Page 87 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 2 solar energy to earth and the seasons 51
OCEAN
-20 -20
OCEAN INDIAN OCEAN
0 1500 3000 KILOMETRES ROBINSON PROJECTION
-114 L
▲Figure 2.10 Daily net radiation patterns at the top of the atmosphere. Averaged daily net radiation flows measured at the top of the atmosphere by the earth radiation Budget experiment (erBe). Units are W·m−2. [Data for map courtesy of gsFC/nAsA.]
ARCTIC
ATLANTIC OCEAN
PACIFIC PACIFIC H H OCEANH
77.4 88.0 88.3
< -100
-100 to -80
-80 to -60
-60 to -40
-40 to -20
-20 to 0
0 to 20
20 to 40
40 to 60
60 to 80
>80
slight maximum-insolation periods of approximately 430 W·m−2 occur at the spring and fall equinoxes, when the subsolar point is at the equator.
Global Net Radiation Figure 2.10 shows patterns of net radiation, which is the balance between incoming short- wave energy from the Sun and all outgoing radiation from Earth and the atmosphere—energy inputs minus energy outputs. The map uses isolines, or lines connect- ing points of equal value, to show radiation patterns. Fol- lowing the line for 70 W·m−2 on the map shows that the highest positive net radiation is in equatorial regions, es- pecially over oceans.
Note the latitudinal energy imbalance in net radia- tion on the map—positive values in lower latitudes and negative values toward the poles. In middle and high lati- tudes, poleward of approximately 36° N and S latitudes, net radiation is negative. This occurs in these higher lati- tudes because Earth’s climate system loses more energy to space than it gains from the Sun, as measured at the top of the atmosphere. In the lower atmosphere, these polar energy deficits are offset by flows of energy from tropical energy surpluses (as we see in Chapters 4 and 6). The larg- est net radiation values, averaging 80 W·m−2, are above the tropical oceans along a narrow equatorial zone. Net radia- tion minimums are lowest over Antarctica.
Of interest is the –20 W·m−2 area over the Sahara region of North Africa. Here, typically clear skies—which permit great longwave radiation losses from Earth’s surface—and light-coloured reflective surfaces work together to reduce
net radiation values at the thermopause. In other regions, clouds and atmospheric pollution in the lower atmosphere also affect net radiation patterns at the top of the atmo- sphere by reflecting more shortwave energy to space.
This latitudinal imbalance in energy (discussed in Chapter 4) is critical because it drives global circulation in the atmosphere and the oceans. Think of the atmosphere and ocean forming a giant heat engine, driven by differ- ences in energy from place to place that cause major circula- tions within the lower atmosphere and in the ocean. These circulations include global winds, ocean currents, and weather systems—subjects to follow in Chapters 6 and 8. As you go about your daily activities, let these dynamic natural systems remind you of the constant flow of solar energy through the environment.
Having examined the flow of solar energy to the top of Earth’s atmosphere, let us now look at how seasonal changes affect the distribution of insolation as Earth or- bits the Sun during the year.
The Seasons
Earth’s periodic rhythms of warmth and coolness, dawn and daylight, twilight and night, have fascinated humans for centuries. In fact, many ancient societies demon- strated an intense awareness of seasonal change and for- mally commemorated these natural energy rhythms with festivals, monuments, ground markings, and calen- dars (Figure 2.11). Such seasonal monuments and calen- dar markings are found worldwide, including thousands
-100
-90
-80
-110
-70
60
60
30 50
-40
-50
-10 10
70
80
-30
-20
0
20
40
70
80
70
80
60
50
40
30
10
-20 -40
-10 -30
-110
0
-80
-60
-90
-50 -70
70
-100
20
-60
-100