Environmental Engineering Reference
In-Depth Information
late 1980s. Wild et al. (2005) put it at 0.68 W/m
2
per
year for the clear-sky flux, and Pinker, Zhang, and Dut-
ton (2005) used satellite records to compute an overall
annual rise of 01.6 W/m
2
irradiance at the top of the atmosphere. Total insolation
is primarily the function of cloudiness: the highest values,
in excess of 250 W/m
2
, are in the cloudless high-
pressure belts of subtropical deserts (fig. 2.3). A fairly
regular poleward decline is unmistakable, as are the rela-
tively low values in the tropics: insolation in the U.S.
Corn Belt, the Great Plains, or the Southeast is higher
than throughout most of southern Nigeria and the Ama-
zon Basin. But tropics have little seasonal or daily varia-
tion. Manaus on the Amazon (3
S) has a daily average
of 195 W/m
2
, with the highest values in August just
16% above, and the lowest ones in April 13% below, the
mean. Ames, Iowa (42
N) averages 167 W/m
2
, just 15%
less than Manaus, but its December flux is 41% below,
and its July insolation 55% above, the mean.
Implications for agriculture are obvious: plants matur-
ing in two to six months will, when provided with ade-
quate moisture and nutrients, perform no worse, and
those requiring long days will do much better, in temper-
ate regions. Radiative impoverishment of the tropics is
seen even in the peak midday fluxes. Jakarta's 550-580
W/m
2
is no different from the summer fluxes in Edmon-
ton or Yakutsk. Local cloudiness can also create large dif-
ferences within short distances. Oahu's Koolau Range
has an annual mean of 150 W/m
2
, and the Pearl Har-
bor, just 15 km downwind, averages 250 W/m
2
. Insola-
tion averages just short of 170 W/m
2
for the oceans and
about 180 W/m
2
on land, a flow totaling about 88 PW.
Measurements of surface solar radiation found that this
flux was decreasing at 0.23%-0.32%/a between 1958
and 1992, and cloud changes, anthropogenic aerosols,
and volcanic eruptions were suggested as the major cause
of this global dimming, which amounted to 6-9 W/m
2
over land (Pinker, Zhang, and Dutton 2005). This trend
did not persist; brightening has been observed since the
(0.1%) between 1983 and
2001.
All radiation that is absorbed by the Earth's diverse
surfaces must be returned to space in order to maintain
planetary thermal equilibrium. The most notable charac-
teristic of this outgoing longwave (LW) radiation is its
shifted wavelength. Whereas the Sun radiates as a black-
body of about 5800 K with most of the flux between 0.1
mm and 4 mm and the peak at 500 nm, the terrestrial ra-
diation ranges between 4 mm and 100 mm with the peak
flux at 10 mm, far into the IR zone. If the Earth were a
perfect blackbody radiator, its effective temperature (T
E
)
would be simply a function of its albedo (a) and its or-
bital distance (a expressed in AU)
T
E
¼
278
ð
1
a
Þ
0
:
25
a
0
:
5
;
and it would radiate at 255 K, the temperature that
would leave all of its water frozen. This is not the case,
because several atmospheric gases selectively absorb some
of the outgoing radiation and reradiate it both down-
ward and upward.
This absorption of IR radiation is commonly known
as the greenhouse effect, and Svante Arrhenius (1896)
published its first detailed elucidation. In the absence of
water vapor on the Earth's two neighbors, it is the pre-
sence of CO
2
that generates a greenhouse gas (GHG)
effect—a very strong one on Venus, resulting in average
surface temperature of 750 K (477
C), and a very weak
one on Mars (surface at 220 K). Thanks to GHG absorp-
tion the Earth's actual average surface temperature of
