The atmosphere in action - Fundamentals of the Physical Environment - page 117

changes constantly, although there is a tendency towards
concentration in certain zones in the northern hemi-
sphere. Labrador, Newfoundland and Greenland are
associated with these areas of activity, experiencing cool,
southward-moving flows of air ( Figure 6.14 ). Britain,
western Canada and Scandinavia, in contrast, tend to be
influenced far more by warm northward-moving air, a
phenomenon that greatly improves their climate.
All these transfers of energy through the atmosphere
are highly variable, and major differences in the intensity
and character of transfers occur over time. Thus the flows
of energy represent net increments, often produced by
individual, temporary processes. It is for this reason that
it is difficult to detect the nature of energy transfer direct
from the general circulation pattern.

compared with 60 C in the southern hemisphere) and so
the driving force of the winds - the pressure gradient - is
reduced.

ENERGY TRANSFER IN THE
ATMOSPHERE

The pattern of energy transfer in the atmosphere is
complex, and we can consider here only some of the
general components of the pattern. As a starting point, let
us look at the simplified model of what happens in the
tropics ( Figure 6.12 ).
The circulation within the tropics consists of two cells.
Air blows in towards the low-pressure belt of the equator
(the equatorial trough) across the subtropical seas. As it
does so evaporation of water from the ocean utilizes vast
quantities of energy so that the sensible heat transfer to
the atmosphere is often smal l ( Figures 3.10 and 3.11 ). The
trade winds approaching the equator rise as they meet the
equatorial trough, creating a cloudy zone which can often
be seen on satellite images ( Plate 6.2 ). The ascent of this
air is not a continuous, widespread phenomenon, but
occurs mainly in association with localized, often intense
and short-lived updraughts such as in thunderstorms. As
the air rises and cools, the water vapour condenses and
releases latent heat. The increased height of the air also
represents an increase of potential energy.
The equatorial air then diverges and flows polewards,
so the potential energy is exported to higher latitudes. The
cycle is completed as radiational cooling causes subsidence
of the air. In the process the air dries and warms as the
potential energy is converted to sensible heat. It also
checks the rise of convection currents in these subtropical
desert areas, producing cloudless skies. Over these arid
areas very little evaporation occurs, energy loss is limited
and the incoming radiation heats the ground surface,
which then heats the atmosphere. Thus much more of the
energy is in the form of sensible heat. During the night
this energy is reradiated back to space, for the dry air is
unable to intercept much outgoing long-wave radiation.
As a result, the net surplus of radiation is fairly small.
In temperate and polar areas the processes of energy
transfer are more difficult to decipher. There is no general,
cellular circulation of air, as in the tropics, but instead a
complicated pattern in which individual, rotating storms
play an important part. Within these storms warm air
masses rise, releasing latent heat and gaining potential
energy. They then become intermixed with descending
cold air and gain sensible heat ( Figure 6.12 ). The rotating
storms are moving, so the position of this intermixing

WIND PATTERNS

The general circulation of the atmosphere reflects the
operation of the atmospheric system as a whole. It is
clear, however, that the system is composed of many
important subsystems and it is these - the main wind belts

Labrador Current

Cold ocean
water

Surface low
forming

Warm ocean
current

Gulf
Stream

Figure 6.14 An area of frequent depression formation along
the steep temperature gradient between the Labrador current
and the Gulf Stream off the north-eastern United States.