Ju~E 1968 NOTES AND CORRESPONDENCE 52I.that a network comprised of 1 or more observation sitesmi-2 is necessary to measure adequately the arealextent of damaging hail. REFERENCESButchbaker, A. F., D. Henkle, L. J. Hagen and E. Rodakowski, 1966: Modification of convective storms in southwestern North Dakota during 1965. Fargo, North Dakota State University, Agric. Eng. Dept., 23 pp.Changnon, S. A., Jr., 1964: Surface features of two intersecting hailstorms. Nubila, 6, 74-86.--, P. Schickedanz and H. Danford, 1967: Hail patterns in Illinois and South Dakota. Proc. Conf. Severe Local Storms, Amer. Meteor. Soc., 325-335.Carte, A. E., 1963: Hail studies in the Pretoria-Witwatersrand area. C.S.I.S.-RF is News Letter, 145, 136-140.Decker, F. W., and L. D. Calvin, 1961: Hailfall of 10 September 1959 near Medford, Oregon. Bull. Amer. Meteor. Soc., 42, 475-480.Frisby, E. M., 1963: Hailstorms of the Upper Great Plains of the United States. J. Appl. Meteor., 2, 759-766.Koscielski, A., 1967: Hail occurrences in Perkins County, South Dakota. Rept. 67-6, Inst. Atmospheric Sciences, South Dakota School of Mines and Technology, 10 pp.Schleusener, R. A., 1962: The 1959 hail suppression effort in, Colorado and evidence of its effectiveness. Nubila, 5, 31-59. , J. D. Marwitz and W. L. Cox, 1965: Hailfall data from a fixed network for the evaluation of a hail modification experiment. J. A ppl. Meteor., 4, 61-68.Smith, M. D., J. S. McCafferty and D. H. Steffe, 1967: Physical characteristics of hail in western Nebraska. Chadron,. Nebraska, Rept. 67-1, Contract NSF-GP 5483, Chadron State College, 12 pp.Stout, G. E., and S. A. Changnon, 1966: Final report by the task chief on precipitation distributions. Project Hailswatl~ Final Rept., Vol. 1, 2 pp.Summers, P. W., and A. H. Paul, 1967: Some climatological characteristics of hailfall in central Alberta. ~?roc. Conf. Severe Local Storms, Amer. Meteor. Soc., 315-324.Wilk, K. E., 1961: Radar investigations of Illinois hailstorms. Urbana, Illinois, Sci. Rept. 1, AF 19(604)-4940, Geophysic~ Research Directorate, AFCRL, 42 pp.A Note on the Role of Sky Radiance in Infrared ThermometrfSHERWOOD ~g. IDSO AND RAY D. JACKSONU. S. Water Conservation Laboratory, Phoenix, Ariz.1 January 1968 In a recent study of evaporation from a wet soilsurface utilizing radiometrically determined surfacetemperatures, Conaway and van Bavel (1967a) raisethe question of whether accounting for sky radiance ~ Contribution from the Soil and Water Conservation ResearchDivision, Agricultural Research Service, U. S. Department ofAgriculture.justifies the additional effort required. They demonstrate that neglecting the sky radiance altogether doesindeed result in a significant error in surface temperature. In contrast to Fuchs and Tanner (1966), however~they furthermore imply that sky radiance is quitevariable and that it should be monitored frequentlywhenever temperature measurements are made by4. 0.00 i O ~.. _*o.~o [K O /K a- +_0.20 -' I~J tK + 0.30 =:) I- m ~o.4o ~ Z o. so ~ 0.60 ~ ~ ~ I I ~ I ~ I 1.00 0.99 0.98 0.97 0.96 0.95 0.94 0.9~ 0.92 '0.91 0.90 EMITTANCE F]~. 1. The error in radiometrically determined surface temperatures as a function of surface emittance for surfacetemperatures of 0, 20, 40 and 60C, caused by assuming clear sky radiance constant at 1.00 mW cm-~, when it actuallyvaries by ~ 15~.522 JOURNAL OF APPLIED METEOROLOGY VOLU~E 7infrared thermometry (Conaway and van Bavel, 1967b).In that report they present experimental evidence fordiurnal fluctuations in clear sky radiance which varyfrom less than 1.0 to over 4.1 mW cm-2 in the 600-1300crn-~ waveband. Theoretical calculations show theselarge variations in the energy transmitted by theinfrared thermometer's filtering system to be physicallyimpossible, however, and experiments have revealedthat they were due to instrument malfunction (Idsoand Jackson, 1968). In a continuous 6-day experiment where clear skyradiance was determined every 10 rain, and where thecalibration of the infrared thermometer (Barnes Engineering Companf Model IT-3) was also checked every10 min, we found the radiance of the clear sky to varyby no more than 4-15-/o. A second 6-day experimentabout a month later (mid-November) gave similarresults. Assuming these maximum variations of 4-15%to be typical, we have constructed a nomogram (Fig. 1)from which the temperature error caused by ignoringthese variations can be estimated for a surface ofknown emittance between the temperatures 0 and 60 C.The nomogram is based upon the equations of Conawayand w~n Bavel (1967a), assuming the clear sky radiancesensed by an IT-2 or IT-3 infrared thermometer to be1.004-0.15 mW cm-~. 2 Trade names and company names are for the convenience ofthe reader and do not imply preferential endorsement of a particular product or company over others by the U. S. Departmentof Agriculture. For the Adelanto clay loam soil investigated byConaway and van Bavel (1967a), the surface emittancewas 0.980 and the surface temperature varied between10 and 30C. Thus, for their situation, the error induced by assuming the sky radiance constant shouldhave been no greater than about +0.08C. Since theradiometric technique of temperature measurement isstated by them and also Fuchs and Tanner to beaccurate to only 0.2C, their question concerning frequent monitoring of sky radiance would thus seem towarrant a negative reply. Under cloudy conditions orfor surfaces of lower emittance, however, their affirmative answer may be well taken, depending also uponthe purpose for which surface temperature is required.Fig. 1 should be helpful in evaluating these cases andmay save much time and effort involved in makingnonessential sky radiance measurements.REFERENCESConaway, J., and C. H. M. van Bavel, 1967a: Evaporation from a wet soil surface calculated from radiometrically determinedsurface temperatures. J. Appl. Meteor., 6, 650-655.----, and----, 1967b: Radiometric surface temperature mea surements and fluctuations in sky radiant emittance in the600 to 1300 cm-x waveband. Agron. J., 59, 389-390.Fuchs, M., and C. B. Tanner, 1966: Infrared thermometry of vegetation. Agron. J., 58, 597-601.Idso, S. B., and R. D. Jackson, 1968: The significance of fluctua tions in sky radiant emittance for infrared thermometry. Agron. J., 60 (in press).Glycol Contamination in Nucleation Counters F. KIRK ODENCRANTZNaval Weapons Center, China Lake, Calif.4 January 1968 and 12 February 1968 Traces of vapor contamination may produce drasticeffects on the habit and electrical charges produced onice crystals grown from a supercooled fog (Odencrantz,1968a). They may also modify the measured value of thenucleation efficiency of AgI pyrotechnic smokes (Oden- crantz, 1968b). Many chambers utilize 1,2-ethanediol(glycol) to prevent frost formation in the chamber. Thisimpurity can produce a significant change in the measured value of the nucleation efficiency at temperatureswarmer than -- 12C, a region that is important for cloudseeding to produce precipitation. The experimental values of the nucleation efficiencies of AgI pyrotechnicsmokes are dependent on the equipment and techniquesused for the measurement; caution should therefore beused if the results are to be extrapolated to atmosphericconditions. Samples of a standard silver iodate pyrotechnic weretested in a 24-ma cold chamber that normally does nothave glycol contamination. Frost is removed from thewalls of the chamber between experiments with cold dryair. The background count of ice crystals in the chamberis at least a factor of 1000 below the normal count in thechamber. Glycol was evaporated from a small electrically heated cup to produce deliberate contamination inthe chamber. At the colder temperatures some of thisglycol condensed in the air and formed a thin fog ofdroplets which were visible in the beam of a spotlight.Steam was then added to the chamber, and the supercooled fog was allowed to come to thermal equilibriumwith the walls of the chamber. If glycol droplets wereinitially present in the chamber, a large number of wateror water-glycol droplets (~ 20 u diameter) settled to thefloor of the chamber during this equilibration time. Itwas not determined how much of the contaminant remained as a vapor, how much was associated with thewater droplets, or how much was removed by precipi