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001245 -7- RAD:IATIVE HEAT TRANSFERS Introduction Available information on the four fluxes of radiation near the earth's surface (downward shortwave, upward shortwave, downward longwave, and upward longwave) sheds relatively little light on dependability of the data, on representativeness of observations, or even on instrument accuracy. While radiation measurements long have been recognized as important in many geophysical processes, they have not been made in net- works for very long. The older records are likely to be highly individual in type of instrument and installation, being parts of agricultural studies in which only local readings were needed. Recently the Weather Bureau's observational network for downward shortwave radiation has expanded) energy-balance approaches in experimental studies of snow melting, evaporation, and evapotranspiration have become widely accepted) and the International Geophysical Year and associated program have greatly stimulated interest in radiation. :In most areas of hydrologic operations, radiation stations are still scarce, but it is to be, hoped that in time small networks will be installed in the larger drainage basins of the West, somewhat as networks of precipitation-intensity stations were installed in the thirties for operational purposes. Effects of station location Radiation stations are few in number and there is a strong temp- tation to extend their records over areas much more extensive than is warranted. Exposure and location of stations become important for this reason. Because radiation sensors must be connected with instruments to record continuously a small electrical output, the recording equipment is not only bUlky, fragile, and expensive but also needs frequent checking and servicing. This requirement tends to confine radiation measurements to ,stations having technical personnel) most of these are in lowlands or valleys. Mouptains are poorly sampled. For example, although the present network for downward shortwave radiation includes stations at considerable altitudes (Albuquerque, Ely, and Grand Junction), these are in valleys. A few mountain stations have operated over short periods, but none is now functioning continuously. Some comparisons suggest that extrapo- lation of lowland records to mountains may be difficult except on a day- to-day basis, with observations of cloudiness at both places. For example, ,Davis and Soda Springs, California, have entirely different regimes of winter radiation, the valley being filled with fog for long periods while the mountains are sunny. The summer regimes differ oppositely, the mountains being commonly cloudy.