In this second part of the book we will consider how energy moves in the atmosphere. In particular, we discuss three forms of energy— radiation (discussed in this chapter), 'sensible heat' (Chapter 3), and energy absorbed in evaporating water (Chapter 4). They are connected by what is called the 'energy balance', considered in Chapter 5. We begin by discussing the radiant energy from the Sun.
It is not obvious how solar energy reaches the Earth because the intervening distance of about 150 million kilometres is practically a vacuum. So the Sun's heat is not carried in a wind, nor is conduction possible. Instead, the energy is transferred by electromagnetic radiation (Note 2.A). Such radiation differs from sound, which requires air to carry the vibrations, but is similar in being characterised by the velocity and wavelengths involved. The radiation travels at a speed close to 3X108 m/s (in vacuum) and therefore takes 8.3 minutes to travel from the Sun to the Earth. The wavelengths of this radiation form a segment of the entire range (i.e. spectrum)
of electromagnetic radiation, which spans radio to cosmic rays (Figure 2.1).
In this chapter, we are concerned especially with visible light and radiant heat. Light has wavelengths of less than a millionth of a metre. (This is called a micrometre or micron, with the symbol 'pm'. Sometimes the unit used is a nanometre, written 'nm', equal to a thousandth of a micron.) On the other hand, radiation from bodies at the much lower Earth temperatures has wavelengths about twenty times as great, the difference reflecting the ratio of the temperatures of the Sun and Earth, effectively 5,770 and nominally 288 degrees Kelvin, respectively.
The radiation from a body involves a range of wavelengths, and the body's temperature determines both the dominant wavelength (given by Wien's Law, see Note 2.B) and the overall rate of energy emission, given by the Stefan—Boltzmann equation (Note 2.C). The consequence is that a diagram of the amounts of energy of various wavelengths from each of the Sun and the Earth shows different humps, the one for solar radiation being higher and to the left, since the Sun is hotter (Figure 2.2). Its
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