Page 142 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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part I The Energy–atmosphere System
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▲Figure 4.15 Global net radiation at Earth’s surface. Distribution of mean annual net radiation (nET R) at the surface in watts per square metre (100 W·m−2 = 75 kcal·cm−2·yr−1). The highest net radiation, 185 W/m2 per year, occurs north of the equator in the arabian Sea. Tem- peratures for the five cities noted on the map are graphed in Figure 5.5. [Based on M. i. Budyko, The Heat Balance of the Earth’s Surface (Washington, DC: U.S. Department of Commerce, 1958), p. 106.]
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Areas of inadequate data
a global scale, average annual net radiation is positive over most of Earth’s surface (Figure 4.15). Negative net radiation values probably occur only over ice-covered surfaces poleward of 70° latitude in both hemispheres. Note the abrupt differences in net radiation between ocean and land surfaces on the map. The highest net radiation value, 185 W · m−2 per year, occurs north of the equator in the Arabian Sea. Aside from the obvious interruptions caused by landmasses, the pattern of values appears generally zonal, or parallel, decreasing away from the equator.
The seasonal rhythm of net radiation throughout the year influences the patterns of life on Earth’s surface. Sea- sonal net radiation shifts between the solstice months of December and June and the equinoxes, with net radiation gains near the equator at the equinoxes shifting toward the South Pole during the December solstice and toward the North Pole during the June solstice.
net Radiation Expenditure As we have learned, in order for the energy budget at Earth’s surface to balance over time, areas that have positive net radiation must somehow dissipate, or lose, heat. This happens through nonradiative processes that move energy from the ground into the boundary layer.
• The latent heat of evaporation (LE) is the energy that is stored in water vapour as water evaporates. Water absorbs large quantities of this latent heat as it changes state to water vapour, thereby removing this heat energy from the surface. Conversely, this heat en- ergy releases to the environment when water vapour
changes state back to a liquid (discussed in Chapter 7). Latent heat is the dominant expenditure of Earth’s entire NET R, especially over water surfaces.
• Sensible heat (H) is the heat transferred back and forth between air and surface in turbulent eddies through convection and conduction within materials. This activ- ity depends on surface and boundary-layer temperature differences and on the intensity of convective motion in the atmosphere. About one-fifth of Earth’s entire NET R is mechanically radiated as sensible heat from the sur- face, especially over land. The bulk of NET R is expended as sensible heat in these dry regions.
• Ground heating and cooling (G) is the flow of energy into and out of the ground surface (land or water) by conduction. During a year, the overall G value is zero because the stored energy from spring and summer is equaled by losses in fall and winter. Another factor in ground heating is energy absorbed at the surface to melt snow or ice. In snow- or ice-covered landscapes, most available energy is in sensible and latent heat used in the melting and warming process.
On land, the highest annual values for LE occur in the tropics and decrease toward the poles. Over the oceans, the highest LE values are over subtropical lati- tudes, where hot, dry air comes into contact with warm ocean water. The values for H are highest in the sub- tropics. Here vast regions of subtropical deserts feature nearly waterless surfaces, cloudless skies, and almost vegetation-free landscapes. Through the processes of la- tent, sensible, and ground heat transfer, the energy from
Antarctic Circle
BAY OF SEA
ARABIAN
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ROBINSON PROJECTION 60°
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Satellite
Global Net Radiation
1. Salvador, Brazil
2. New Orleans, Louisiana 3. Edinburgh, Scotland
4. Montréal, Canada
5. Barrow, Alaska
(see Figure 5.5)