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<br /> <br />-12- <br /> <br />Some of the relationships of downward fluxes of radiation involve <br />properties of clouds not always observed; however, current work in cloud <br />physics may make it possible to extend the validity of these relation- <br />ships. Another class of relationships includes those that might be <br />developed between radiation fluxes and conditions at the earth's surface, <br />such as daily maximum air temperature. These relationships are valid <br />as long as the balance between radiative and advective forces remains the <br />same; unfortunately, the climate-generating forces are not always in <br />balance but may shift radically with changes in daily weather. The <br />,balance between radiative and advective influences also varies spatially <br />with changes in ventilation--that is, the intermingling of surface and <br />upper air caused by topographic roughness. Therefore, relationships <br />between any radiation flux and meteorological measurements derived at a <br />particular station cannot be extended far from that station. <br /> <br />These considerations suggest that attempts to fill gaps in radiation <br />data will be most successful for a few missing days of record, where <br />relationships to various meteorological factors can be developed at that <br />place and in that weather situation. Attempts to fill long gaps in a <br />record become risky. Attempts to fill spatial gaps between radiation <br />stations, much-needed as they are by reason of coarseness of the network, <br />should be confined to the same climatic region, to weather situations <br />bringing the same type of cloudiness, and to the same topographic areas. <br /> <br />In conclusion, it may be said that the radiative heat transfers <br />offer at least as much danger that the hydrologist will fall into errors <br />of use and interpretation as does any other hydrologic factor. Also, <br />that the temptation is so large because data are sparse. In hydrologic <br />factors such as precipitation, many sources of error have been identified <br />by past workers, who also have developed procedures for minimiZing them. <br />Radiation has a shorter history; not all the possible errors have yet <br />been identified, and few have been studied enough that procedures for <br />minimiZing them are available. The hydrologist wishing to employ <br />radiative-heat factors in his investigation should be prepared to work <br />in developing and testing procedures to assure quality of data"and, to <br />fill gaps in the record. <br /> <br />Selected references <br /> <br />Brunt, David, 1941, Physical and dynamical meteorology, 2d edition: <br />Cambridge University Press. (Chapter 5 deals with radiation.) <br /> <br />Budyko, M. I., 1958, The heat balance of the Earth's surface; Trans- <br />lated by N. A. Stepanova from Teplovoi Balans Zemnoi Poverkhnosti, <br />Gidrometeorologicheskoi Izdatel'stvo, Leningrad, 1956: Dept. of <br />Commerce, Office of Technical Services, Washington, D. C., <br />PB 131692. (Section 3 deals with the radiation balance.) <br /> <br />Fritz, S., and MacDonald, T. H., 1949, Average solar radiation in the <br />United States: Heating & Ventilating, v. 46, p. 61-64. (Discusses <br />relation of average daily insolation amounts to cloudless-day <br />insolation.) <br />