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001470 Lifitation of instruments and recording equipment This cti.scussion does not try to go deeply into problems of instru- ment desi~, manufacture, and calibrationl it may be said, however, that radiation equipment is not entirely standal'dized. There is worldwide agreement on two kinds of instruments but not yet on others, although the increasing llse of energy-balance concepts and the impetus of the Inter- national Ge;ophysical Year will bear fruit eventllally in instruments of greater COn\parabili ty and reliability. The best source for infoI'lllation on radiation instruments is an Instruction Manual publishe(l. by" the U.S. National CO/1llRittee for the 1.G.Y. (Annals, Vol. 5, part 6l London, Pergamon Ptess, 1958. See Meteor. Abstracts for Sept. 1958, item 9.9-33.) .Interested readers are referred to this manual. . At this point it is well to state explicitly what until now has been ilnplicit--namely, (1) shortwave and longwave radiations are physically distinct :flows of energyl (2) each has its unique relationships with clouds and :other meteorological factors, and (3) each is influenced different~ by topography, forest cover, and other physical features of the region upon which it is incident. Thus, each enters geophysical relations :ih Ii different manner. It is risky to lUlllp them in analysis. Moreover, ih studying hydrologic phenomena at the earth's 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 proce'ss, all four have to be considered. Some studies make use of one instrument alone--that for measuring the next e:X;change of radiation of all wavelengths, a net exchange which is the resup.tantof the four separate, more or less independent fluxes 'Of upward ahd downward shortwave and upward and downward longwave. These values of n;e.t radiant energy available at a given site are useful. However, since it is, difficult to break them down into their shortwave and 10llgWave components," they are not easily applied to problems in whi.ch the separate :flows are :tnf1"uenced differently by meteorological, vegetal, or topographic factors. For example, the same net radiant energy may ocourona cloudy day with SI1(all dOllmfard shortwave and large downward longwave radiation, as on a cle:arday of bright sunshine and small downward :flux of longwave radiation. : Yet a given hydrologic process may proceed entirely differently on these t1lO days--for example, snow responds differently to short,...ave thlU1 it does to longwave. The simplicity of a single instI'Ulnent, with perhaps onJ.y an ineXpensive totalizing recorder, should be weighed by the 1nVesti.. gator against (1) the incomplete understanding he can derive from the single observat:l.on on the one hand and (2) on the other hand, the real complexity :ofthe four :flows of radiant energy and their differing receptions by" vegetation" snow, and soil. IV.J