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Arthur L. Rangno

Abstract

A six-season, randomized-by-season cloud seeding experiment consisting of three seeded seasons and three non-seeded seasons was conducted by Colorado State University (CSU) during the middle and late 1960's in the Wolf Creek Pass region of the San Juan Mountains of southwest Colorado. The results of the seeding have been reported in a series of papers as having produced statistically significant increases in precipitation at Wolf Creek Summit when the 500 mb temperature was ≥−23°C. Furthermore, it has been reported that increases in precipitation produced statistically significant increases in the runoffs from three target watersheds when compared to the runoffs from three control watersheds.

In this paper the results of the Wolf Creek Pass Experiment (WCPE) are reexamined. It is shown that the three non-seeded seasons occurred during meteorological conditions which brought “warm aloft” (500 mb temperatures ≥ −23°C) storm days with unusually light precipitation over a wide region of Colorado, northern New Mexico, southern Utah and northern Arizona. This bias produced high values of seed/no-seed precipitation ratios at Wolf Creek Summit which led to the misperception of large increases in precipitation due to cloud seeding.

It is also shown that nearly all central and southwest Colorado watersheds with similar exposures to the target watersheds for the WCPE had high runoffs during the three seeded seasons compared to the three control watersheds chosen. Hence, the increases in runoff reported from the three target watersheds were part of a large-scale pattern due to natural causes rather than to cloud seeding.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

No abstract available.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Evidence is presented for the production of high concentrations of rather uniformly-sized ice crystals in a supercooled stratus cloud by a commercial, turbine, propeller-driven aircraft.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

The physical hypotheses for the cloud seeding experiments carried out by Colorado State University in the Colorado Rockies (Climax I and II and Wolf Creek Pass) in the 1960's are critically examined. Airborne measurements over the Rockies have shown that the concentrations of ice particles in the natural clouds are often much greater than originally assumed; consequently, the conditions under which it might be possible to increase precipitation by artificial seeding are probably much more limited than previously supposed. There appears to be no firm evidence to support the contention that 500 mb temperatures are a good measure of cloud-top temperatures over the Rockies. Finally, examination of a more extensive data set than previously used fails to substantiate the claim that precipitation over the Rockies decreases as 500 mb temperatures increase above certain values.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Evidence is presented that the passage of an aircraft through supercooled clouds can produce high concentrations of ice particles (> 1000 L−1 at −8°C in one case). These Aircraft Produced Ice Particles (APIPs) are characterized, initially, both by their high concentration and very uniform size distribution. The ice particles are contained in a cylindrical-like volume of air that is initially oriented along the flight track of the aircraft; the diameter of the cylinder is ∼300 m after 5 min of elapsed time. Possible mechanisms for APIPs are discussed.

In view of these findings, care must be taken in the interpretation of data on clouds that have been penetrated by aircraft. It seems likely that some previous observations of abnormally high concentrations of ice particles in clouds were due to APIPs. Also, since APIPs mimic some of the effects produced by deliberate artificial seeding, it could complicate the evaluation of cloud seeding experiments.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

No abstract available.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

No abstract available.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Several aspects of the study by Elliott et al. (1978) of the Colorado River Basin Cloud Seeding Pilot Project are discussed. Issues addressed include the design of the project, the diffusion of the seeding agent, anomalous ice nucleus concentrations in the target area, and statistical analysis of the randomized experiment.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Two statistical experiments, carried out in Israel, appeared for a time to have provided a unique demonstration of the ability of cloud seeding to increase rainfall. In this paper the authors examine the possibility that both experiments were compromised by type I statistical errors (i.e., “lucky draws” or false positives). It is concluded that in the first Israeli experiment a type I statistical error produced the appearance of statistically significant effects of artificial seeding on rainfall 1) in the buffer zone and the center target area, 2) in the coastal region of Israel, a few kilometers downwind of the seeding, and 3) in portions of Lebanon, Syria, and Jordan.

Analysis of the second Israeli experiment using the original crossover design produced a null result. However, when the two target areas were evaluated separately, naturally heavier rainfall over a wide region on days when the north target area was seeded produced the appearance of increases in rainfall due to seeding in the north target area, and when the south target area was seeded, the appearance of decreases in rainfall due to seeding was produced.

Target-control (as contrasted with crossover) evaluations of the second Israeli experiment for the north target area alone foundered when control stations were selected from a relatively small region of anomalously low seed/no-seed ratios that was situated within a much larger region of high seed/no-seed ratios, which included Lebanon, Jordan, and most of Israel. Thus, the north target area seed/no-seed ratios are not an isolated, seeding-induced anomaly. On the contrary, it is the low seed/no-seed ratios of the northern coastal control stations, selected after the experiment began, that are anomalous in a regional context and are virtually the only stations that yield an apparently statistically significant effect due to seeding in the north target area.

It is concluded that neither of the Israeli experiments demonstrated statistically significant effects on rainfall due to seeding.

Considerations of the rainfall climatology of Israel, recent reports concerning the microstructure of clouds in Israel, aid the relatively small amount of seeding carried out in the first Israeli experiment support the view that seeding was unlikely to have had significant effects on rainfall. Contrary to previous reports, clouds in Israel contain large cloud droplets, precipitation-sized drops, and considerable concentrations of natural ice particles at quite high temperatures, all of which should obviate attempts to increase rainfall by artificial seeding in wintertime air masses.

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Arthur L. Rangno and Peter V. Robbs

Abstract

Two statistical experiments, carried out in Israel, appeared for a time to have provided a unique demonstration of the ability of cloud seeding to increase rainfall. In this paper the authors examine the possibility that both experiments were compromised by type I statistical errors (i.e., “lucky draws” or false positives). It is concluded that in the first Israeli experiment a type I statistical error produced the appearance of statistically significant effects of artificial seeding on rainfall 1) in the buffer zone and the center target area, 2) in the coastal region of Israel, a few kilometers downwind of the seeding, and 3) in portions of Lebanon, Syria, and Jordan.

Analysis of the second Israeli experiment using the original crossover design produced a null result. However, when the two target areas were evaluated separately, naturally heavier rainfall over a wide region on days when the north target area was seeded produced the appearance of increases in rainfall due to seeding in the north target area, and when the south target area was seeded, the appearance of decreases in rainfall due to seeding was produced.

Target-control (as contrasted with crossover) evaluations of the second Israeli experiment for the north target area alone foundered when control stations were selected from a relatively small region of anomalously low seed/no-seed ratios that was situated within a much larger region of high seed/no-seed ratios, which included Lebanon, Jordan, and most of Israel. Thus, the north target area seed/no-seed ratios are not an isolated, seeding-induced anomaly. On the contrary, it is the low seed/no-seed ratios of the northern coastal control stations, selected after the experiment began, that are anomalous in a regional context and are virtually the only stations that yield an apparently statistically significant effect due to seeding in the north target area.

It is concluded that neither of the Israeli experiments demonstrated statistically significant effects on rainfall due to seeding.

Considerations of the rainfall climatology of Israel, recent reports concerning the microstructure of clouds in Israel, aid the relatively small amount of seeding carried out in the first Israeli experiment support the view that seeding was unlikely to have had significant effects on rainfall. Contrary to previous reports, clouds in Israel contain large cloud droplets, precipitation-sized drops, and considerable concentrations of natural ice particles at quite high temperatures, all of which should obviate attempts to increase rainfall by artificial seeding in wintertime air masses.

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