
• Alternatively, some people find the rule ‘righty tighty, lefty loosey’ a useful reminder of the direction of rotation – high pressure is like tightening a screw (righty tighty) and low pressure like loosening a screw (lefty loosey) (Figure 2).

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