Environmental Engineering Reference
In-Depth Information
Kinetic energy is proportional to the square of the velocity of the wind ( V 2 /2).
Therefore strong winds have more kinetic energy than gentle winds, as the damage they
cause indicates. In fact, on a global scale, hurricanes and other strong winds at the surface
are relatively rare, so the quantity of energy in the form of kinetic energy is limited. Even
in the regions of strongest winds it probably reaches no more than 0ยท5 per cent of the total
energy content of the atmosphere.
The actual flows of energy can be shown more simply if we consider them as part of a
large system in which we distinguish the inputs and outputs with feedback between the
different subsystems (Figure 3.11 for radiant energy). The energy is transferred in a
variety of forms and, during these transfers, it undergoes numerous transformations.
While the general principles of flow are known, the figures quoted are, in most cases,
best estimates. Measurements have been taken at a number of places, but in insufficient
quantities to give a reliable global figure. It is little use giving a global average based on a
few clustered observations. This has been one of the problems in determining the
magnitude of any 'enhanced' greenhouse effect. Satellite observations have helped (and
led to appreciable changes in estimates of Earth's albedo) but there are still numerous
flows which are imperfectly known. Long-wave emission by the atmosphere, the
separation into direct and diffuse radiation and sensible and latent heat transfer are the
main problems, as conditions vary quickly, and, until measurements become more
comprehensive, some of the figures are little more than intelligent guesses. The actual
value of the flows will depend, in part, on the nature of the assumptions made about
them. What we can be sure about, both theoretically and from satellite measurements, is
that what energy comes into the earth/atmosphere system must eventually leave .
EFFECTS UPON TEMPERATURE
Let us now consider briefly the effects that these energy inputs and outputs have upon
temperature.
THE DAILY PATTERN
If we consider a clear day in the spring in an area in, say, London, sunrise will be at about
6.00 a.m. local time. Temperatures then are low, for during the night the ground has been
losing
heat
by
radiational
cooling.
Slowly,
as
the
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