An Introduction to the Atmospheric Circulation System Learning Objectives: To be familiar with the 3 cell concept of atmospheric circulation; To be able to describe and explain how the Hadley Cell system is responsible for air circulation in the tropics.

1.1 The General Atmospheric Circulation System Consists of three cells in each hemisphere (6 cells from N – S poles). Differences in atmospheric pressure are created at the earth’s surface; areas of high & low pressure. High pressure = air falls, low pressure = air rises. (Remember = warm air always rises, it is less dense than colder air). Three cells are known as the Hadley cell, the Ferrel cell and the Polar cell. Figure 1: Simple diagram showing the three cells found in each hemisphere.

Figure 2: Complete Global Model of Atmospheric Circulation. Idealised representation of the general circulation of the atmosphere showing the positions of Polar Front; ITCZ (Inter Tropical Convergence Zone); Subtropical Jets (STJ) Polar Front Jets (PFJ).

1.2 The Hadley Cells The Hadley Cells are responsible for the seasonal changes in climate of the regions that experience a wet and dry climate. The cells are responsible for tropical air circulation. The cell circulation system can be broken down into four separate components: Over the equator, between the two cells, there is an area of low pressure, known as the inter-tropical convergence zone or ITCZ. Above the equatorial latitudes, the sun is directly overhead and heats the surface of the earth rapidly during the day. This causes high evaporation rates. The hot air rises by convection currents and an area of low pressure develops. As this air rises, it cools and condenses into cumulonimbus clouds. Heavy rainfall results.

2) At high altitudes the air moves towards the poles 2) At high altitudes the air moves towards the poles. This air usually circulates as upper westerly winds around the planet due to the deflection of the rotation of the earth. This is known as the Coriolis effect. Overall, air moves polewards. As air moves from high to low pressure in the northern hemisphere, it is deflected to the right by the Coriolis force. In the southern hemisphere, air moving from high to low pressure is deflected to the left by the Coriolis force.

3) The colder air begins to sink at 30 degrees N & S, to the earth’s surface. As the air falls, it warms and any moisture in the air evaporates. At the surface, high pressure is created, with cloudless skies. These areas are known as subtropical anticyclones. 4) Some of the falling air returns towards the equatorial areas as consistent winds known as the trade winds. Due to the Coriolis effect, they blow from a north easterly direction in the northern hemisphere and from the southeast in the southern hemisphere. The trade wind systems move air towards the equator, forming the ITCZ. Have YOU got it?

Task: Describe and Explain the Hadley Cell Sequence. 2 Use the worksheet to help you to draw and give a full description of what happens at each point of the cell. Don’t forget detail on changes in pressure, latitudes and forces. 3 1 4