Page 314 - Fundamentals of Air Pollution 3E

270 17. The Physics of the Atmosphere

terminology of winds, which uses the direction from which they come,
these near-surface winds are referred to as easterlies (Fig. 17-26), also called
trade winds. Since most of the earth's surface near the equator is ocean,
these winds absorb heat and moisture on their way to the equator.
Where the trade winds from each hemisphere meet is a low-pressure
zone, the intertropical convergence zone. This zone of light winds or doldrums
shifts position with the season, moving slightly poleward into the summer
hemisphere. The rising air with high humidity in the convective motions
of the convergence zone causes heavy rainfall in the tropics. This giant
convective cell, or Hadley cell, absorbs heat and the latent heat of evapora-
tion at low levels, releasing the latent heat as the moisture condenses in
the ascending air. Some of this heat is lost through infrared radiation from
cloud tops. The subsiding air, which warms adiabatically as it descends in
the vicinity of 30° latitude (horse latitudes), feeds warm air into the mid-
latitudes. Although the position of the convergence zone shifts somewhat
seasonally, the Hadley cell circulation is quite persistent, resulting in a
fairly steady circulation.

B. Mid-Latitudes

Because at higher latitudes the coriolis force deflects wind to a greater
extent than in the tropics, winds become much more zonal (flow parallel
to lines of latitude). Also in contrast to the persistent circulation of the
tropics, the mid-latitude circulations are quite transient. There are large
temperature contrasts, and temperature may vary abruptly over relatively
short distances (frontal zones). In these regions of large temperature con-
trast, potential energy is frequently released and converted into kinetic
energy as wind. Near the surface there are many closed pressure sys-

Fig. 17-26. Near-surface winds for various latitude belts.