Geoscience Reference
In-Depth Information
2
Earth is one of the smaller of the eight major planets
forming the solar system. Each planet is distinct in terms
of its physical geography. The different distances of the
planets from the sun, their different sizes and composition
ensure that each world is unique. They all depend for
virtually all their energy upon the nearest star, which we
call the sun. A basic understanding of what energy is, how
it moves and how it can be transformed is required. In this
chapter we examine the nature of energy and how it is
emitted by the sun. Variations in sun-Earth relations
create changes in the pattern and distribution of energy
at the top of our atmosphere. Finally we study the
mechanisms that carry this energy to all parts of Earth's
global system. No further reference will be made to the
other planets but similar physical laws operate there.
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 2.1
)
- varies considerably, so that solar
radiation comprises a range of electromagnetic wave-
shortest waves, called gamma rays, carry most energy,
whilst radio waves carry least. Only a small 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 distance from the sun to Earth of about
150 million km, it takes this energy about 8·3 minutes to
reach us.
When solar radiation is absorbed, it is converted from
radiant to other forms of energy. Much is altered to
thermal
(
heat
)
energy
. In reality, it warms Earth's surface
and the atmosphere by exciting the molecules of which
they are composed. In simple terms, the radiant 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 with nuclear (or atomic) energy. Neither of
these has any great significance with regard to environ-
mental 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-
wavelength
(
Q
)
crest to the next crest or from one trough to the next trough
is known as the wavelength (
, Greek, lambda). It is an
important indicator of the properties of the electromagnetic
radiation.
