Article

Status of possibilities of artificial precipitation

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

Cloud seeding with dry ice or silver iodide is designed to increase the ice-crystal content of clouds containing supercooled liquid water droplets, and thereby to permit the growth of precipitation elements (snowflakes or raindrops) by diffusion from water droplets to the induced ice crystals. Uncertainties as to need for additional ice crystals in clouds of appreciable water content make it difficult to estimate the potential value of cloud seeding in producing increased precipitation. Tests thus far have shown that it is often possible to produce slight amounts of precipitation from the bases of clouds the rain reaching the ground on some occasions. Further research, both in the field and hi the laboratory, will be needed to determine quantitatively the significance of cloud seeding in augmenting the nation's water resources. A short bibliography is furnished.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
Article
Full-text available
It is generally acknowledged that drizzle or light rain can fall from clouds which do not reach freezing level and cases have recently bean described in which moderate to heavy rain has been observed to fall from such clouds. A simple theory is developed to account for the phenomenon, based on the initial growth of cloud droplets by condensation followed by the growth of a small fraction of their number by coalescence. These grow in their ascent through the cloud until they are large enough to remain in suspension in the upward air current, after which they fall as min. It is shown that for a given set of cloud conditions the maximum height reached by the drops increases with increasing vertical air velocity and that the size of the drops emerging from the base of the cloud is nearly a linear function of the height attained. The time for the precipitation to appear, on the other hand, is an inverse function of the upward air velocity. Experimental observations of rain from non-freezing clouds have distinguished two main types. The first of these shows an increase in drop diameter or rainfall intensity downward through the cloud, as would be expected if the drops followed a variety of trajectories within the cloud. The second type is one in which the drop trajectories tend to coincide, in which case there would be a maximum in the raindrop density and the rain water content at some defined height within the cloud. This has been verified qualitatively by radar observations and flight experiments.
Article
On account of the scarcity of data on the number of nuclei over the oceans, I thought it would be interesting to carry out new measurements of the nucleus content of the air during a voyage from New York to Le Havre, France, and back, aboard the SS America of the United States Lines. The last observations in the North Atlantic were made between 1928 and 1934. It seemed also worth while to see if the contamination of the atmosphere had increased since that time. The instrument used was an Aitken pocket dust counter (modified model, after G. Luedeling), which was recalibrated according to the findings of V. F. Hess and C. O'Brolchain [see 1 of "References" at end of paper].
Article
The results of cloud-seeding experiments are reviewed briefly and it is concluded that, taken alone, they do not constitute an adequate basis for a sound appraisal of the potentialities of the cloud seeding techniques. An attempt is made to extend and supplement the observational evidence with the aid of existing knowledge of cloud physics and synoptic meteorology. It is found that significant amounts of precipitation can fall only from clouds which are continuously replenished. It is concluded that the thermal effects of seeding are seldom sufficient to stimulate the vertical development of a cloud. Since the activation-temperatures of the artificial ice-nuclei are believed to be higher than the activation-temperatures of natural ice-nuclei there is a possibility that seeding may advance the onset of precipitation or release precipitation from clouds which do not extend to the activation-temperature of the natural ice-nuclei. It appears that the seeding of clouds which are already releasing precipitation may either decrease or increase the total precipitation; an unequivocal answer is not possible due to the lack of pertinent information regarding natural precipitation processes. On the basis of the analysis of the physical processes and general knowledge of synoptic meteorology it is concluded that it is unlikely that cloud seeding will cause large-scale effects. Further research on cloud physics and cloud seeding is recommended.
Article
Tests involving exposure of AgI smoke to bright sunlight show a decrease in concentration of effective nuclei (at −20°C) of approximately two orders of magnitude per hour. The concentration of effective nuclei is increased greatly (as much as two orders of magnitude) by the addition of a little ammonia vapor to the AgI smoke. Smoke samples which have been deactivated completely by exposure to ultraviolet light can be caused to form large numbers of ice crystals by the addition of ammonia vapor. If ammonia is added before exposure to light, the rate of decay is the same or greater, and the effectiveness cannot be restored by further addition of ammonia. The effect of ammonia is believed to be due to the adsorption of ammonia on the silver iodide surfaces or to the formation of an ammine of silver iodide.
Article
Cloud seeding operations took place over a 10,000 square mile area in Central Arizona during the first four months of the winter of 1951. The natural precipitation for these months in this area was estimated by a simple correlation method utilizing an adjacent unseeded region. The actual precipitation consistently exceeded the estimated natural precipitation. The apparent precipitation increases were roughly proportional to the amount of cloud seeding equipment used. Although the correlation technique applied is rather inexact, it seems sufficiently accurate to make the consistent precipitation increases suggestive of a large positive effect from cloud seeding.
Article
Preliminary experiments have been made on seeding natural supercooled clouds with silver iodide smokes. It is believed that in many cases positive effects were observed. In a majority of the experiments it is impossible to prove beyond doubt that the effects are the result of the seeding. However, certain of these experiments demonstrate conclusively that modification of natural clouds with silver iodide smokes can be achieved. Areas of supercooled ground fog several hundred feet in diameter have been changed to ice at a temperature of − 5°C by small scale releases of silver iodide smoke from the ground. Similarly areas up to a mile in diameter have been filled with small ice crystals by releasing the smoke at − 20°C when the air is supersaturated with respect to ice. On December 21, 1948, a supercooled stratus cloud layer approximately 1,000 feet thick at a temperature of − 10° was seeded from an airplane with silver iodide smoke produced by dropping three or four pounds of burning charcoal which had been impregnated with one percent by weight of silver iodide. For purposes of comparison, and in order to definitely establish the position of the seeding, dry ice seedings were made about three miles away on either side. The results of the silver iodide seeding are clearly visible in photographs taken from the airplane. About six square miles of the supercooled cloud layer were transformed into ice crystals as the result of seeding with somewhat less than one ounce of silver iodide.
Article
Photolytic decomposition of silver-iodide crystals has been observed when the crystals were exposed to light of wave lengths less than 4300Å, as indicated by darkening of the exposed crystals. Qualitative observations indicate exposed silver iodide crystals undergo reversible photolysis, although the exact conditions under which this takes place is not well understood. When silver iodide nuclei were similarly exposed to light, the ability to form ice particles, when injected into a cloud of super-cooled water droplets, was found to be essentially destroyed. It is believed that, as a result of photolysis of the exposed silver iodide nuclei, the physico-chemical nature of surface of the nuclei has been altered to minimize effectively the surface-structure sensitive process of ice nucleation.
Article
On account of the relative scarcity of data on nuclei over the oceans, the author repeated his earlier observations (1948) in July and August 1951 on the S.S. America from New York to Le Havre and back. The average number of nuclei found per cubic centimeter for the western half of the Atlantic was 1,512 on the eastward trip and 1,229 on the westbound trip. On the eastern half of the Atlantic, the respective figures were 462 and 887. The mean (total) was 956 in July and 1,019 in August. Both figures are somewhat higher than in 1948.
Article
In experiments carried out near Sydney, supercooled water clouds were " seeded " with ice crystals by dropping pellets of solid carbon dioxide into them. Up to August 25, 1948, 20 experiments were made under conditions sufficiently well defined for the experiments to be significant. In 15 of them precipitation is believed to have been released artificially. Of 11 clouds with tops colder than -7°C., 10 precipitated. The depth of the treated clouds ranged from 1000 to 15,000 ft. In none of these did natural precipitation occur within 20 miles. Further evidence that the effect is genuine is given by the fact that both the likelihood of success in inducing precipitation and the time between treatment of the cloud and the appearance of precipitation at its base, varied consistently with the cloud characteristics. The observations are consistent with the view, as postulated by Bergeron, that the precipitation grew from ice crystals (formed by the dry ice pellets).
Article
This paper describes five experiments in which granulated dry ice was dropped into layers of cumulus cloud at temperatures below freezing. In each case precipitation of snow or ice crystals occurred, and part or all of the cloud dispersed. Displays of haloes and similar phenomena were observed.
Article
Generators are described which are capable of producing large numbers of silver iodide smoke particles for nucleating natural clouds. These generators are of two kinds, the charcoal-burning type and the spray-nozzle type. The charcoal-burning generator produces silver iodide smokes by burning wood charcoal which has been impregnated with silver iodide. The spray-nozzle generator produces smokes by burning silver iodide solution atomized from a small spray nozzle with compressed hydrogen gas. The construction and operation of these smoke generators is described. In addition, a larger generator employing a turbo-jet burner is described. The preparation of the silver iodide solutions used in these smoke generators is outlined.
Article
The question of the modification of natural clouds by artificial seeding is examined in terms of current theoretical considerations as to the physical-chemical and meteorological effect of seeding agents on clouds. The results of experimental laboratory studies made in this country and abroad on the formation of the ice phase and precipitation are reviewed. The results of the joint Air Force-Weather Bureau seeding trials then are studied in light of these theoretical considerations. The combined theory and experiments point to little likelihood of large-scale weather control using present techniques.
Article
During the last decade many experiments have been designed to investigate the initial processes of ice formation in the atmosphere. The results of these experiments are examined for clues as to the identity of the responsible nuclei and the following tentative conclusions are reached: (a) The nuclei which cause freezing between 0°C and −32°C are mainly solid, insoluble particles which are wetted by water and produce ice particles through the freezing of water drops. (b) The nuclei which become operative in the range −32° to −11°C consist of droplets of sea salt solution, ice and salt crystallising out on contaminating foreign particles at a temperature depending on the efficiency of these particles and the rate of expansion of the ambient air. (c) The nuclei effective at just below −41°C consist of droplets of pure salt solution and possibly of “ gaseous” nuclei formed industrially or by the action of ultra-violet light on the gases of the upper atmosphere.
Article
Calculations are made from which it appears that the rate of loss of heat from ice particles growing by coagulation in dense supercooled clouds may be insufficient to allow all the acquired water to freeze. The larger coagulation-elements within cumulonimbus clouds may therefore have liquid skins at levels where the temperature has fallen to −20° or −30°C. The rain-drop multiplication process envisaged by Langmuir may thus proceed also at temperatures well below 0°C, and the effect may also be important in the production of ice nuclei by‘splinter’ formation and in the generation of thunderstorm electricity.
Article
The theory of the formation of rain by coalescence leads logically to the suggestion that, if water drops of suitable size are sprayed into the base of a convective cloud, the drops will ascend in the upward air current, will grow by coalescence with the cloud droplets and may ultimately fall out as rain. Calculations show that under appropriate conditions the drops could attain a diameter greater than 1 mm and emerge from the base of the cloud from 20 to 60 min after water had been sprayed into it. Eleven experiments have been carried out to test the possibility of the method. In 10 cases, distinct changes occurred following the introduction of a water spray into the selected cloud. In 4 of these, virga fell from the cloud and it dissipated soon afterward. In 4 cases, rain or hail appeared shortly after spraying. On only one occasion were no results observed. The results suggest that the method might be useful for the artificial stimulation of rainfall, but further experiments are required to fully establish the mechanism.
Artificial rain, a review of the subject to the close of 1889
  • Ward
Artificial nucleation of clouds
  • Ronald Frith
  • Frith
An analysis of cloud-seeding operations in north-central Oregon
  • R T Beaumont
  • Beaumont
Verification studies of commercial cloud-seeding operations in Wyoming for the period
  • Miller
Comments and communications—Langmuir's article on precipitation control
  • Hall
Water resources development, final research report to the State of New Mexico Economic Development Commission
  • Holmes
Rain making and other weather vagaries
  • W J Humphreys
  • Humphreys
First partial report on the artificial production of precipitation—stratiform clouds, Ohio
  • Coons
On the results of recent experiments in the artificial production of precipitation
  • Haurwitz
The microphysics of clouds, Rep. Progress in Physics
  • Mason
Report of cloud-seeding experiments in the Santa Ana River Watershed and Escondido Drainage Area
  • John A Battle
  • Battle
Report on experiments at Kongwa on artificial stimulation of rain
  • Davies
Preliminary report on analysis of climatological data for the spring cloud-seeding period over north-central Colorado
  • Sol D Resnick
  • Resnick
The natural and artificial formation of snow in the atmosphere
  • J Schaefer
  • Schaefer
Compendium of meteorology, Amer
  • Malone
Experimental meteorology
  • Vincent J Schaefer
  • Experimental Meteorology
  • Schaefer