Environmental Engineering Reference
In-Depth Information
zone in which most weather systems develop,
and run their course. These factors, together
with the high level of human intervention in this
part of the atmosphere, ensure that many of the
global environmental problems of current
concern—including acid rain, atmospheric
turbidity and global warming—have their
origins or reach their fullest extent in the
troposphere.
The tropopause marks the upper limit of the
troposphere. Beyond it, in the stratosphere,
isothermal conditions prevail; temperatures
remain constant, at or about the value reached
at the tropopause, up to an altitude of about 20
km. Above that level the temperature begins to
rise again reaching a maximum some 50 km
above the surface, at the stratopause, where
temperatures close to or even slightly above 0°C
are common. This is caused by the presence of
ozone, which absorbs ultraviolet radiation from
the sun, and warms the middle and upper levels
of the stratosphere, creating a temperature
inversion. The combination of that inversion
with the isothermal layer in the lower
stratosphere creates very stable conditions, and
the stratosphere has none of the turbulence
associated with the troposphere. This has
important implications for the environment.
Any foreign material introduced into the
stratosphere tends to persist there much longer
than it would if it had remained below the
tropopause. Environmental problems such as
ozone depletion and atmospheric turbidity are
aggravated by this situation. Much of the
impact of nuclear winter would result from the
penetration of the tropopause by the original
explosions, and the consequent introduction of
large volumes of pollutants into the
stratosphere.
Temperatures again decrease with height
above the stratopause and into the mesosphere,
falling as low as -100°C at the mesopause, some
80 km above the surface. The thermosphere
stretches above this altitude, with no obvious
outer limit. In this layer, temperatures may
exceed 1,000°C, but such values are not
directly comparable to temperatures in the
stratosphere and troposphere, because of the
rarified nature of the atmosphere at very high
altitudes. Knowledge of the nature of the
thermosphere, and its internal processes, is far
from complete, and linkages between the upper
and lower layers of the atmosphere remain
speculative. For the climatologist and
environmentalist the most important structural
elements of the atmosphere are the troposphere
and stratosphere. The main conversion and
transfer of energy in the earth/atmosphere
system takes place within these two layers of
the lower atmosphere, and interference with
the mechanisms involved has contributed to the
creation and intensification of current
problems in the atmospheric environment.
THE EARTH'S ENERGY BUDGET
Virtually all of the earth's energy is received from
the sun in the form of short-wave solar radiation,
and balancing this inflow is an equivalent amount
of energy returned to space as long-wave
terrestrial radiation. The concept is a useful one,
but it applies only to the earth as a whole, over
an extended time scale of several years; it is not
applicable to any specific area over a short period
of time. This balance between incoming and
outgoing radiation is referred to as the earth's
energy budget.
The earth intercepts only a small proportion
of the total energy given out by the sun—
perhaps as little as one five-billionth (Critchfield
1983)—and not all of that reaches the earth's
surface. The estimates differ in detail, but it is
generally accepted that only about 50 per cent
of the solar radiation arriving at the outer edge
of the atmosphere is absorbed at the surface
(Lutgens and Tarbuck 1982). That proportion is
split almost equally between direct solar
radiation and diffuse radiation, which has been
scattered by water vapour, dust and other
aerosols in its passage through the atmosphere
(see Figure 2.5). Of the other 50 per cent, some
30 per cent returns to space as a result of
reflection from the land and sea, reflection from
clouds or scattering by atmospheric aerosols.
