QL1 is the long-wave atmospheric radiative energy;

QL1 is the long-wave terrestrial outgoing radiation for which is emissivity, σ is the Stefan-Boltzmann constant, and T is absolute surface radiative temperature;

QH is the sensible heat energy;

QE is the latent heat energy;

QS is the energy stored in soil or water;

QM is heat rejection from man-made combustion and processing; and

A is the albedo.

Only the terms QI, QL1, and QH are not directly affected by changes in the surface. All other terms are altered, some of them quite substantially. But even the sensible heat transfer is indirectly affected because all the surface changes alter the surface roughness, which partially governs heat transfer. Scientists engaged in the study of micrometeorology and microclimatology have long been aware of these changes (Geiger, 1961), but these seemingly small-scale phenomena were generally ignored in discussions of climatic change until they reached proportions that began to affect meteorological mesoscale phenomena.

Although the quantitative changes in the physical characteristics may seem to be small, the consequences can be large because of the magnitude of the energies involved. The first notable parameter that changes is the albedo (A), which is the percentage of incident radiation reflected. Table 5.1 gives a small sample of measured albedos of natural and man-made surfaces.

It should be noted, as a generalization, that albedos of cultivated vegetation are higher than those of the forests they replaced. Similarly, much of the urbanized area has a lower albedo than the fields that were there before. The differences in the albedos are immediately reflected in the surface temperatures (Lorenz, 1973a). This, in turn, of course, affects the outgoing long-wave radiation. But it is not only the albedo but also the emissivity that is changed by man’s activities. This also is reflected in the long-wave radiation term of the energy equation. A few characteristic values of emissivity are listed in Table 5.2.

Very profoundly affected by man’s works are the heat conductivity and heat capacity of surficial materials. Veg

TABLE 5.1 Selected Values of Surface Albedosa


Albedo (%)


Albedo (%)

Fresh snow


Dry, ploughed field





Dried grass


Wet fields


Deciduous forest


Growing grain


Evergreen forest


Stubble field












Urban areas


aSee Barry and Chambers (1966), Chin (1967), Kung et al. (1964), Nkemdirim (1972). Oguntoyinbo (1974), Stanhill et al. (1966), and Stewart (1971).

TABLE 5.2 Emissivities of Selected Surfaces (in the Long-Wave Spectral Band 8–14 µm)



Growing crops and grass

0.98 (Fuchs and Tanner, 1966)

Natural freshwater

0.97 (Davies et al., 1971)


0.95 (Lorenz, 1966)


0.94 (Lorenz, 1966)

Land (dry)

0.91 (Gorodetsky and Filipov, 1967)


0.90 (Buettner and Kern, 1965)

etated surfaces usually have considerably lower values of these parameters than building materials such as brick, stone, concrete, and asphalt. Representative values, for example for thermal diffusivity, are

Dry vegetation

0.002 cm2 sec1,


0.02 cm2 sec−1.

They are an order of magnitude apart. Add to this the fact that man artificially compacts the soil under roads and parking lots to increase the bearing strength and it is easily envisaged that considerably more of the incoming energy from sun and sky is transferred to deeper layers and stored. This will then become available for transfer back to the surface during intervals of net outgoing energy flux. Thus the factor QS is the one most profoundly disturbed by urbanization.

Not far behind is the change produced by the evaporation term, QE. This is also prominently interfered with in farming operations. Not only is the evapotranspiration of forests different from that of cultivated fields, but the widespread use of irrigation has a profound influence on the local heat balance, Indirectly, shelter belts affect this factor too. Yet, the rural alterations are small compared with those produced in urban areas. There, vegetation, and consequently evapotranspiration, is sharply reduced. But the water, and hence evaporation balance, is even more radically altered by the large amount of impermeable surface, with every effort being made to drain precipitation away as rapidly as possible. The reduced evaporation will, of course, reduce the energy needed for this purpose.

This and the other changes in physical parameters are responsible for a major portion of the urban heat island. The other is the heat rejection from man-made combustion processes, QM. However, it is essential to realize that the other elements of the energy balance are quite sufficient to produce a substantial urban heat island. Measurements of surface temperatures by infrared emission techniques from the air (Lorenz, 1973b) show that on sunny days in the summer the urban surface is materially warmer than that of the surroundings. These larger-scale surveys closely resemble infrared thermometer measurements from the surface (Landsberg and Maisel, 1972; Kessler, 1971). These measurements show that (up to a conversion of 50 percent of the surface buildup) the midday temperature difference

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement