BULLETIN of the American Meteorological Society

Published Monthly except July and August at Prince and Lemon Streets, Lancaster, Pa.f William E. Hardy, Department of Meteorology, Oklahoma A & M College, Stillwater, Oklahoma, Editor Robert G. Stone, Route 1, Box 540, Clinton, Maryland, Consulting Editor

VOL. 36 APRIL, 1955 No. 4

Rainfall and Stardust

MILDRED B. OLIVER AND VINCENT J. OLIVER

6511 Flanders Drive, Hyattsville, Md.

ABSTRACT

Bowen's cosmic cloud-seeding hypothesis of rainfall singularities is examined using rain- fall data from the African tropics. These data are presented for January and evaluated for the entire year. Some of the physical uncertainties remaining to be investigated are discussed.

ETEOROLOGISTS have tended to look hemisphere, Bowen evolved the idea that unusu- ally heavy deluges of rain may be due to the seed- M ing of cumuliform clouds by particles derived somewhat askance at the subject of from meteor showers. He found at times, even singularities, exceptions as they seem above inversions, certain abnormally high concen- to be to the general laws of atmospheric phe- trations of nuclei which could not be accounted nomena. But the evidence is piling up that for by any known terrestrial source, e.g., by ver- singularities do occur. Particularly in the light tical transport of dust from the ground in turbu- of the careful research of Namias, Wahl,1 and lent eddies, by volcanic eruptions, etc. [2]. There- Brier 1 in this country and of other's elsewhere, fore, he began to look for some extra-terrestrial we now have adequate documentation for the source for the observed nuclei and concluded that statistical existence of such things as "January the only really possible source was the debris from thaw," "index cycles," and the like. But the cause meteor showers. It so happens that many promi- and meaning of singularities is still very much in nent meteor showers recur annually, suggesting question. BOWEN'S THEORY that there would be a maximum of nuclei on the Recently, E. G. Bowen [1] suggested a new same dates every year and that the recurrent approach to the causality of rainfall singularities, maxima of nuclei might be responsible for rainfall which at first glance seems plausible enough. singularities through a natural cloud seeding. However, the physical mechanism for producing The physical mechanism proposed by Bowen to unusual rains which he proposes contains several explain meteoric seeding may be summarized problematical features. roughly as follows. After meteor showers strike From his measurements of the concentration of the atmosphere in the neighborhood of 85 km, the ice-crystal nuclei in the atmosphere of the southern smaller particles (1-4 microns in diameter) drift 1 See Bull. Amer. Met. Soc., Nov. 1952, p. 380; id., Oct. 1954, p. 378. downward through the atmosphere at an average

t Entered as second class matter September 24, 1945, at the Post Office at Lancaster, Pennsylvania, under the Act of August 24, 1912. Acceptance for mailing at special rate of postage provided for in paragraph (d-2), section 34.40, P. L. and R. of 1948, authorized September 24, 1945. Address all business communications, purchase orders and inquiries regarding the Society to the Executive Sec- retary, 3 Joy Street, Boston 8, Mass. See inside back cover for complete information regarding publications, officers and activities of the Society. 147

Unauthenticated | Downloaded 09/26/21 10:13 AM UTC 148 BULLETIN AMERICAN METEOROLOGICAL SOCIETY rate of, roughly, 10,000 feet per day. At the end TABLE II. METEOR-RELATED RAINFALL PEAKS IN of about 30 days many of them will descend to the KENYA (1937-1948) levels of tall cumulus clouds (40,000 to 50,000 Average lag 32 days with a coefficient of variation of feet). If cumuliform clouds in the water phase 5 percent. should build up to these levels and intercept the (2) (3) (4) (5) meteoric dust, seeding would be rapid; the re- (1) Date of Date of Lag of Years in Name of Meteor Kenyan (3) After Which Heavy Meteor sult—a cloudburst. The induced rainfall is po- Shower Rainfall (2) in Rain Fell Shower tentially heavy enough so that if meteoric seeding Maximum Peak Days This Date occurred at any one place only once in every ten Quadrantids Jan. 3 Jan. 30 27 1947 e-Aquarids May 1-11 June 2 30 1943-46-47 to twenty years, it could produce a singularity in f-Perseids June 3 July 3 32 1940 Arietids June 8 July 12 35 1942, 1948 the daily rainfall record. Such a frequency is 54-Perseids June 25 July 30 35 — /S-Taurids July 2 Aug. 5 34 1945, 1948 reasonably likely in view of the fact that many j'-Geminids July 12 Aug. 17 36 1945 Start of Perseids July 17 Aug. 17 31 1938 meteor showers recur on a given date every year, 0-Aurigids July 25 Aug. 24 30 1939 5-Aquarids July 28 Aug. 29 32 1940 others every few years. Furthermore, implicit in Perseids Aug. 12 Sep. 9-11 28-30 1946 the extra-terrestrial origin of these nuclei is the Giacobirids Oct. 9 Nov. 9 31 1941, 1946, 1947 Orionids Oct. 20-23 Nov. 21 30 — provocative idea that meteor-related rainfall singu- Taurids Nov. 3-10 Dec. 10 30 1946, 1948 Geminids Dec. 13-14 Jan. 12-15 30-32 1947 larities would occur simultaneously over the entire Ursids Dec. 22 Jan. 22 32 1941 earth, wherever sufficiently tall cumuli existed at the proper time. This means that the supposedly noted by Bowen in the southern hemisphere, and meteor-related singularities found by Bowen in the peak rains of Southern Rhodesia and Kenya. the southern hemisphere should exist on the very The January curves shown in FIGURE 1 look par- same dates in the northern hemisphere and even ticularly similar. But the detailed statistical in- at the equator. vestigation of the lag of the peak rains in Kenya

STATISTICAL INVESTIGATION It happens that the authors have been investi- gating the rainfall of East Africa, with special reference to rainfall in Southern Rhodesia and in the Kiambu district of the Rift Valley of Kenya. The part of Rhodesia considered is highland country between 16° and 18° S, nearer the equator than the areas of the southern hemisphere whose rainfall Bowen has studied. Kiambu district lies almost at the equator (1°S, 37°E) at elevations between 4000 and 6000 feet, on a relatively low plain between two ranges of mountains. In both these countries it is known that some of the heavy rainfall comes from cumulonimbus clouds extend- ing up to great heights, clouds potentially sus- ceptible to seeding by meteoric nuclei since they extend well above the freezing level. TABLES I and II and FIGURE 1 show the cor- respondence between the maxima of the principal annual meteor showers, the rainfall singularities

TABLE I. METEOR-RELATED RAINFALL PEAKS Australian data (1859-1950) from E. G. Bowen. Ken- yan data (1937-48) from Bulletin of Daily Rainfall, British East African Meteorological Service, Nairobi.

Name Date of Date of Date of Date of FIG. 1. January rainfall regime. Abscissae are date in of Meteor Australian Kenyan Rhodesian Meteor Shower Rainfall Rainfall Rainfall January for all curves. Upper left curve: small dashes Shower Maximum Peaks Peaks Peaks represent average rainfall intensity for the month, large Quadrantids Jan. 3 Jan. 31-Feb. 1 Jan. 30 Jan. 30-31 dashes are twice the average intensity. Curves for Chile Geminids Dec. 13-14 Jan.12-13 Jan. 12 Jan.14 Ursids Dec. 22 Jan.22-23 Jan.24 Jan. 22 and Australia from E. G. Bowen. Curves for Kenya and Rhodesia, M. B. Oliver and V. J. Oliver.

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tween the dates of occurrence of the two phe- nomena is significantly better than that obtained by chance. (Since such cases are numerous in meteorological and climatological research, the method used here is also one of general interest.) In applying the method to our problem, a peak rain was defined as one whose intensity was more than twice the average intensity (i.e., total rainfall divided by number of rainy days) for the month. By using this method of selection for peak rains, those peaks made up of numerous but light rains were eliminated; those peaks caused by infrequent deluges retained. Only the maxima of activity of prominent meteor showers were used and when- ever two occurred within five days of each other, they were treated as one. Our problem, then, is FIG. 2. Quantitative lag relationship between meteor to correlate two sets of dates: (1) the dates of showers and selected heavy rains in Kenya. the peak rains, and (2) the dates of the meteor shower maxima, as defined above. We wanted after meteor showers for all twelve months turned in particular to know whether the 30-day lag out to be less promising. Bowen noted gives a significantly better fit be- This statistical investigation was carried out by tween the two sets of dates than chance would a lag method, suggested to us by G. W. Brier [3]. give. If we assume that there is a lag between the Following Brier's suggestion, we used two cir- occurrence of meteor showers and the occur- cular discs, one slightly larger than the other, and rence of peak rains, this method quickly solves marked along the edge of each the 365 dates of the the problem of just exactly which lag gives the year. On one disc we marked the dates of the best fit between the two sets of occurrences. The peak rains in Kenya, on the other the dates of the method can be used for this purpose in cases where meteor showers. Then, superimposing the two one of the related phenomena occurs much more discs concentrically with the two January firsts frequently than the other and in cases where lined up, we counted the number of days between neither phenomenon occurs at regular intervals. the date of each meteor shower and the nearest For these cases there is no mathematical statistic rainfall peak. The sum of these numbers, to be which tells us whether a lag noted empirically be- referred to hereafter as the deviation factor, is a

FIG. 3. Quantitative lag relationship between meteor showers and rainfall peaks in Kenya. Curve "A" using all rainfall peaks. Curve "B" using major rainfall peaks.

Unauthenticated | Downloaded 09/26/21 10:13 AM UTC 4 BULLETIN AMERICAN METEOROLOGICAL SOCIETY measure of the correlation between the two sets January (succeeded by much drier weather the of phenomena for zero days lag. next two weeks), which might be due to cloud If there is actually a 30-day lag between meteor seeding by the Geminids of early December. On showers and rainfall peaks, the deviation factor the other hand, all these features of the East will be smallest after we rotate one of the discs so African rainfall regime have been explained in an as to superimpose January thirty-first of the rain- apparently adequate way by migrations of the fall date disc on January first of the meteor shower heat low across Africa in response to the motion disc. Since there is no satisfactory statistical of the sun relative to the earth (M. El-Fandy method for showing that a minimum in the devia- [4]). tion factor for a 30-day lag is the only or the best minimum, it is necessary to compute the deviation CONCLUDING EVALUATION factors for all possible lags, up to 365 days. Our statistical investigation of the rainfall re- FIGURE 2 shows partial results obtained with gime in East Africa does not lend support to the Kenyan data. There is, indeed, a marked Bowen's theory of meteoric seeding for this sec- minimum of the deviation factor with a 32-33 day tion of the world. But this could be because dur- lag. Although this would favor Bowen's theory, ing at least one third of the year (May through the even lower value of the deviation factor at 60 August) most of the rain comes from "warm" days is disquieting. In FIGURE 3 we have Brier clouds, clouds not susceptible to seeding. We are and Enger's curve [3] of the deviation factor for unable to ascertain the frequency or annual distri- an entire year, using substantially the same data bution of thunderstorms in this part of Kenya (using all rainfall peaks). The many minima (information vital in evaluating the importance of and maxima in FIGURE 3 make it look as though our statistics) ; however, in favor of Bowen's any correlation based on these data between me- hypothesis is the fact that the January correlation teor shower maxima and peak rains is not much between meteors and heavy rains is the best for better than that which could be produced by any of the twelve months and in January thunder- chance. As further amplification of this fact, we storms are known to occur. computed the deviation factor in the lag between In addition to the uncertainty as to the physical the occurrence of meteor showers and droughts, characteristics of the rainfall involved in our stud- with partial results shown in FIGURE 2. There is ies, there are important uncertainties in the physi- no minimum in the deviation factor for the cal mechanism proposed by Bowen. Perhaps the droughts at about 30 days, but the curve in gen- wide fluctuations in ice crystal nuclei are due to eral has peaks and dips in it similar to the devia- terrestrial, rather than extra-terrestrial influences tion-factor curve for the peak rains. (The devia- [ 5 ]. Computations of the rate of fall of meteoric tion factors for the droughts are greater numeri- dust particles of the size Bowen discusses show cally because of differences in the numbers of these particles will slow down markedly as they droughts and peak rains per year.) reach the lower portions of our atmosphere and In conclusion, it may be said that these rain- would take about a year to fall the last 10,000 feet. fall statistics for East Africa do not favor Bowen's Wexler's [6] report of the spread of dust from the theory. explosion of Krakatoa shows that this dust re- mained in the atmosphere in large enough quanti- PROLONGED METEOR SHOWERS ties to affect the color of the sky for at least three Neither the method applied above, nor Bowen's years and Whipple [7] has expressed the opinion work sheds any light on the effect of prolonged that the amount of cosmic dust daily entering the meteor showers. Although many meteor show- atmosphere is increased by only a small percentage ers have a span of but a few hours or of a single during meteor showers. It follows that unless the day, three prominent annual showers persist for meteoric dust is washed out of the atmosphere several weeks. Dust particles from these lengthy there is always a large concentration of it in the meteor showers might well be expected to make lower atmosphere and, as clouds penetrate these for increased general raininess associated with the nucleated layers, the dust is carried up in the entire duration of the meteor shower spell. There clouds. Therefore nearly all clouds contain these is actually an increase of raininess in early Sep- particles and it is difficult to see how seeding can tember, which might be due to the Perseids of late be dependent on the presence of newly arriving July and early August; raininess in late Novem- meteroic dust particles, or how rain can be re- ber, which might be due to the Orionids of Oc- lated to the time the meteors enter the atmosphere. tober; and rainy weather the first two weeks in Another related physical uncertainty concerns

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whether or not ice crystals form on meteoric dust have appeared in the Australian Journal of Phys- at all, and, if they do, on what range of particle ics, Vol. 7, 1954, by W. C. Swinbank (p. 354) and sizes. Since the rate of fall of the particles is a by J. Neumann (p. 522). In particular, the latter function of their size and density, no rigorous shows that Bowen's rainfall singularities can be computations are possible until we determine what readily explained as due to random statistical size dust particles can be effective seeding agents. effects. It is easily possible that the dust of the correct REFERENCES size would fall too slowly or rapidly for Bowen's [1] Bowen, E. G., "The Influence of Meteoritic Dust on observed thirty-day lag. Moreover, the disruptive Rainfall," Australian Journal of Physics, Vol. 6, No. 4, pp. 490-7, 1953. effect of vertical motions in the atmosphere on the [2] Bowen, E. G., Seminar discussion at U. S. Weather rate of fall of the dust has not been taken into Bureau, Washington, D. C., Jan. 8, 1954. account. [3] Brier, G. W., Unpublished manuscript. U. S. Weather Bureau, Washington, D. C., June 1954. Until it is demonstrated that the fluctuations of [4] El-Fandy, M., "Effects of Topography and Other meteoric nuclei in the atmosphere vary in a manner Factors on the Movement of Lows in the Middle similar to the observed heavy rainfall variataions, East and Sudan," Bull. Amer. Met. Soc., Vol. 31, No. 10, Dec. 1950. we feel meteorologists would do well to consider [5] Schaeffer, V. J., "The Concentration of Ice Nuclei Bowen's "stardust" theory as an interesting but in Air Passing the Summit of Mt. Washington," Bull. Amer. Met. Soc., Vol. 35, No. 7, 1954. as yet unproven explanation for rainfall singu- [6] Wexler, H., "Spread of the Krakatoa Volcanic Dust larities. Cloud as Related to the High Level Circulation," Added in Proof: Since this article was written, Bull. Amer. Met. Soc., Vol. 32, No. 2, Feb. 1951. [7] Whipple, F., Seminar Discussion at U. S. Weather several serious criticisms of Bowen's hypothesis Bureau, Washington, D. C., Jan. 1954.

methods used range from balloons, rockets, and radio, to REVIEW spectroscope, and noctilucent clouds. A particularly valuable and readable summary of past and present work in the ozone layer is given. The symposium on radiation is given over to the fun- Proceedings of the Toronto Meteorological Confer- damental problem of calculating the radiative flux in an ence 1953. Edited and Published under the direction atmosphere composed of several radiating gases whose of the Council of the Royal Meteorological Society, pressure, temperature and concentration vary with height. 1954. Pp. 294; 5 plates, numerous figures, $4.50 (30 Investigations included here deal with spectral models Sh.). [$2.40 for members of RMS and AMS.] designed for both regular and random distribution of lines. This publication presents the results of a joint meeting Meteorologists contemplating the problems of the de- of the American and Royal Meteorological Societies held veloping field of aviation meteorology in the arctic will in Toronto in September 1953. Touching, as it does, on welcome the wealth of material appearing in the arctic nearly all problems of modern meteorology it is some- section. Papers include not only the recently acquired thing of a compendium in itself. It contains nearly 50 experience of arctic meteorologists, and studies of physi- papers. Papers, by invitation, include the results of cal and climatological problems but also the down to recent solid research and also numerous summaries of earth problems of establishing and operating arctic sta- the present state of development of various phases of the tions and selecting their personnel. Of special interest is science. During the full week deliberations nine sym- the set of cyclone and anticyclone tracks prepared by posia were held on the topics: Ozone and the High At- project AROWA. mosphere ; Radiation and Other Energy Transformations; With the wealth of upper air data now available the Arctic Meteorology; The General Circulation; Modern concepts of angular momentum flux, originally intro- Cyclone Theory; High-Level Forecasting; Climatic duced by Jeffries, are being developed and quantitative Change; Microclimatology and Micrometeorology; Cloud measurements made. The circulations required to explain Physics and Induced Precipitation. Discussions are also these transfers are considered. Other studies describe the added in many cases. mean flow patterns over the hemisphere, their seasonal In the first symposium the papers give the results of variations, the subtropical jet stream, and the equatorial recent studies of the high atmosphere, generally from high level easterlies. 15-100 km. These include the distributions and varia- A notable feature of the cyclone theory symposium, is tions of temperature, wind, ozone, and turbulence. The (Continued on page 158)

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