What is the global circulation system?

The global circulation system (GCS) is a model that explains how heat, wind and precipitation are found in different parts of the world. It also explains why different ecosystems and biomes are located where they are and why tropical storms are only found in certain areas.

There are 5 key ideas you need to understand to fully understand the GCS.

Idea 1: How is global circulation linked to latitude?

Image showing lines of latitude around the world
Lines of latitude around the world.

The first is how latitude works. Latitude is a set of imaginary lines that run around the Earth from east to west. They can be used for locating places, in this case, we use them for showing how far away we are from the equator.

The equator is a line that runs around the centre of the Earth and runs through countries such as Brazil, the Democratic Republic of Congo and Indonesia.

The equator is written as 00 and as you move away from the equator (either north or south) the number increases. The north pole is at 900 N and the south pole is at 900 S. (it is 900 because it is at right angles to the equator if you imagined cutting through the world).

Idea 2: Differential heating

For the next section, we are going to look at two opposite places in the world, the equator and the poles (north or south, it doesn’t really matter at this point!)

The climate and surface temperature at the equator and the poles are very different, this is due to the suns insolation (the amount of solar energy hitting an area) being different between these two places. The equator gets a higher level of insolation as the solar insolation is more concentrated at the sun is directly overhead. The poles get much less insolation due to the curve of the earth, making the same amount of insolation spread over a larger area.  

Solar intensity around the world
The suns energy is spread out over a larger area at the poles, compared to the equator

Idea 3: Air pressure

clear sky, high pressure
High pressure give clear skies and no clouds
grey sky, low pressure, clouds forming
Low pressure creates clouds and rain

Air pressure is the next step in understanding the Global atmopsheric circulation model. When we talk about air pressure we are talking about how much air is at the surface.

How do we get areas of high and low pressure?

Remembering back to how the sun heats at different latitudes, the equator gets most solar insolation which warms the air, causing it to expand and rise away from the surface, this leaves low pressure at the surface as some of the air has risen up. As this air rises it cools, and condenses to form clouds, which causes rainfall. This effect causes tropical rainforests (which are located on the equator) to have a warm and wet climate.

In areas of high pressure, the air sinks to the ground so no clouds can form, causing these areas to have very little rainfall. This causes deserts to form where areas of high pressure occur.

Key points:

Low pressure: rising air, less air at the surface and clouds form

High pressure: sinking air, more air at the surface, no clouds can form.

Idea 4: Hadley cells

To understand how high and low pressure work together to form the GCS model, we go back to the equator again! Air rises and when it hits the top of the atmosphere, it has to move apart. Some goes north, and some goes south. It then moves until it reaches approximately 30° N or S where it cools and sinks back to Earth, causing areas of high pressure (deserts). Trade winds pull most of this air back towards the equator causing a loop of moving air that is called a Hadley cell. There is a separate Hadley cell north and south of the equator.

Hadley Cell diagram

Idea 5: Ferrel and polar cells

The final idea of the GCS is the ferrel and polar cells. Some of the heat energy from the Hadley cells is pulled north or south by winds known as westerlies. The westerlies pull the air from 30° to approximately 60° north or south. Here it mixes with colder polar air which forces the warmer (less dense) air to rise and condense to form clouds around this area. Some of the warmer air returns towards the equator forming the next cell called a ferrel cell.

Between 60° and 90° there is less heat energy being transferred so the final cell of the three cell model, the polar cell is cooler than the ferrel and Hadley cells. Winds in this cell often stay circulating around the poles.  

The UK sits underneath where the ferrel and polar cells meet and as the cells move slightly over time, the weather conditions can change depending on which cell is determining the weather at the time. This gives the UK a varied climate compared to some other areas around the world.

Key points:

Hadley cell: nearest the equator, largest heat energy, creates deserts and tropical rainforests.

Ferrel cell: middle cell, medium heat energy, variable climate areas.

Polar cell: highest latitude cell, low heat energy, creates polar conditions.

The three cells combine together to form a conveyor belt that moves heat energy from the equator to the poles and controls the wind and rainfall patterns around the world. 

How these can combine to form extreme weather is the subject of the next lesson. 

Global atmospheric circulation

What might the examiner ask?

Explain how the Hadley cell works and why air sinks at 30 degrees N/S, what this causes (high pressure) and how high solar insolation gives high temperatures, forming an area of dry, warm deserts.

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