Page 293 - Fundamentals of Air Pollution

I. Sun, Atmosphere System, and Heat Balance 249

D. Outgoing Longwave Radiation
Because most ultraviolet radiation is absorbed from the solar spectrum
and does not reach the earth's surface, the peak of the solar radiation which
reaches the earth's surface is in the visible part of the spectrum. The earth
reradiates nearly as a blackbody at a mean temperature of 290 K. The
resulting infrared radiation extends over wavelengths of 3-80 /am, with a
peak at around 11 /am. The atmosphere absorbs and reemits this longwave
radiation primarily because of water vapor but also because of carbon
dioxide in the atmosphere. Because of the absorption spectrum of these
gases, the atmosphere is mostly opaque to wavelengths less than 7 /am
and greater than 14 /am and partly opaque between 7 and 8.5 /am and
between 11 and 14 /am. The atmosphere loses heat to space directly through
the nearly transparent window between 8.5 and 11 /am and also through
the absorption and successive reradiation by layers of the atmosphere con-
taining these absorbing gases.
Different areas of the earth's surface react quite differently to heating
by the sun. For example, although a sandy surface reaches fairly high
temperatures on a sunny day, the heat capacity and conductivity of sand
are relatively low; the heat does not penetrate more than about 0.2-0.3 m
and little heat is stored. In contrast, in a body of water, the sun's rays
penetrate several meters and slowly heat a fairly deep layer. In addition,
the water can move readily and convection can spread the heat through a
deeper layer. The heat capacity of water is considerably greater than that
of sand. All these factors combine to allow considerable storage of heat in
water bodies.

E. Heat Balance
Because of the solar beam's more direct angle of incidence in equatorial
regions, considerably more radiation penetrates and is stored by water near
the equator than water nearer the poles. This excess is not compensated
for by the outgoing longwave radiation, yet there is no continual buildup
of heat in equitorial regions. Figure 17-3 shows the annual mean incoming
and outgoing radiation averaged over latitude bands. There is a transfer
of heat poleward from the equatorial regions to make up for a net outward
transfer of heat near the poles. This heat is transferred by air and ocean
currents as warm currents move poleward and cool currents move equa-
torward. Considerable heat transfer occurs by the evaporation of water in
the tropics and its condensation into droplets farther poleward, with the
release of the heat of condensation. Enough heat is transferred to result in
no net heating of the equatorial regions or cooling of the poles. The pole-
ward flux of heat across various latitudes is shown in Table 17-3.
Taking the earth as a whole over a year or longer, because there is no
appreciable heating or cooling, there is a heat balance between the incoming