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<br /> <br />-10- <br /> <br />and the net exchange of allwave,radiation (difference between upward <br />allwave and downward allwave). From the se four measurements, 10ngW'ave <br />radiation upward and downward can, at least in theory, be determined by <br />subtraction. <br /> <br />It may be possible to dispense with continuous measurements of <br />upward shortwave radiation by measuring albedo at intervals. However, <br />albedo varies in response to meteorologioal and other forces, often over <br />a large range; its regime must be understood before available handbook <br />values of albedo can be applied to many practical problems. <br /> <br />Instrumental limitations of the. radiation sensors, beyond thcse <br />caused by such special oonditions as pollution by aerosols or tcpographic <br />shading, include the effects of (1) temperature on output of thermo- ' <br />oouples, (2) wind on unshieldedallwave instruments, (3) dU,st and dew <br />on glass shields of shortwave sensors and on polyethylene ,shields of <br />10ngW'ave sensors; and (4) precipitation on both shielded and unshielded <br />sensors. There remains a question whether polyethylene films change <br />transparency during exposure to the weather. For further discussion of <br />such limitations, reference should be made to the I.G.Y. Manual. <br /> <br />Rapid variation of the radiation fluxes with passage of clouds has <br />brought about the use of continuously recording potentiometers that <br />produce great mileages of strip charts, often in more detail thana <br />given problem may require. Digitizing, tape-punching recording systems <br />may use longer intervals between measurements; also, they offer the <br />possibility of computer processing of the record. Since few studies <br />have been published on variation of radiation in different types of <br />weather, each investigator should examine his own problem to be sure he <br />is getting needed detail without incurring unneeded costs for chart <br />rsduction. Attachment of electrical 'or mechanical integrators to the <br />potentiometer increases the investment in recording equipment but reducee <br />the operational cost of chart reduction, if daily totals are needed or <br />s~fice. In some installations, it may be possible to dispense with <br />the intermediate strip chart and go direct to a counter giving daily <br />totals. <br /> <br />Radiation sensors usually are Calibrated by the manufacturer. <br />Maintaining calibration in field use is a problem, sometimes requiring <br />shipment back to the manufacturer, or comparison against a standard <br />instrument taken from station to station of a network. Homemade <br />calibration equipment is also in use; Users of records should assure <br />themselves that instruments have been kept in calibration. <br /> <br />Since the downward fluxes of shortwave and longwave radiation <br />vary on a large geographic scale, they may need tb 1)e measured at. only <br />one place in a region of homogeneous cloudiness. The upward flux of <br />shortwave radiation (if taken as albedo) and of longwave radiation <br />vary abruptly with changes in surface cover and in temperature, but <br />less abruptly with time than with distance. Thus, they may be amenable <br />to aerial observation. <br />