Page 85 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 2 solar energy to earth and the seasons 49
INPUT
Shortwave radiation Sun to Earth
• ultraviolet
• visible
• shortwave infrared
OUTPUT
Longwave radiation
Earth to space
• thermal infrared
▲Figure 2.7 Earth’s energy budget simplified.
object like the Sun emits a much greater amount of en- ergy per unit area of its surface than does a similar area of a cooler object like Earth. Shorter wavelength emis- sions are dominant at these higher temperatures.
Although cooler than the Sun, Earth also acts as a blackbody, radiating nearly all that it absorbs (also shown in Figure 2.6). Because Earth is a cooler radiating body, it emits longer wavelengths, mostly in the infrared portion of the spectrum, centred around 10.0 mm. Atmo- spheric gases, such as carbon dioxide and water vapour, vary in their response to radiation received, being trans- parent to some while absorbing others.
Figure 2.7 illustrates the flows of energy into and out of Earth systems. To summarize, the Sun’s radiated en- ergy is shortwave radiation that peaks in the short visi- ble wavelengths, whereas Earth’s radiated energy is long- wave radiation concentrated in infrared wavelengths. In Chapter 4, we see that Earth, clouds, sky, ground, and all things that are terrestrial radiate longer wavelengths in contrast to the Sun, thus maintaining the overall energy budget of Earth and atmosphere.
Incoming Energy at the Top
of the Atmosphere
The region at the top of the atmosphere, approximately 480 km above Earth’s surface, is the thermopause (see Figure 3.1). It is the outer boundary of Earth’s energy system and provides a useful point at which to assess the arriving solar radiation before it is diminished by scattering and absorption in passage through the atmosphere.
Earth’s distance from the Sun results in its intercep- tion of only one two-billionth of the Sun’s total energy output. Nevertheless, this tiny fraction of energy from the Sun is an enormous amount of energy flowing into Earth’s systems. Solar radiation that is intercepted by Earth is insolation, derived from the words incoming solar radia- tion. Insolation specifically applies to radiation arriving at Earth’s atmosphere and surface; it is measured as the rate of radiation delivery to a horizontal surface, specifi- cally, as watts per square metre (W·m−2).
Solar Constant Knowing the amount of insolation incoming to Earth is important to climatologists and other scientists. The solar constant is the average inso- lation received at the thermopause when Earth is at its average distance from the Sun, a value of 1372 W·m−2.* As we follow insolation through the atmosphere to Earth’s surface (Chapters 3 and 4), we see that its amount is reduced by half or more through reflection, scattering, and absorption of shortwave radiation.
Uneven Distribution of Insolation Earth’s curved surface presents a continually varying angle to the incoming paral- lel rays of insolation (Figure 2.8). Differences in the angle at which solar rays meet the surface at each latitude result in an uneven distribution of insolation and heating. The only point where insolation arrives perpendicular to the surface (hitting it from directly overhead) is the subsolar point.
During the year, this point occurs only at lower lat- itudes, between the tropics (about 23.5° N and 23.5° S), and as a result, the energy received there is more con- centrated. All other places, away from the subsolar point, receive insolation at an angle less than 90° and thus ex- perience more diffuse energy; this effect becomes more pronounced at higher latitudes. Remember the photo in this chapter’s Geosystems Now, Figure GN 2.2; the shad- ows are cast not at an angle but directly below the boys hauling water—this was May 1 at 14.8° N latitude.
The thermopause above the equatorial region receives 2.5 times more insolation annually than the thermopause above the poles. Of lesser importance is the fact that, be- cause they meet Earth from a lower angle, the solar rays ar- riving toward the poles must pass through a greater thick- ness of atmosphere, resulting in greater losses of energy due to scattering, absorption, and reflection.
*A watt is equal to 1 joule (a unit of energy) per second and is the standard unit of power in the International System of Units (SI). (See the conversion tables in Appendix D of this text for more information on measurement conversions.) In nonmetric calorie heat units, the solar constant is expressed as approximately 2 calories per square centimetre per minute, or 2 langleys per minute (a langley being 1 cal·cm−2). A calorie is the amount of energy required to raise the temperature of 1 gram of water (at 15°C) 1 degree Celsius and is equal to 4.184 joules.
Earth