1. Define an air mass and explain the formation of air masses.
Definition of air mass
- A huge quantity of air with dimensions of about 500 nm or so with similar properties, especially
in temperature and humidity (moisture) condition.
- This area of high pressure remains over an area for a long time that eventually, it takes the
characteristics of the underlying surface.
Formation of air mass
- The formation of air masses is due to the heating and cooling of the earth’s surface. This
continuous process is established in order to achieve an equilibrium state between the ground
and air.
- Heat is transferred by a combination of:
i) The turbulent convective transport of heat upward into the higher levels of air.
ii) Cooling of air by radiation loss of heat.
iii) Transport of heat by evaporation and condensation process.
- During the heat transferring process, there will be a large divergent flow that tends to destroy
temperature contrasts. As a result, a surface with relatively uniform temperature and moisture
is produced.
- Hence, any large body of air floating above this surface will develop into an air mass with similar
properties as the underlying surface.
2. Explain the significance of a source region and describe the characteristics of a region acting as
the source region for an air mass.
Significance of a source region
- The surface of a source region will determine the original characteristics (temperature and
moisture properties) of an air mass floating above it.
- When an air mass leaves its source region and begins to travel, it modifies its original
characteristics and adopts some of the properties from the surface over which it travels.
- The difference of temperature and moisture that the air mass experience when it travels from
one source region to another will affect the weather condition that develops within it.
Characteristics of a source region
- Air mass can either originate from a polar or tropical region and can develop over land
(continent) or ocean.
- If it is of polar (P) origin, the air mass will be cold and if of tropical (T) origin, the air mass will be
warm.
- If it forms over a continent (c), it will be dry and if it forms over an ocean (m), it will be moist.
3. Discriminate between the source region characteristics of
a. Artic Maritime and Continental Air
b. Polar Maritime and Continental Air
c. Tropical Maritime and Continental Air
d. Equatorial Air
Temperature Moisture
Air Mass Type Source Region
Characteristics Characteristics
Artic or Antartic (A or Poles Extremely cold Very dry due to
AA) extreme cold
Polar Continental (cP) Sub polar land Very cold Very dry due to the
cold
Polar Maritime (mP) Sub polar ocean Very cool due to high Moderately moist due
latitude and to cool temperature
moderating influence and evaporation from
of the sea and the the water surface
warm ocean current
Tropical Continental Sub tropical latitude Very warm Dry
(cT) land
Tropical Maritime (mT) Sub tropical latitude Very warm Very humid due to the
ocean evaporation from the
warm water surface
Equatorial (E) Equator Hot Extremely humid
Continental is not differentiated from maritime because much of equatorial land is covered with
humid tropical rainforests.
4. Describe the subsequent modification of an air mass by the nature of the surface over which it
travels.
When an air mass travels from one source region to another, its subsequent modification is affected
by:
- Surface temperature
- Surface moisture
- Topography
- Trajectory
- Age
a) Surface Temperature
- Air mass properties and stability can be modified due to the difference of temperature between
the air mass and the surface.
- When warm air mass moves over a colder surface, it will be cooled from below and will become
extremely stable in the lower layers.
- When cold air mass moves over a warmer surface, it will be heated from below and becomes
unstable and consequently spreads to higher layers.
b) Surface moisture
- The moisture content of an air mass may be modified by the addition of moisture (evaporation)
or by the removal of moisture (condensation and precipitation).
- The movement of air mass from land to ocean increases the moisture content of the lower
layers by evaporation from the underlying water surface.
- The movement of air mass from ocean surface to land decreases the moisture content of the
lower layers by condensation and precipitation.
c) Topography
- Evident primarily in the mountainous regions.
- Moisture is removed from the air mass through precipitation on the windward side thus
decreasing its stability.
- When it descends on the leeward side of the mountain, the air becomes warmer and drier and
stability increases.
d) Trajectory
- The trajectory path (cyclonic and anticyclonic) that an air mass follows will affect its stability.
- If it follows a cyclonic trajectory (convergence at the lower levels and divergent at the upper
levels), its stability in the upper levels is decreased. This reflects a cyclonic relative vorcity.
- If it follows and anticyclonic trajectory (divergence at the lower levels and convergence at the
upper levels), its stability in the upper levels is increased. This reflects an anticyclonic relative
vorcity.
e) Age
- Time that an air mass spent on its journey from its source and how long it has been in contact
with the surface over which it travels.
- Determines the amount of modification that takes place within an air mass.
- Air mass that moved into a new region from its source region and stagnated for some time will
acquire the properties of the underlying surface of the new region and lost its original
characteristics.
5. Describe stable and unstable air masses.
Stable air mass
An air mass where vertical motion is suppressed within it and has a tendency to return to its original
state after a disturbance occurred within it.
Unstable air mass
An air mass where vertical currents (convection) is developed within it and has a tendency to move
further away from its original state after a disturbance occurred within it.
- The stability of an air mass indicates the weather, cloud types that will form, type of
precipitation to be expected and occurrence of turbulence and visibility of the lower layer.
- It can be changed by either thermodynamic (heat loss / gain or moisture increase / decrease) or
mechanical (movement) means.
