Altering The Energy Balance

As examples of changing an energy balance, we will consider the effects of preventing evaporation at the surface, altering the ground's albedo, and differences due to the orientation of sloping ground.

A layer of oil on a lake greatly reduces evaporation, and a clear-plastic cover has the

Local time (hours)

Figure 5.9 Comparisons of the available energy Rn with the amounts used in evaporation L.E, heating of the ground G and heating of the air H from a wheat crop in Canberra, Australia. At any moment, Rn equals the sum [LE+G +H].

Local time (hours)

Figure 5.9 Comparisons of the available energy Rn with the amounts used in evaporation L.E, heating of the ground G and heating of the air H from a wheat crop in Canberra, Australia. At any moment, Rn equals the sum [LE+G +H].

same effect on a ground surface (Table 5.3). This leaves all the incoming net radiation available to warm the surface and the adjacent air. The warmer surface increases three fluxes: (i) the upwards terrestrial radiation (reducing the incoming net radiation Rn, (ii) the sensible-heat flux to the atmosphere H, and (iii) the conduction of heat downwards into the ground or lake G. So a new balance arises automatically, with a reduced Rn matched by an increased (H+G).

Carbon powder spread onto clean snow dramatically increases the absorption of incoming radiation and therefore promotes the

3 40

3 40

January rainfall: mm

Figure 5.10 Effect of summer rainfall at Alice Springs on the daily-maximum temperature.

clearing of snow. The albedo of snow is thus reduced from about 80 per cent to 5 per cent, so that the surface absorbs almost five times as much shortwave radiation, warming the surface. This promotes sublimation, and melting and runoff occur when temperatures reach 0°C.

Also, a lowering of albedo accelerates evaporation in getting salt from sea water within shallow ponds exposed to the Sun. Adding

Table 5.3 Example of the effects of a clear impermeable plastic cover on components of the energy balance; the signs of the energy fluxes (W/m2) are consistent with Figure 5.1

methylene-blue dye to the brine reduces its albedo, so that more solar radiation is absorbed, increasing the net radiation (Section 2.8), thereby making more energy available for evaporation.

Coating the leaves of an orange tree with (white) kaolinite to increase the albedo has been found to lower leaf temperatures in the daytime by 3-4 K, and so the rate of evaporation falls, conserving water. Likewise for leaves of a rubber plant, though to a smaller extent.

The direction in which sloping land faces governs the amount of radiation received, and therefore the amounts of energy going into heating the air and perhaps evaporation. Table

5.4 gives results showing substantially less heating of the air against a slope facing away from the Sun, whereas evaporation differed little. (Of course, there may have been different rainfalls on the two slopes to complicate matters.)

5.5 THE ENERGY BALANCE OF THE HUMAN BODY

The climate affects our feelings of comfort (Notes 3.D, 3.E, 3.J and 4.H) by challenging homeostasis, the maintenance of a steady bodycore temperature. This depends on balancing the various energy inputs and outputs of the core of the body, the balance being controlled by a part of the brain called the hypothalamus. Body temperatures rise as the result of (i) solar radiation, (ii) shivering, (iii) exercise and (iv) metabolic processes, i.e. normal body functioning. Cooling occurs either through panting (i.e. evaporation from the lungs) or an increase of blood circulation to the skin (i.e. the skin flushes), to carry more heat away from the core, followed by (i) sensible-heat flux from the skin to the air, (ii) longwave radiation to the surroundings, and (iii) the evaporation of perspiration (Notes 4.D and 4.H). Clothing provides insulation that reduces all these three processes. Also the body reduces them automatically when we feel cold by constricting the blood vessels near the skin, thus reducing the transport of heat from the body's core (Figure 5.11).

Sweat

Humans are outstanding amongst animals in their ability to sweat profusely from the skin, because of hairlessness. An adult can sweat up to 3 litres an hour or so, representing a loss of almost 4 per cent of body weight hourly. A loss of 2 per cent of body weight causes great thirst, and 8 per cent makes the tongue swell so that speech and then breathing become difficult. Hard work in the open at 30°C may take 10 litres a day. However, a typical figure for someone at rest is only 0.8 1/d, about half of which evaporates in the lungs and then is exhaled.

Sweating starts when the skin temperature exceeds about 30°C. This can occur, for instance, when the air temperature is only 3°C, if the body is being heated by 640 watts of exercise. On the other hand, the sweating mechanism collapses if the body temperature reaches 41°C, and then further heating is

Table 5.4 Example of the effect of a slope's orientation to the Sun on the energy balance of a hillside's bare soil; the signs of the energy fluxes (W/m2) are consistent with Figure 5.1

ENVIRONMENT

ENVIRONMENT

Human Core Temperature
Figure 5.11 Processes involved in the energy balance of a human body maintaining homeostasis, i.e. keeping the body-core temperature steady. There are four different layers—the central core, the skin, the clothing and the ambient environment—and heat fluxes between them are indicated in italics.

unchecked. Difficulty in sweating is why the elderly are more vulnerable to high temperatures (Note 3.C).

One's survival in a hot climate depends on the availability of water to replenish what is lost as sweat. With daily maxima above 37°C in a desert, one can survive without water for only about four days by resting still, or two days by travelling at night and resting in the daytime. A night traveller who drank four litres daily could survive for five days.

Loss of a litre hourly by sweating represents

400 W/m2 from a nominal skin area of 1.7 m2, so the cooling by sweating is considerable. We can compare it with metabolic heating of 44 W/m2 when sleeping, 120 W/m2 if strolling, 460 W/m2 when running at 10 km/h and 520 W/m2 during heavy manual labour. At the other extreme, shivering can generate up to 250 W/m2.

That example of the human body concludes our consideration of energy balances. It demonstrates again how universally applicable and how illuminating the concept is.

NOTES

5.A Why doesn't the world get hotter? 5.B Does a car's colour influence its temperature?

5.C Factors governing the daily minimum temperature 5.D Estimation of evaporation 5.E Sol-air temperatures

5.F Energy balances of the southern hemisphere

Part III WATER

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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