A level Geography A2 revision Tropical climates Air masses

1. AIR MASSES
• Air masses
• An air mass is a large body of air in which the horizontal gradients of the main
physical properties, such as temperature and humidity, are fairly gentle
• The concept of an air mass is generally applied only to the lower layers of the
atmosphere. An air mass can cover an area up to tens of thousands of km2 .
• Since the lower layers of the atmosphere acquire these properties via the
earth‘s surface, it is possible to recognise air mass source regions in which air
masses pick up their distinctive features.

2. AIR MASSES
• Origin of air masses
• Air masses derive their temperature and humidity from the regions over which
they lie.
• These regions are known as source regions.
• The principal ones are:
• areas of relative calm such as semi-permanent high pressure areas
• where the surface is relatively uniform, including deserts, oceans and ice-fields.
• Although air masses can become considerably modified as they travel, it is usual
to describe the main types in terms of their region or source region.
• NB: When air masses, which have different characteristics, are brought together
they do not mix freely with each other. They tend to remain separate with
slopping boundary surfaces between them. These boundary surfaces are called
―fronts‖ and they usually separate masses of air with different temperature
and humidity features

3. AIR MASSES
• Classification of Air masses
• The initial classification of air masses was made by Tor Bergeron in 19 28.
Primarily they are classified first by the latitude of the source area (which largely
controls its temperature)
• secondly by whether the source area is continental (dry) or maritime (moist)
• A maritime tropical (mT) air mass is one that is warm and moist.
• A third subdivision refers to the stability of the air mass - that is, whether it has
cooled and become more stable or whether it has become warmer and less
stable

4. AIR MASSES

5. AIR MASSES

6. AIR MASSES
Air masses can be modified when they leave their sources as Figure 7. 3
illustrates

7. AIR MASSES
• Modification of air masses
• AE air masses move from their source regions, they may be changed due to:
• internal changes and
• the effects of the surface over which they move.
• Thermodynamic changes involve the addition and subtraction of heat into or
out of the air mass.
• Air masses are heated or cooled from below, all depending on the surface they
are passing through.
• Heating from below increases instability, while cooling at the surface may result
in a temperature inversion, which in turn actually limits the vertical spread of
the cooling.
• Thus air masses effectively lose heat through radiation heat loss.

8. AIR MASSES
• As air masses move from their source regions they can be changed by the area over
which they move.
• For example, a warm air mass that travels over a cold surface is cooled and becomes
more stable.
• Hence, it may form low cloud or fog but is unlikely to produce much rain.
• By contrast, a cold air mass that passes over a warm surface is warmed and becomes
less stable.
• The rising air is likely to produce more rain.
EXAMPLE
Continental polar air mass that is cold and dry, is modified as it moves towards the
equator, that is, toward warm landmasses.
It is heated from below, resulting in the steepening of its lapse rates. This creates
instability and convective activities.
Clouds are formed but they are generally of little significance since the air is dry.

9. AIR MASSES
• On the other hand, if Maritime air moves pole ward, that is, over cooler land
and water surfaces, the air mass is cooled from below and this results in a
temperature inversion.
• Air becomes stable and they result in the trapping of pollutants if it is moving
over industrialised countries.
• Advection fogs are also common where such a warmer air mass moves over
cooler surfaces.