Environmental Engineering Reference
In-Depth Information
Plate 6.2
Streaks indicating surface flow patterns in a
rotating fluid subject to heating at the outer side wall and
cooling at the inner side wall. At low rates of rotation (left)
the flow is symmetrical about the axis of rotation. As the
rotation rate increases (centre) the flow develops jet streams
and waves. At higher rates (right) the flow is highly irregular,
with a resemblance to the cyclonic and anticyclonic eddies
found in the westerly circulation. Photo: courtesy of Dr R.
Hide.
(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 small (Figures 3.8 and 3.10). 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 6.3). 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
