Page 130 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

94 part I The Energy–atmosphere System Energy Pathways and Principles
Insolation, or incoming solar radiation, is the single en- ergy input driving the Earth–atmosphere system, yet it is not equal at all surfaces across the globe (Figure 4.3). Consistent daylength and high Sun altitude produce fairly consistent insolation values (about 180–220 watts per square metre; W·m−2) throughout the equatorial and tropical latitudes. Insolation decreases toward the poles, from about 25° latitude in both the Northern and the Southern Hemispheres. In general, greater insolation at the surface (about 240–280 W·m−2) occurs in low-latitude deserts worldwide because of frequently cloudless skies. Note this energy pattern in the subtropical deserts in both hemispheres (for example, the Sonoran desert in the American southwest, the Sahara in North Africa, and the Kalahari in South Africa).
Scattering and Diffuse Radiation Insolation encoun- ters an increasing density of atmospheric molecules as it travels toward Earth’s surface. These atmospheric gases, as well as dust, cloud droplets, water vapour, and pollut- ants, physically interact with insolation to redirect ra- diation, changing the direction of the light’s movement without altering its wavelengths. Scattering is the name for this phenomenon, which accounts for a percentage of the insolation that does not reach Earth’s surface but is instead reflected back to space.
Incoming energy that reaches Earth’s surface after scat- tering occurs is diffuse radiation (labeled in Figure 4.1). This weaker, dispersed radiation is composed of waves travelling in different directions, and thus casts shadow- less light on the ground. In contrast, direct radiation travels in a straight line to Earth’s surface without being scattered or otherwise affected by materials in the atmosphere. (The values on the surface-insolation map in Figure 4.3 combine both direct and diffuse radiation.)
Have you wondered why Earth’s sky is blue? And why sunsets and sunrises are often red? These com- mon questions are answered using a principle known as Rayleigh scattering (named for English physicist Lord Rayleigh, 1881). This principle applies to radiation scat- tered by small gas molecules and relates the amount of scattering in the atmosphere to wavelengths of light— shorter wavelengths are scattered more, longer wave- lengths are scattered less.
Looking back to Chapter 2, Figure 2.5, we see that blues and violets are the shorter wavelengths of visible light. According to the Rayleigh scattering principle, these wavelengths are scattered more than longer wave- lengths such as orange or red. When we look at the sky with the sun overhead, we see the wavelengths that are scattered the most throughout the atmosphere. Although both blues and violets are scattered, our human eye per- ceives this colour mix as blue, resulting in the common observation of a blue sky.
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▲Figure 4.3 Insolation at Earth’s surface. average annual solar radiation received on a horizontal surface at ground level in watts per square metre (100 W·m−2 = 75 kcal·cm−2·yr−1). [Based on M. i. Budyko, The Heat Balance of the Earth’s Surface (Washington, DC: U.S. Department of Commerce, 1958).]
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