Transcript Atmosphere-Circulation

A2 Module 4: Global Change
An Introduction to the Atmospheric
Circulation System
Learning Objectives:
1) To be familiar with the 3 cell concept of
atmospheric circulation;
2) 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
•
•
1)
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. 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.
Task: Describe and Explain
the Hadley Cell Sequence.
2
3
1
4
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.