Environmental Engineering Reference
In-Depth Information
recorded, implying that solar activity was low. Recently the number of sunspots has been
much higher, associated with a more active sun. Global temperatures have increased
during this period and it has been argued that changes in solar activity have a significant
effect on global temperatures
amounts are received through reflection from the moon, and extraterrestrial material
passes into the atmosphere and even down to the surface as meteorites, but we can ignore
these contributions. Just as the input of energy is dominated by radiant energy from the
sun, so the output of energy from Earth is almost entirely radiant energy, although this
time with somewhat different properties. Much of the energy has been radiated or emitted
by Earth and its atmosphere, but some is solar radiation reflected from clouds or from
Earth's surface without any major modification. As the overall energy level of Earth is
not changing, we can assume that there must be a balance between the energy input to
and energy output from the globe as a whole.
CONCEPTS OF ENERGY
Before discussing the quantities of energy received by Earth, it is necessary to consider,
briefly, the nature of energy and the ways we measure it. Energy exists in a variety of
forms. We are familiar with electrical energy in the home and increasingly with nuclear
(or atomic) energy. Neither of these has any great significance with regard to
environmental processes. More important as far as environmental processes are
concerned are radiant, thermal, kinetic, chemical and potential energy.
Radiant energy is the most relevant to our discussion here, for it is in this form that
the sun's energy is transmitted to Earth. The heat from the sun excites or disturbs
electrical and magnetic fields, setting up a wave-like activity in space, known as
electromagnetic radiation. We can think of this radiation as streams of particles or
photons. The length of the waves - that is, their distance apart (Figure 2.2) - varies
considerably, so that solar radiation comprises a range of electromagnetic wavelengths
from 0·2 µm to 5·0 µm (Figures 2.3 and 2.7). The shortest waves, called gamma rays,
carry most energy, whilst radio waves carry least. Only part of the spectrum is visible to
the human eye, reaching us as light, but all waves transmit some energy from the sun to
Earth. Assuming a mean
Figure 2.2 Electromagnetic radiation: the distance from one
crest to the next or from one trough to the next is known as
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