Met Office College – Course Notes
Global Circulation
Contents
1
Global Circulation
1.1
Origin of the Atmosphere
1.2
The troposphere
1.3
Water in the atmosphere
2
Causes of differential heating
3
The Hadley, Ferrel and Polar Cells
4
High and Low Pressure
5
Land Sea Temperature Differential
6
Topography
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1 Global Circulation
1.1
Origin of the Atmosphere
Our atmosphere is a gaseous envelope surrounding the earth which is retained by
gravitational attraction and largely rotates with it. In comparison to the dimensions of
the earth, the atmosphere is extremely thin. 99% of its mass lies below 30 km that is
0.005 of the earth’s radius. Its chemical and physical properties, together with its fields
of motion, mass and moisture, constitute the subject matter of meteorology.
1.2
The troposphere
About 80% of the atmosphere lies within the troposphere. Here the average
temperature decreases with height and it is where all of our weather occurs. The
troposphere’s temperature structure is primarily due to the heating of the earth’s
surface. The sun heats the ground which heats the air in contact with it by conduction
and convection. The troposphere’s upper boundary is called the tropopause
The position and temperature of the tropopause varies. It is higher and colder in the
tropics and lower and warmer in higher latitudes. This becomes an important factor
for thunderstorm development. Higher and colder cloud tops generally produce much
more vigorous thunderstorms.
1.3
Water in the atmosphere
Water in its gaseous state (water vapour) is a relatively small and variable constituent
of the atmosphere (0-4% in the troposphere). Because it can exist in the solid, liquid
and gaseous states in the range of temperatures encountered, it helps to determine
the temperature distribution over the earth via latent heat processes. The following
are the different phase changes possible:
1.
Changes of state requiring heat input:
Solid to liquid
Melting (fusion)
Liquid to vapour
Evaporation (vaporisation)
Solid to vapour
2.
Sublimation
Changes of state resulting in heat release:
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Liquid to solid
Freezing
Vapour to liquid
Condensation
Vapour to solid
Deposition
Table 1: Changes of state of water vapour
Latent heat is defined as: The quantity of heat absorbed or emitted, without change of
temperature, during a phase change of unit mass of material.
2 Causes of differential heating
Differential heating from south to north occurs at the earth’s surface as the result of a
number of factors:

The main cause is the differences in solar elevation according to latitude and
season.

Reflectivity (albedo). The ice sheets of the Arctic and Antarctic reflect incoming
heat effectively.

Poleward of 40 latitude (north of Central Spain), the outgoing heat on average
exceeds incoming heat.
3 The Hadley, Ferrel and Polar Cells
High temperatures over the equator and low temperatures over the poles result in
organised circulations. However, there is an extra complication in that the earth is
rotating. This has the effect of splitting the circulation between the equator and the
pole into three cells. Within the equatorial region, surface air rises and flows polewards. At about 30° latitude (Morocco) the air starts to descend before completing the
circulation with the returning branch flowing equator-wards at the surface. The earth’s
spin acts upon this, adding an easterly component. The resulting persistent winds are
named the trade winds, because of the important role they played in opening up the
New World to trade. They blow from the northeast in the Northern Hemisphere and
from the southeast in the Southern hemisphere (Fig 1).
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Figure 1: Idealised representation of the general circulation of the atmosphere.
Descending air marks the position of the polar and subtropical high-pressure areas;
ascending air marks the position of the equatorial and mid-latitude low-pressure areas.
This cell is named after the English meteorologist George Hadley (1685—1768).
Between the Hadley cell and the poles exist two more circulations. From about 30 to
about 60 exists the Ferrel circulation and a weak polar cell between the Ferrel cell and
the pole.
4 High and Low Pressure
Ascending air where the Ferrel and Polar cells meet results in low pressure. The
convergence of the north-easterlies and south-westerlies results in the polar front
which moves position daily. There are two main areas of rising air; in the tropics and
along the polar front (seen in fig. 1). As might be expected, it is in these areas that the
greatest average rainfall occurs.
Along the polar front, areas of low pressure known as depressions or, simply ‘lows’,
are formed.
There are also two main compensating areas of descending air: in Polar regions and
subtropical regions. These give rise to the subtropical and polar high-pressure areas
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and precipitation amounts are relatively small. The major deserts of the world are
found in the subtropical high-pressure belt, and the polar regions are also rather dry.
5 Land Sea Temperature Differential
The land will generally be warmer than the sea in summer and colder that the sea in
the winter.
The patterns of mean sea level pressure change from season to season due to
differential heating between the land and sea. Land surfaces cool more than the sea in
winter, and colder, denser air at the surface leads to higher pressure. Similarly in
summer the land warms more than the sea, and warmer, less dense air rises leading to
lower pressure.
Reasons for differential heating between land and sea surfaces:

Land is a poor conductor, so heating is confined to a shallow layer leading to a
large temperature rise at the surface.

The sea is semi-transparent, so that heat penetrates the surface, spreading the
heating through a greater depth. The top layer overturns, which also has the
effect of spreading the heating.

Water has a high specific heat capacity, therefore a larger amount of heat is
required for a given temperature rise.

Much of the heat absorbed by the sea goes into the process of evaporation rather
than producing a temperature rise.
6 Topography
Topography also affects weather globally, with mountain barriers such as the Rocky
Mountains or the Andes being two of the most influential. Mountain ranges have a
profound effect on the distribution of precipitation, with windward slopes being wet
and areas to the lee being dry. In the UK, the Pennines, Cumbrian mountains, Scottish
Highlands and Welsh mountains have the same effect but on a smaller scale.
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