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0014 71IV.4 In most geophysical experiments in the field, three or four flUXes . of radiation are measured. The cost of a recorder for each fluxc~ be reduced where a multi-channel recording system is available. The four fluxes usually measured are: shortwave downltard (insolation, incident solar radiation); shortwave upward (reflected from the surface); allwave downward (liemispherical radiation of both short and long wavelengths) j and the net exchange of allwave radiation (difference between upward . allllllve and d01lllW1lrd allllllve). From these four measurements, longwave radiation It>li'llrd and dOWIlllllrd can, at least in theory, be detel'lllined by subtraction. It m~ be possible to dispense with continuous measurements of ,. llpWllI'd shoi;lt..,.~ 1'lldiation by m~asuring albedo at intervals. However, albedo varfes in response to MeteorOlOgical and other forces, often over a large range; its regime must be understood before available handbook values of' albedo can be applied to many practical prOblems. Inst~ental limitations of the radiation sensors, beyond those caused by" $1ch special conditions as pollution by" aerosols or topographic shading, iriOlude the effects of (1) tomperature on output of thermocouples, (2) wind on 1lli.shielded allwave instruments, (3) dust and dew on glass shields of :shortwave sensors and on polyethylene shields of longwave . sensors; arid (4) precipitation on both shielded and unshielded sensors. There remains a question whether polyethylene fillns change transparency during exposure t.o the weather. For further discussion of such limitatio~, reference should be made to the I.G.Y. Manual. Rapid variation of the radiation fluxes with passage of clouds has brought about the use of continuously recording potentiometers that produce gr~atmileages of strip charts, often in more detail th~ a given prob1;emmay require. Digitizing, tape-punching recording systems may use longer intervals between measurements; also, they offer the possibility of computer processing of the record. Since few studies have been published on variation of radiation in different types of weather, each invest:;tgator should examine his own problem to be sure he is getting' needed deta"ilwi thout incurring unneeded costs for chart redw:tion. Attachment 'of electrical or mechanical integrators to the potentiometer increases the investment in recording equipment but reduces the operationJl cost of oha:r-tredu:ction, if daily totals are neededorsllf'fiee. In some installatiO)1s, it mtly be possible to dispense with the intermediate strip chart and go direct to a counter giving daily totals. . Radia'l<ion sensors usually are calibrated by the manllf'acturer. Maintain1ngcalibration in field use is a problem, sometime.s requiring shipment back to the manufacturer, or pOlllparison against .a standtu"d instrument 'taken from station to station of a nett(Ork. Homemade calibration equipment is also in use.. Users of records should assure themselves .that instruments have been kept in calibration.