Environmental Engineering Reference
In-Depth Information
In both air and sea, however, the transfers are not oneway. If that were the case, we
would be faced with a buildup of air and water in the higher latitudes and a slow
emptying of the tropical areas. Clearly this does not happen; the warm air and ocean
currents that flow towards the poles are replaced by a counter-flow of cooler air and
water moving equatorwards. In the case of the sea, the flow tends to occur at depth, for
the cool water sinks. In the air, the pattern is a little more complex. The transfer of latent
heat (that is, energy tied up within water vapour) occurs mainly in the lowest 2 km or 3
km of the atmosphere. It is closely related to the surface wind network. The sensible heat
transfer (that is, of warm air masses) occurs both close to the ground surface and also at
high altitudes (around 10 km). Both flows, however, are balanced by counter-flows of
cooler air from higher latitudes.
Thus the three main processes of energy transfer at a global scale are:
1 The horizontal transfer of sensible heat by warm air masses.
2 The transfer of latent heat in the form of atmospheric moisture.
3 The horizontal convection of sensible heat by ocean currents.
Of these three processes, the first and third are most important, each accounting for about
40 per cent of the total annual energy flow. However, methods used to calculate the
values are imprecise and there is considerable spatial variability to complicate matters.
We shall consider the detailed processes involved in these transfers in Chapter 3, and
will see there the factors that lead to the spatial distribution of these transfer
Figure 2.15
The average annual latitudinal distribution of
the components of the poleward energy transfer in the earth -
atmosphere system.
Source: After Sellers (1965).
mechanisms, but it is worth noting here that marked latitudinal variations in the three
processes occur. Sensible heat transfers by the atmosphere, for example, are at a
