Weather

Basics

2. Circulation Systems

Worksheet 1:
Atmospheric Pressure and Hadley Cells

hadley cells, image source: NASA, animation by Julia Heres

This figure shows the atmospheric circulation patterns which give rise to the major bands of high and low pressure around the Earth.

The pressure cells between the equator and 30°N and 30°S are known as Hadley Cells after George Hadley who suggested their existence way back in 1735. 

These cells transport heat from the equator to the colder temperate and polar regions of the Earth. 

Exercise:

What is the Hadley Cell Circulation?
What is the relationship between the Hadley Cells, the rotation of the Earth, wind drift and the trade winds?

Explanation:

Temperature in the tropical region is high throughout the year. 
The annual distribution of rainfall varies greatly throughout the globe.  Near the equator, it rains heavily all year and rainfall amounts can be three times greater than the amounts we see in Europe. 

It rains much less in the tropics (23.5 °N or 23.5 °S) and in some regions there is no rainfall at all for years.

Large amounts of solar radiation at the equator warms the moist air at the surface. This warm air rises into the atmosphere and, as it rises, it cools.  As the air cools, it can no longer hold as much water vapour and the water condenses out and forms big towers of clouds.  This process often results in daily thunderstorms.  As the warm air rises, the pressure at the ground decreases.  This process occurs all around the equatorial region. This is why an area of low pressure is seen in this region and it is known as the equatorial low pressure belt.

At higher altitudes the rising air drifts towards the North and towards the South and cools.  Close to the tropics (23.5 °N and 23.5 °S) the air is cold enough to sink to the ground.  This sinking air is very dry, all its water has been lost as rain over the equator.  The air warms up as it sinks towards the hot ground and this causes a hot high pressure region to form.  This high pressure belt is known as the horse latitudes or the subtropical calms. 

Air moves from high pressure regions to low pressure regions to try to even out pressure differences around the globe.  As a result, air moves from both the northerly and southerly tropical regions back towards the equator.

These consistent winds are known as the trade winds. Sailors have used them for many years to help them cross the oceans.

As a result of the Earth's rotation, the trade wind in the northern hemisphere drift to the right and are called the north east trade winds. In the southern hemisphere they drift to the left and are called the south east trade winds.   These warm dry winds absorb moisture. At the equator they merge and close the circle of the trade winds.

This whole atmospheric circulation pattern is called a Hadley-Cell.

Experiment:

The winds from the south drift towards the left, those from the north drift to the right.  As  a result, the winds appear to come either from a northeastly or southeastly direction. 
Do you believe this?

It is quite simple to simulate the effect of the rotation of the Earth:

You will need a globe (or a big water-melon, a ball or just a tomato).

Draw the equator and the northern and southern tropics onto the globe using a pen or some chalk.

Remember, the Earth rotates in an anti-clockwise direction looking from the north pole.

Start to rotate the Earth and, while it is rotating, draw the winds coming directly from the tropics to the equator.
Do the winds travel in a straight line or not?

Movies: Now watch these three movies on the experiment.

Attention:
If you cannot see anything, please install the Microsoft Media Player (TM) or the RealPlayer (TM) first.
If you have a slow modem it is better to save the file on your hard disk and run it later. If you have a fast connection, then just run the movie.

Movie No.1:

If the Earth didn't rotate, the winds would blow straight from the tropics to the equator.  But, as everyone knows, the Earth does rotate!  Watch the next two movies to see how the winds drift to the east and to the west.

Movie No. 2:

This movie shows the effects of the Earth's rotation on the northern hemisphere. As you can see, the wind drifts and does not blow in a straight line.

Movie No. 3:

This movie shows the effects of the Earth's rotation on the winds in the southern hemisphere.

Compare your lines on your globe with the films and with the picture.

Does this mean that all sailors meet at the equator?

About this page:

author: Dr. Yvonne Schleicher - University of Nürnberg, Germany
educational reviewing: Dr. Helmut Schrettenbrunner and Julia Heres - University of Nürnberg, Germany
last update: 2003-09-15