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001245 -9- Limitation of instruments and recording equipment This discussion does not try to go deeply into problems of instru- ment design, manufacture, and calibration; it may be said, however, that radiation equipment is not entirely standardized. There is worldwide agreement on two kinds of instruments but not yet on others, although the increasing use of energy-balance concepts and the impetus of the Inter- national Geophysical Year will bear fruit eventually in instruments of greater comparability and reliability. The best source for information on radiation instruments is an instruction manual published by the U.S. Special Committee for the International Geophysical Year (1958). At this point it is well to state explicitly what until now has peen implicit--namely, (1) shortwave and longwave radiations are physically distinct flows of energy; (2) each has its unique relationships with clouds and other meteorological factors; and (3) each is influenced differently by topography, forest cover, and other physical features of the region upon which it is incident. Thus, each enters geophysical relations in a different manner. It is risky to lump them in analysis. Moreover, in studying hydrologic phenomena at the earth1s surface, upward flows must be separated from downward flows. Thus, four fluxes make up the radiation picture. To understand the role of radiation in a hydro- logic process, all four have to be considered. Some studies make use of one instrument alone-.that for measuring the net exchange of radiation of all wavelengths, a net exchange which is the resultant of the four separate, more or less independent fluxes of upward and downward shortwave and upward and downward longwave. These values of net radiant energy available at a given site are useful. However, since it is difficult to break them down into their shortwave and longwave components, they are not easily applied to problems in which the, separate flows are influenced differently by meteorological, vegetal, or topographic factors. For example, the same net radiant energy may occur on a cloudy day with small downward shortwave and large downward longwave radiation, as on a clear day of bright sunshine and small downward flux of longwave radiation. Yet a given hydrologic process may proceed quite differently on these two daYS--for example, snow responds differ- ently to shortwave than it does to longwave, The simplicity of a single instrument, with perhaps only an inexpensive totalizing recorder, should be weighed by the investigator against (1) the incomplete understanding he can derive from the single observation on the one hand and (2) on the other hand, the real complexity of the four flows of radiant energy and their differing receptions by vegetation, snow, and soil. In most geophysical experiments in the field, three or four fluxes of radiation are measured. The cost of a recorder for each flux can be reduced where a multi-channel recording system is available. The four fluxes usually measured are: shortwave downward (insolation, incident solar radiation); shortwave upward (reflected from the surface); allwave downward (hemispherical radiation of both short and long wavelengths);