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Vol. 57, No. 9, September 1976 Dat a from initial flights on an NCAR jet aircraft showed that the instrument operated well, but compara- tive analyses have not yet been performed. A fixed-path version, built for the GAT E experiment, is still in regu- la r use and has performed reliably for about 2000 flight hours . A sample of data from the GATE instrument is compare d with a dew-point hygrometer in Fig. 8. Durin g a field test at Haswell, Colo., in September 1975, the variable-path hygrometer was exposed for 2 weeks to rain, wind, and dust. Th e self-calibration pro- cess was performed twice a day; the rest of the time the pat h length was left constant. Throughout the period, accuracy remained better than 1°C dew point. 8. Conclusion I n summary, the variable-path Lyman-alpha hygrometer is proving to be a field-worthy instrument capable of combinin g high time-space resolution with good stability over a wide range of humidities. Acknowledgments. The valuable help of James Till- FIG. 8. In-flight data from Lyman-alpha hygrometer ma n of the University of Washington, who consulted on compare d to a dew-point hygrometer. th e source design and suggested the UH approach, and Bryan Lee, who designed the electronic circuitry, is h test period lasted 13 months. In spite of the large gratefull y acknowledged. changes in gain (a factor of 6 in the first case), the in- strumen t maintained average accuracies better than References ±0.5° C dew point. Buck, A. L., 1971: Models for absorption hygrometry. Pro- Fortunately , the window degradation has proved to ceedings of the Technical Program, Electro-Optical Systems be reversible, since it occurs on the outside of the source Design Conference—1971 West, Chicago Industrial and an d the detector. Window transmission can be largely Scientific Conference Management, Inc., 88-95. restored by washing with alcohol or acetone, and it can , 1975: Error sensitivity of fixed- and variable-path be completely restored by light polishing with some fine Lyman-alph a hygrometers. NCAR TN/EDD-103, Boulder, abrasive compound. (We use 0.02 /mi alumina particles.) Colo., 45 pp. Th e hygrometer operates well in fog or clouds so long Dieke, G. H., and S. P. Cunningham, 1952: A new type of as there is no condensation on the windows. Calcula- hydroge n discharge tube. / . Opt. Soc. Amer., 42(3), 187-189. Samson, A. R., 1967: Techniques of Vacuum Ultraviolet tions by Tillman (1965) show that scattering from the Spectroscopy. New York, John Wiley and Sons, 348 pp. droplet s found in heavy fog would produce an error Spedding , F. H., A. S. Newton, J. C. Wharf, O. Johnson, of only 0.004%, and so can be ignored. A film of liquid R . W. Nottorf, I. B. Johns, and A. H. Daane, 1949: Ura- water, however, prevents the hygrometer from function- niu m hydride, 1, Preparation, composition and physical ing. properties . Nucleonics, l 4-15. Erro r analysis showed that the data reduction algo- Tillman , J. E., 1965: Water vapor measurements utilizing rith m was fairly insensitive to the accuracy of the oxygen th e absorption of vacuum ultraviolet and infrared radia- ter m but was very sensitive to that of the path length. tion . Humidity and Moisture, Measurement and Control in T o introduce less than 0.1 °C dew-point error, one must Science and Industry, vol. 1, Principles and Methods of typically know air density to 10% and path length to Measuring Humidity in Gases, R. E. Ruskin, Ed., New 0.005 cm. York, Reinhold , 428-133. Several corrections should be made in the August 1976 BULLETIN . In the Table of Con- tents, the AMS 56th Annual Business Meeting should be 57th; on page 1012 (top left photo) , the caption should read, "From left to right are Robert Simpson; winners, Mary McGarr y and Steven Payne; and Joanne Simpson"; and also on page 1012, the photos i n the right column should be interchanged.
Bulletin of the American Meteorological Society – American Meteorological Society
Published: Sep 1, 1976
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