outgoing flux of IR energy radiated from ground, plants, clouds, and sky — is of equal and often greater importance in determining the effective heat load than is solar radiation. This fact is clarified in Table 26 where values of the IR radiation from the ground and sky are compared with relevant values of solar radiation. The incident solar radiation can be very large for very short periods of time, as shown in Table 26A where representative maximum values of solar insolation at the surface of the Earth are shown, together with the solar constant, which is included for comparison purposes. A high mountain top near the equator may experience insolation values as high as 1325 W/m2 for very short durations, and solar insolation on a sunny summer noon at sea level in the mid-latitudes may be as great as 700 W/m2. These, however, are maxima, and the average insolation during the day will be much less, even in summer. Although the contribution of solar radiation to the energy environment can be large at times, it is, in fact, usually a minor contributor to the total. The latter includes, in addition, all the IR energy radiated from warm objects in the environment — soil, rocks, structures, leaves, and shrubs — as well as IR energy from the sky. The IR energy radiated from these objects can be calculated by means of the Stephan-Boltzmann equation, since all these objects closely resemble blackbodies. Table 26B lists some representative values of the IR energy sources. Extremely large amounts of energy are available from very cold frozen ground. Thus, at -20°C such ground radiates over 10 times the maximum estimated energy that will be lost from the center of the SSPS receiving antenna. In contrast, a warm rock in a desert can reach a temperature of 60°C, and when it does it will be radiating IR at a rate equivalent to the maximum solar insolation, or 700 W/m2. IR energy is radiated downward from the sky in a somewhat more complicated fashion. Clouds radiate as blackbodies and, consequently, the energy available from them depends on the temperature of the radiating surface, which varies with cloud type and cloud height. IR energy is also available from CO2, water vapor, and ozone in the atmosphere, and consequently on cloudless days will depend on vapor pressure and the temperature throughout the atmosphere. The IR available from the sky has been frequently measured. The lowest radiant temperature of the Alaska winter nighttime sky observed during one series of experiments was -80°C, observed on a few cloudless nights (50). Thus the net IR loss from the ground corresponds to nearly twice the maximum energy loss in the center of the receiving antenna. Table 27, from Gates (50), indicates the manner in which the solar and IR components of the energy environment combine to affect organisms, in this case a horizontal leaf which absorbs energy on two surfaces. The absolute energy flux to which such a leaf is subjected is given in the second column from the right. The values range from 1846 W/m2 on a sand dune during a sunny day to 670 W/m2 on the same sand dune on a clear night. The coolest place shown — the interior of a deciduous forest - is characterized by an absolute energy flux of more than 900 W/m2 during a summer day. Although these values are for representative hypothetical environments in mid latitudes in summer, consideration of the data of Table 26 indicates that even in winter the
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