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. Unauthenticated | Downloaded 09/26/21 10:13 AM UTC VOL. 36, No. 4, APRIL, 1955 149 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.