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Roger L. Steele and Charles I. Davis

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Lewis O. Grant and Roger L. Steele

The output of effective ice nuclei from silver iodide generators can vary by at least three to four orders of magnitude as a function of the type of generator device employed and its operational efficiency. A detailed consideration of each phase of the process of calibrating silver iodide generators shows that with care meaningful calibrations to within a factor of 1.5 are possible.

The repeatability of actual calibration experiments together with space closed volume experiments support the validity of generator testing procedures.

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Alan W. Gertler and Roger L. Steele

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The IR extinction of clouds has been measured with a CO2 laster transmissometer (λ = 10.6 μm). Using an established linear relationship (based on the Chýlek approximation to the Mie theory) between extinction and liquid water content, the liquid water contents of clouds varying in both droplet size and concentration have been determined. These values are compared with liquid water contents derived from simultaneous cloud collection. The agreement between the two techniques is excellent having a correlation coefficient of 0.98 and corresponding favorable 95% confidence limits. The results indicate that this technique should prove extremely useful for the field measurement of the liquid water content of clouds and fogs.

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Charles I. Davis and Roger L. Steele

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A variety of devices that produce artificial nuclei have been used in weather modification experiments. These fall into two broad classifications, i.e., pyrotechnics and steady-state flame-type generators.

The effectiveness-temperature relationships for these classes were measured and are presented in graphical form. In summary, the steady-state systems are in general one order of magnitude more effective than the pyrotechnics at temperatures below −12C. Above this temperature, the effectiveness of the pryotechnics approaches and in some cases exceeds that of the steady-state systems. These different characteristics can be explained by particle size effects.

Other performance parameters are examined. Air-fuel ratio is one of these since effectiveness varies markedly near the stoichiometric air-fuel ratio for steady-state systems. A system which employs isopropylamine as the AgI carrier is more sensitive to changes in air-fuel ratio than the NaI-acetone AgI carriers. Another important performance parameter is AgI burn rate. In general, the effectiveness of a pyrotechnic decreases rapidly and then levels off as the burn rate increases. This same effect has been observed in steady-state systems.

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Roger L. Steele, C. P. Edwards, Lewis O. Grant, and Gerhard Langer

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The NCAR acoustical ice nucleus counter was calibrated against a Bigg-Warner Weather Bureau type chamber modified as a mixing chamber. The mixing chamber was in turn calibrated against the CSU-NSF isothermal diffusion cloud chamber. This work was carried out using a 300-liter aluminized mylar bag into which known samples of silver iodide nuclei were introduced. Nuclei were transferred from the bag to the NCAR counter in a carrier gas, at a flow rate of 10 liters min−1. It was found that the NCAR counter measured from 16–52% of the count given by the mixing chamber. An NCAR unit was modified with a velvet liner to test the feasibility of eliminating the glycol system, and measurements were made as described above. The modified unit did not count reliably.

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Joanne Simpson, William L. Woodley, Howard A. Friedman, Thomas W. Slusher, R. S. Scheffee, and Roger L. Steele

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The development, testing and use of an airborne pyrotechnic cloud seeding system is described. Pyrotechnic flares producing 50 gm of silver iodide smoke each were developed by two industrial corporations and laboratory tested for nucleation effectiveness in the Colorado State University cloud chamber. A delivery rack and firing system were developed, under ESSA supervision, and installed on its B-57 jet aircraft. Night flight tests were made of reliability, burn time and flare trajectory.

The flare system was used in a Florida cumulus seeding experiment in May 1968 conducted jointly by ESSA and the Naval Research Laboratory, with the participation of the U.S. Air Force, the University of Miami Radar Laboratory, and Meteorology Research, Inc. A randomized seeding scheme was used on 19 supercooled cumuli, of which 14 were seeded and 5 were studied identically as controls. Of the 14 seeded clouds, 13 grew explosively. Seeded clouds grew 11,400 ft higher than the controls, with the difference significant at better than the 0.5% level. Rainfall from seeded and control clouds was compared by means of calibrated ground radars. Large increases in rainfall were found from seeded clouds, but at a significance level ranging from 5–20% depending on the statistical test used. A single successful repeat of the experiment could result in rainfall differences significant at the 3% level with the most stringent test.

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