AIR MASSES AND FRONTS
Temperature, in the form of heating and cooling,
plays a key roll in our atmospheres circulation.
Heating and cooling is also the key in the formation of
various air masses. These air masses, because of
temperature contrast, ultimately result in the formation
of frontal systems. The air masses and frontal systems,
however, could not move significantly without the
interplay of low-pressure systems (cyclones).
Some regions of Earth have weak pressure
gradients at times that allow for little air movement.
Therefore, the air lying over these regions eventually
takes on the certain characteristics of temperature and
moisture normal to that region. Ultimately, air masses
with these specific characteristics (warm, cold, moist,
or dry) develop. Because of the existence of cyclones
and other factors aloft, these air masses are eventually
subject to some movement that forces them together.
When these air masses are forced together, fronts
develop between them. The fronts are then brought
together by the cyclones and airflow aloft. This
produces the classic complex frontal systems often seen
on surface weather maps.
LEARNING OBJECTIVE: Determine the
conditions necessary for the formation of air
masses and identify air mass source regions.
An air mass is a body of air extending over a large
area (usually 1,000 miles or more across). It is
generally an area of high pressure that stagnates for
several days where surface terrain varies little. During
this time, the air mass takes on characteristics of the
underlying surface. Properties of temperature, moisture
(humidity), and lapse rate remain fairly homogeneous
throughout the air mass. Horizontal changes of these
properties are usually very gradual.
CONDITIONS NECESSARY FOR AIR MASS
Two primary factors are necessary to produce an air
mass. First, a surface whose properties, essentially
temperature and moisture, are relatively uniform (it
may be water, land, or a snow-covered area). Second, a
large divergent flow that tends to destroy temperature
contrasts and produces a homogeneous mass of air. The
energy supplied to Earths surface from the Sun is
distributed to the air mass by convection, radiation, and
Another condition necessary for air mass formation
is equilibrium between ground and air. This is
processes: (1) turbulent-convective transport of heat
upward into the higher levels of the air; (2) cooling of
air by radiation loss of heat; and (3) transport of heat by
evaporation and condensation processes.
The fastest and most effective process involved in
establishing equilibrium is the turbulent-convective
transport of heat upwards. The slowest and least
effective process is radiation.
processes, evaporation and condensation contribute in
conserving the heat of the overlying air. This occurs
because the water vapor in the air allows radiation only
through transparent bands during radiational cooling
and allows for the release of the latent heat of
processes. Therefore, the tropical latitudes, because of
a higher moisture content in the air, rapidly form air
masses primarily through the upward transport of heat
by the turbulent-convective process. The dryer polar
regions slowly form air masses primarily because of the
loss of heat through radiation. Since underlying
surfaces are not uniform in thermal properties during
the year and the distribution of land and water is
unequal, specific or special summer and/or winter air
masses may be formed. The rate of air mass formation
varies more with the intensity of insolation.
EFFECTS OF CIRCULATION ON ALL AIR
There are three types of circulation over Earth.
However, not all of these are favorable for air mass
development. They are as follows:
systems have stagnant or slow-moving air, which
allows time for air to adjust its heat and moisture
content to that of the underlying surface. These