Page 150 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

114 part I The Energy–atmosphere System
1. Give several examples of each type of heat transfer. Do you observe any of these processes on a daily basis?
■ Identify alternative pathways for solar energy on its way through the troposphere to Earth’s surface— transmission, scattering, refraction, and absorption— and review the concept of albedo (reflectivity).
The molecules and particles of the atmosphere may redirect radiation, changing the direction of the light’s movement without altering its wavelengths. This scattering represents 7% of Earth’s reflectivity, or albedo. Dust particles, pollut- ants, ice, cloud droplets, and water vapour produce further scattering. Some incoming insolation is scattered by clouds and atmosphere and is transmitted to Earth as diffuse radiation, the downward component of scattered light.
The speed of insolation entering the atmosphere changes as it passes from one medium to another; the change of speed causes a bending action called refraction. Mirage is a refraction effect in which an image appears near the horizon when light waves are refracted by layers of air at different temperatures (and consequently of dif- ferent densities). Reflection is the process in which a por- tion of arriving energy bounces directly back into space without reaching Earth’s surface. Albedo is the reflec- tive quality (intrinsic brightness) of a surface. Albedo can greatly reduce the amount of insolation that is available for absorption by a surface. We report albedo as the percent- age of insolation that is reflected. Earth and its atmosphere reflect 31% of all insolation when averaged over a year. Absorption is the assimilation of radiation by molecules of a substance, converting the radiation from one form to another—for example, visible light to infrared radiation.
scattering (p. 94) diffuse radiation (p. 94) refraction (p. 95) mirage (p. 95) reflection (p. 96) albedo (p. 96) absorption (p. 96)
2. What would you expect the sky colour to be at an altitude of 50 km? Why? What factors explain the lower atmosphere’s blue colour?
3. Define refraction. How is it related to daylength? To a rainbow? To the beautiful colours of a sunset?
4. List several types of surfaces and their albedo
values. What determines the reflectivity of a
surface?
5. Using Figure 4.6, explain the differences in albedo
values for various surfaces. Based on albedo alone, which of two surfaces is cooler? Which is warmer? Why do you think this is?
6. Define the concept of absorption.
■ Analyze the effect of clouds and aerosols on atmo- spheric heating and cooling, and explain the green- house concept as it applies to Earth.
Clouds, aerosols, and other atmospheric pollutants have mixed effects on solar energy pathways, either cooling or heating the atmosphere. Global dimming describes the
decline in sunlight reaching Earth’s surface owing to pol- lution, aerosols, and clouds and is perhaps masking the actual degree of global warming.
Carbon dioxide, water vapour, methane, and other gases in the lower atmosphere absorb infrared radiation that is then emitted to Earth, thus delaying energy loss to space—this process is the greenhouse effect. In the atmo- sphere, longwave radiation is not actually trapped, as it would be in a greenhouse, but its passage to space is de- layed (heat energy is detained in the atmosphere) through absorption and reradiation by greenhouse gases.
Cloud-albedo forcing is the increase in albedo, and therefore in reflection of shortwave radiation, caused by clouds, resulting in a cooling effect at the surface. Also, clouds can act as insulation, thus trapping longwave ra- diation and raising minimum temperatures. An increase in greenhouse warming caused by clouds is cloud- greenhouse forcing. Clouds’ effects on the heating of the lower atmosphere depend on cloud type, height, and thickness (water content and density). High-altitude, ice- crystal clouds reflect insolation, producing a net cloud greenhouse forcing (warming); thick, lower cloud cover reflects about 90%, producing a net cloud albedo forcing (cooling). Jet contrails, or condensation trails, are pro- duced by aircraft exhaust, particulates, and water vapour and can form high cirrus clouds, sometimes called false cirrus clouds.
global dimming (p. 97) greenhouse effect (p. 98) greenhouse gases (p. 98) cloud-albedo forcing (p. 98) cloud-greenhouse forcing (p. 98) jet contrails (p. 98)
7. What is the effect of aerosols on heating and cooling of Earth’s atmosphere?
8. What are the similarities and differences between an actual greenhouse and the gaseous atmospheric greenhouse? Why is Earth’s greenhouse effect changing?
9. What role do clouds play in the Earth–atmosphere radi- ation balance? Is cloud type important? Compare high, thin cirrus clouds and lower, thick stratus clouds.
10. In what ways do jet contrails affect the Earth– atmosphere balance? Describe some recent scientific findings.
■ Review the Earth–atmosphere energy balance and the patterns of global net radiation.
The Earth–atmosphere energy system naturally balances itself in a steady-state equilibrium. It does so through energy transfers that are nonradiative (convection, con- duction, and the latent heat of evaporation) and radiative (longwave radiation travelling between the surface, the atmosphere, and space).
In the tropical latitudes, high insolation angle and consistent daylength cause more energy to be gained than lost, producing energy surpluses. In the polar regions, an extremely low insolation angle, highly reflective surfaces, and up to 6 months of no insolation annually cause more energy to be lost, producing energy deficits. This imbalance of net radiation from tropical surpluses to