Environmental Engineering Reference
In-Depth Information
Now, we know that there are places on earth such as arctic regions that
rarely if ever reach 15° C and other places such as equatorial tropical regions
and deserts that routinely exceed this temperature.What scientific value,
then, is there to an estimate of 15° C? There are two values. First, this tem-
perature represents a baseline
average
for the entire planet.This average ac-
counts for regional temperatures that may fall below or exceed this value.
Second, the recognition that this is an average and not applicable to the en-
tire globe generates questions about why there are regional differences in
temperature to begin with.The answer lies again in considering the nature
of the sun as producer and emitter of radiant energy and the earth as ab-
sorber of that energy.
Like the sun, the earth is a ball.The earth is, however, far enough from
the sun that all beams of the sun's radiation can be considered to be paral-
lel to each other.The combination of parallel incoming radiation and the
curvature of the earth results in differential intensities of heat radiation that
strike the earth's surface at different locations. If we measured the amount
of solar radiation striking a fixed-size plot near the equator and at the pole,
we find that the intensity of radiation striking the plot near the equator is
far higher than that striking the same-size plot near the pole.That is to say,
the intensity of radiation striking a unit area is far higher at the equator than
at the pole (figure 3.1). As a consequence, there is less energy to heat up a
unit area at the pole than at the equator. Less energy absorbed means lower
realized temperatures.
The differential thermal regimes when coupled with the earth's rotation
then generate prevailing winds and ocean currents.Warm water or air flows
to colder regions; colder water or air circulates back to warmer regions.This
circulation pattern, coupled with the earth's geographic formations (e.g.,
mountains, flatland, etc.), determines the kind and amount of precipitation
that occurs at different geographic locations.
Ecosystem Types
The combination of temperature and moisture determine patterns in the
distribution of life on earth. A method to quantify pattern in global life
zones or ecosystem types using such climatic data was developed by
Holdridge (1947).These ecosystem types range from tundra and taiga in
polar and alpine regions, temperate deciduous and coniferous forests in the
northern and southern latitudes, tropical rainforest, savanna grasslands, and
deserts in the midlatitudes. Figure 3.2 demonstrates that different regions
