Geoscience Reference
In-Depth Information
Air mass
Symbol
Properties
Mean temperature
Specific humidity
(°C)
(g kg
-1
)
Arctic/Antarctic
A
Very cold, dry
-20
0·1
Polar continental
Pc
Cold, dry (winter)
-10
1·4
Polar maritime
Pm
Cool, moist
5
4·4
Tropical continental
Tc
Warm, dry
25
11.0
Tropical maritime
Tm
Warm, moist
20
17.0
Equatorial
E
Warm, very moist
27
19.0
but it is a relatively slow process. In addition, the air is
made more stable, so rainfall is less likely (
Figure 7.2
).
Conversely, air moving towards the equator becomes
warmer as it meets warmer surfaces. As we saw in
Chapter
4,
warming of the lower layers of the air steepens the lapse
rate, making the air less stable and convectional showers
more likely. With instability, the process of transformation
of the air mass is more rapid.
Changes in air masses by these means are particu-
larly marked in the mid-latitudes. Here, depressions
draw in air from several sources; the air is modified by the
new surfaces it encounters and is gradually mixed as it
rises around the cyclone centre. Its precise thermal
properties will depend upon the origin of the air, its
track and the speed of its movement from the source
area.
Hadley cell circulation of the tropics, and hence act to
link the upper and lower atmosphere. As air descends it
gets warmer and drier (
Figure 7.3
),
but in these regions
its descent is restricted by the layer of cool oceanic air
below. The result is a semi-permanent inversion. This
combination of circumstances gives rise to very stable
atmospheric conditions, reducing the possibility of
precipitation. Even over heated desert surfaces the effects
of subsidence dominate, so these anticyclonic belts are
associated with the main dry zones of the world.
In the middle latitudes, anticyclones often develop as
a result of convergence in the upper westerlies, particularly
where the waves in those westerlies have a large amplitude.
The surface anticyclones may intensify within the usual
depression tracks, diverting the cyclones from their
normal routes and giving rise to exceptional patterns of
weather.
Blocking anticyclones
, as they are called, are most
frequent over north-west Europe and the north Pacific.
Blocking in the Atlantic was responsible for the droughts
of the late 1980s and 1995 and the severe winter of
1978-79 over north-west Europe. Unfortunately we do
not yet know enough about the causes of blocking
anticyclones to predict their future development.
Anticyclones are normally associated with dry weather
and light winds. Clear skies or extensive cloud and very
warm or very cold conditions may occur. Which we get
depends upon the time of the year, the degree of moistness
TEMPERATE LATITUDES
Anticyclones
An anticyclone is a mass of relatively high pressure within
which the air is subsiding. The major anticyclonic
belts are in the subtropics, centred about 30° from the
equator. They are located under the descending arm of the
Initial lapse rate
Initial lapse rate
Ht.
Ht.
Effect of
surface
cooling
Effect of
surface
warming
Less
stable
More
stable
Temperature
Temperature
