WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–18
Figure 10: Dry-adiabatic and actual temperature variations with height as a function of time
of day, for clear skies.
inversion near the ground. This usually occurs only with clear skies, which allow the earth to
radiate its energy into space effectively.
When sunlight strikes the earth in the morning, the earth’s surface temperature rises
rapidly, producing the air temperature variations shown in Fig. 10a. The difference between
the actual air temperature and the adiabatic curve would normally reach its maximum before
noon, causing a relatively rapid heating of the air. There will be a strong convective flow of
air during this time because a parcel of air that is displaced upward will find itself warmer and
hence lighter than its surroundings. It is then accelerated upwards under hydrostatic pressure.
It will continue to rise until its temperature is the same as that of the adiabatic curve. Another
parcel has to move down to take the first parcel’s place, perhaps coming down where the earth
is not as effective as a collector of solar energy, and hence causes the atmosphere to be well
mixed under these conditions. This condition is referred to as an unstable atmosphere.
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001