The Nature and the Origin of Nuclei in the .Atmosphere

By K. Isono, M. Komabayasi and A. Ono

Geophysical Institute, Tokyo University (Manuscript Received 23 December, 1959)

Abstract

Concentrations of ice-nuclei active at temperatures of -13•Ž, -15•Ž and -20•Ž were observed with an ice-nucleus counter in Tokyo, Japan. The concentrations varied from day

to day in a wide range. Generally, higher concentrations occurred on days when air came from the continent of Asia and low concentrations occurred when air came from the

Pacific Ocean. Abnormally high concentrations were observed when air reached Japan

from the arid regions of North China and Mongolia where heavy dust storms occurred in front of cold fronts. Although the ice nuclei concentration was low in maritime airmass,

sometimes high values were observed. This was found to be associated with volcanic eruptions. Possibility of production of ice nuclei from sea-spray is also discussed.

The ice-forming abilites of volcanic ash from ten active volcanoes in Japan, of dust

(loess) particles from North China which fell in Japan, and of soil particles and stone meteorite were determined in laboratory.

It is concluded that the principal active ice nuclei in the atmosphere are some kinds

of soil particles (especially, clay minerals) blown up from arid regions and volcanic dust from active volcanoes.

authors (Isono 1955, 1959) examined the nuclei 1. Introduction of crystals and identified their substance

Although the importance of ice-forming from their electron diffraction patterns. He nuclei in the precipitation process has been found that most of them were clay minerals, widely recognized, our knowledge about their probably kaolinite and montmorillonite, and nature, origin and concentration in the atmos- concluded tentatively that the nuclei im- phere has been rather poor. portant in precipitation might be clay minerals In these years, various substances such as and other silicates such as volcanic dust parti- silver iodide and lead iodide have been found cles. Mason (1950, 1955) stated that the natu- to serve as ice-forming nuclei, and the process ral ice-forming particles active at tempera- of ice formation on these nuclei has been tures above about -30•Ž were , insoluble studied by many investigators. As for the ice particles probably originating from ground nuclei naturally present in the atmosphere, such as clay-silicate particles. Bowen (1953) Schaef er (1954) observed their concentration suggested that meteoric dust might be ef- by use of a cold box kept at temperatures ficient nuclei. about -20•Ž on Mt. Washington, and found Isono and Komabayasi (1954) made a sta- that their concentration varied from day to day tistical study on the relation between the in a very wide range and that high counts eruptive activity of Volcano Asama and the occurred when the west wind came from semi- precipitation, concluding that annual and daily arid region of the west of the North America. amounts of rainfall in areas over which vol- He also determined threshold temperatures of canic dust was carried by upper wind were soil and mineral particles as ice-forming nu- larger in years with many eruptions and on clei. Kumai (1951) examined the centre nuclei days with eruption respectively, and that the of snow crystals and found that most of them increase of rainfall amount was probably contained soil particles. One of the present caused by the glaciation of by volcanic dust which was found to act as ice-forming * Division of Meteorology , Contribution No. 124. nuclei at temperatures below -13•Ž.

Vol. 37, No. 6, 1959 212 K. Isono, M. Komabayasi and A. Ono

Smith et al (Smith and Heffernan 1954, 2. Apparatus Smith, Kassander and Twomey 1956) mea- sured the concentration of ice-nuclei by air- Ice-crystals formed on ice nuclei are detected plane and found that the concentration was by means of cooled sugar solution as in the case higher at levels above inversion than near the of Bigg's apparatus (Bigg 1956) . In each test, surface, and suggested that the ice-nuclei had outdoor air is introduced into a copper-walled their origin in the upper atmosphere and were cylinder of two litres in volume by a metal disc, presumed to be meteoric dust as suggested on which a glass dish containing sugar solu- by Bowen (loc. cit.), for that high concentra- tion is set, being pushed down to the bottom tion occurred on days about thirty day after of the cylinder. A sheet of sponge which con- meteoric showers. Murgatroyd and Garrod tains glycerol is fixed on the fringe of the disc

(1957) measured the concentration of ice-nu- mentioned above being closely in contact with clei over England, and found similar facts the inner wall of the cylinder when the disc with respect to height, but he could not find slides down along the wall. When the disc the increase in the concentration about thirty is pushed down, the air which have been in days after meteoric showers. Nemoto et al the cylinder is exhausted through out-let tube (1957) measured -10•Ž nuclei in Tokyo in at the bottom of the cylinder and at the same March 1956 and stated that the concentration time the inner wall is smeared with glycerol. of the ice nuclei increased with increasing After the finish of this procedure, on the velocity of the local wind at the site of the cylinder is put a metal lid with a glass window, observation. Soulage (1958) made observation thus, the cylinder is kept air tight. The of ice nuclei on the coast of Basque, and found cylinder is placed in a tank (volume 3 litres) that high concentrations occurred only when which contains ethylengrycol- mixture air came from ocean (NW to SW wind) and which is cooled by dry ice or liquid carbon that at a site inland 50 km from the coast the dioxide. The temperature of the air in the measured values of the concentration were cylinder and that of refrigerant are measured low. with mercury thermometers. The outdoor Measurements of ice-nucleus concentration air which is introduced into the cylinder is have been made at various stations in the cooled at the rate about 3•Ž per minute. When world which were organized by the Radio- the temperature of the air reaches a pre-de-

physics Laboratory, Australian Common- termind temperature(-13•Ž, -15•Ž and -20•Ž wealth Scientific and Industrial Research Or- respectivery), water vapor is supplied. After

ganization. Some of the results have been a few minutes white hexagonal crystals are reported (Bracewell 1956, Heffernan and formed in the sugar solution as are in the case Bracewell 1959, Bigg 1958 and Kline and Brier of Bigg's counter. The difference between 1958). Heff ernan and Bracewell reported the a predetermined temperature of the air at existence of the peak on 16 January, and Kline which the number of ice nuclei is to be and Brier showed that statistically significant counted and the temperature of the refrigerant peaks existed on days about thirty days after should be as small as possible, otherwise the meteoric shower suggested by Bowen. error of measurement due to the uncertainty In order to get inf ormations about the nature of the temperature would be so large that and origin of the ice nuclei in the atmosphere, the measurement might become unreliable the authors have been measuring the concen- and not reproducible. In the present case, trations of ice nuclei effective at -13°C, -15°C the differences of the temperatures are kept and -20•Ž in Tokyo since April 1958. Some within 1.5•Ž. When ice nucleus counters of results of the measurement have been reported various types such as Schaef er's and Bigg's

in previous papers (Isono et al 1958, 1959). type counters were used, the authors were In the present paper, more detailed and full suffered from the disturbance caused by tiny descriptions and discussions about the results crystals visible and invisible formed on of the measurement made in the period from the inner walls of the test chambers and air April 1958 to May 1959 are presented. inlet tubes and other portions of the apparata: abnormally high counts were often found to

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 213

be due to the frost formation, especially when able concentration is in the present case one humidity of out-door was relatively high. In nucleus per 10 litres, or 0.1 nucleus per litre. the present counter described above, the insta- Possibility is considered to exist that the bility which often arises in the original Bigg's number of the detected ice nuclei might de-

type counter is avoided by smearing inner pend on the way of supply of the water vapour. wall with grycerol with the sponge, by closing In the present measurement, the water vapour the test chamber with the tight lid and by is at the water saturation or slightly above keeping the temperature difference between it at the time of vapour supply. The com-

the air under test and the refrigerant as small parison is now going on between two methods as possible. of measuring of ice nuclei concentration, cool- ing of air in a testing room through its 3. Daily variation of the concentration of wall by use of refrigerant such as in Bigg's ice nuclei method (Bigg 1956), and that by adiabatic ex-

The concentration of the atmospheric ice pansion of air in a testing room such as in nuclei has been measured in Tokyo since April Warner's method (Warner 1957). 1958. Measurement has been made on each In the present routine measurement, the day for three different threshold temperature temperatures of the sampled air have been ranges of nuclei, that is, air samples intro- lowered to -13•Ž, -15•Ž and -20•Ž respec- duced into the counter being cooled to the tively. So, three different kinds of nuclei of temperatures of -13•Ž, -15•Ž and -20•Ž which threshold temperatures are higher than respectively. Through the daily observation, -13•Ž , -15•Ž and -20•Ž have been measured some characteristic features of the variation daily. From this, the spectrum of threshold of the concentration of ice nuclei and a marked temperatures, or the dependence of activation relation between the concentration and the of nuclei on the temperature can be obtained. history of air-masses have been revealed. On As will be described in the later section, the the basis of the data obtained, the origins of temperature dependence of has the atmospheric ice nuclei are considered to been found to be defferent for ice nuclei from have been determined, so far as the ice nuclei different natural sources. The spectrum of over Japan is concerned. Dust storms in the threshold temperature as well as history of arid area of the continent of Asia and the the airmass provides us the important infor- eruptive activity of some active volcanoes in mation concerning their origin. Japan have been found to have a close rela- b) Results of measurement tion to the peak concentration of ice nuclei. The data of daily observation of concentra- In this section, the procedure of measurement tion of atmospheric ice nuclei are described and results of daily measurements are des- below. Situation of weather and the data of cribed. volcanic activities in Japan are added for re- a) Procedure of measurement ference. The measuring apparatus has been placed i) Dust storm of arid area in the Asiatic on the roof of a building, 30 meters above Continent and the increase of the concentra- the ground. Usually the measurement has tion of ice nuclei in Japan. been made in the daytime from eleven o'clock The measurement of ice nuclei was made in the morning to three or four o'clock in for a period from 17th April 1958 to 2nd May the afternoon in Japan Standard Time. This 1958. Data are tabulated in Table 1 and shown time is chosen because the thermal convec- in Fig. 1. tion is considered to be in its maximum in In this period, the day-to-day variation in mixing the air near the ground with the air the concentration of the ice nuclei was very in upper levels, so that the information of large. The ratio of the large concentration the ice nuclei in upper levels may be obtain- to small one was several tens to a hundred. ed as much as possible. Generally speaking, the concentrations of The total volume of the air of which ice -13•Ž nuclei, of -15•Ž nuclei and of -20•Ž nuclei content is measured is 10 litres for one nuclei varied roughly in parallel with each temperature. So the lower limit of the detect- other from day to day.

Vol. 3.7, No. 6, 1959 214 K. Isono, M. Komabayasi and A. Ono

Table 1. Concentration of atmospheric ice nuclei measured in Tokyo (April 17-May 2 in 1958).

come from the area of industry. This is considered to show that the important source which determines the main part of daily varia- tion of concentration of ice nuclei in Tokyo is neither ground in and around Tokyo nor industrial area in Tokyo. The present result contradicts with the views of Nemoto et al (loc. cit.) who state that the ice nucleus con- centration increases with wind velocity and suspects that ice nuclei are brought up by the wind from the ground. However, their result shows, as they point out, that the con- centration of ice nuclei is not lower on rainy days when the ground is wet than on clear days when the ground is dry at the same wind velocity. This fact seems to show that the source of the nuclei is not the ground near Tokyo. The dependence of the nucleus con- Fig. 1. Concentration of ice nuclei measured in Tokyo. Loess clouds arrived in Japan centration found by them may be not due to on 20, 26 and 27 April, the direct physical effect proposed by them, but due to an incidental and indirect correla- The concentration was found to be indepen- tion between the synoptic condition which has dent on the velocity and the direction of the a relation to the nucleus concentration as local wind in Tokyo. (see Fig. 2) Sometimes, described below and to the local wind velocity. at the time of measurement, dust was blown On the other hand, the concentration of ice upward from the nearby ground by strong nuclei was found to be correlated to the large wind, while no increase of the concentration scale synoptic situation around Japan. In order of ice nuclei was observed. No particular to know the history of the airmass which reach- increase of the concentration of ice nuclei was ed Tokyo each day in the period of measure- observed on the day of the wind which had ment, trajectories of air parcel at surface, 850 Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 215 mb and 500mb levels were drawn. The analy- from the arid area of the Asiatic Continent, ses of these trajectories have revealed a rela- the northern part of China, have been found tion between the history of airmass and the to have high concentration. Especially, when concentration of ice nuclei as described below. air masses were ones which had experienced Airmass which stayed over the ocean for a dust storm in this area, the concentrations of time longer than three or four days were found ice nuclei contained in them were very large to have very low concentrations. On the other and of the highest value among all observed hand, the airmases which arrived in Japan in Japan. The air parcel which arrived in

Fig. 2, a) -13•Ž nuclei (open circles) and Fig. 2, b) -20•Ž nuclei for the same period

-15•Ž nuclei (full circles) for a period as (a). from 17 April to 2 May 1953.

Fig. 2. c) -15•Ž nuclei for all period shown in Tables 1, 3, 5 and 6.

Vol. 37, No. 6, 1959 * : The airmass experienced a dust storm. For example, *15 means that the airmass encountered a dust storm on April 15.

216 K. Isono, M. Komabayasi and A. Ono

Fig. 2. d) -20•Ž nuclei for the same period as (c).

Fig. 2. Concentration of ice nuclei versus wind velocity.

Table 2. Histories of airmasses and ice nuclei 0 : less than 0.1 nucleus per litre for -13•Ž concentrations. and -15•‹ nuclei, less than 1 nucleus per litre for -20•‹ nuclei

+ : 0.11/l for -13•‹ and -15° nuclei, 1~10/l for -20•‹ nuclei

++ : 1~10/l for -13•‹ and -15~ nuclei, 10~40/l for -20~ nuclei

+++ : more than 40/l for -20•‹ nuclei

Fig. 3. Division of regions used in Table 2.

C : Continental Japan from southern Siberia had a concentra- M : Maritime tion of medium values. Airmasses which came C-M: Continental air staying over sea within a from southern part of China where climate is week wet had a low ice nucleus concentration. Cn : Continental north (Siberia) see Fig. 3 In Table 2 and in Fig. 3, the relation bet- Cm : Continental middle (northern China) ween the concentration of ice nuclei and the Cs: Continental south (southern China) histories of air masses are tabulated. The trajectories were followed back about a week before the arrival of the airmasses in Tokyo.

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 217

Some typical examples of trajectories are mass experienced dust storm during the travel shown in Figs. 4 a-f. A solid line shows a between 15th and 16th April in northern China. trajectory at 850 mb level. Dotted and broken The air parcel at 850 mb level came to Tokyo lines show those at surface and 500 mb levels straight from the arid area, but this air itself respectively. The numbers indicated along had not met the dust storm. The storms oc- trajectories are the dates of arrival at the curred in that area on 15th and 18th, April, respective locations. The areas where dust while this air passed there on 17th. The air storm was reported are stippled and the dates at 500 mb level passed there on 17th April also, of storm occurrence are indicated. On 20th but this air passed along a way a little north April 1958, the concentrations of -13•Ž, -15•Ž to that of the parcel at 850 mb, with greater and -20•Ž nuclei were very large. As shown velocity. The air between 850 mb and 500 mb in Fig. 4b, dust storms were observed in the levels had experienced dust storm on 18th. drainage of Yellow River on 15th, 16th and The dust storm which often occurrs in this 18th April. The air at the surface level passed area is called Kosa, which is the yellowish around the Korea peninsular and arrived at loess storm of very wide scale. When this Tokyo after another small turning. The air- occurrs, the sky becomes yellowish dark in

Fig. 4. a) 19 Apr. 1958. Fig. 4. b) 20 Apr. 1958.

Fig. 4, c) 23 Apr. 1958. Fig. 4, d) 25 Apr. 1958.

Vol. 37, No. 6, 1959 218 K. Isono, M. Komabayasi and A. Ono

Fig. 4. e) 26 Apr. 1958. Fig. 4, f) 27 Apr. 1958. Fig. 4. Trajectories of airmasses. Spotted areas show where loess storm and loess were observed at weather observatories. - - - - Surface, -850 mb, - 500 mb .

the source region of northern China. Even the following day, 27th, the loess clouds were in Japan, the clear sky becomes a little opaque. observed widely in Japan. As described above, The height of the top of dust layer often the peaks of the concentration of the atmos- reaches to 500 mb level. pheric ice nuclei during the spring season The concentration of ice nuclei attained a coincided with the days of arrival of loess maximum on 20th April, the very day when cloud in Japan. the main part of cloud of loess particles cov- On the other hand, when the air parcel ered Japan. Loess sampled from the ground which had passed over the Pacific Ocean for a of northern China is found to be effective ice long days arrived in Tokyo, the concentration nuclei by the test of cold chamber as is de- was observed to be very low, eventually scribed in the later section. below the lower limit of detection. A typical Similarily as 20th April, on 26th and 27th example is shown in Fig. 4d for 25th April, April, the concentration of ice nuclei was the day before the arrival of loess cloud in observed to reach another maximum. The which at the surface level, air came from trajectories of air parcels at surface, 850 mb south-east after several days travel over the and 500 mb levels are shown in Fig. 4e and 4f. Pacific. The air of upper levels came from In this case also the air mass which covered the south-eastern part of China, which is of Japan was one that had experienced severe wet climate and has no region of arid desert. dust storm in the area of northern China. On The air below 850 mb level came to Japan these two days, according to the flight log of from the south after travel over the Pacific. Japan Air Lines, air was dusty and smoky No ice nucleus could be detected in 10 litres and the visibility was very low below the level of sampled air on that day even at -20•Ž. of 50007000 ft in southwestern Japan and In other cases, that is, when the airmass below the level of 9000~10000 ft in north- came to Japan from the southern Siberia and eastern Japan. Yellow dust was found to ad- when the airmass came from the southern here to the windowpane of the airplane. This China continent, the concentration of ice nuclei indicates the presence of loess clouds in high took medium values. The examples are shown concentration over Japan on this day. On in Fig. 4a. On 19th April, the air of upper 26th, arrival of loess clouds in South Korea levels came from Siberia. Surface air was one was reported by the weather service, and on which had wandered around Sea of Japan.

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 219

Fig. 5. a) 00GMT 25 Apr. 1958,

Fig. 5. b) 00GMT 26 Apr. 1958.

Vol. 37, No. 6, 1959 220 K. Isono, M. Komabayasi and A. Ono

Fig. 5. c) 00GMT 27 Apr. 1958.

Fig. 5. d) 00GMT 28 Apr. 1958. Fig. 5. Surface weather chart. Areas shaded show where loess storm and loess clouds were reported by weather observatories and aircraft.

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 221

The -13•Ž nuclei was not detected. The information about the concentration of ice -15•Ž nuclei and -20•Ž nuclei were 0 .4/l and nuclei in the summer. It has been first ex-

44/1 respectively. On 22nd and 23rd April no pected that the concentration would be very -13•Ž nucleus was detected . Concentration low, since usually the maritime airrnass covers of -20•Ž nuclei was 8/l on 22nd 4/l on 23rd. Japan persistently in this season. However, So it is considered that the air came from the four volcanoes in Japan began to erupt in these southern Siberia contained -20•Ž nuclei, while months, and the concentration of -15•Ž nu- there is no strong source region of -20•Ž nu- clei markedly increased to values above the clei in the area of the southern China. But it peak concentration which occurred at the ar- has not yet been determined whether this dif, rival of cloud of loess. ference in the concentration of nuclei of -20•Ž The observed concentrations of ice nuclei is due to the difference in minerals in soils in this period are tabulated in Table 3 with in Siberia and southern China or due to the inf ormations about the eruptions of volcanoes difference in degree of aridity in the ground in Japan. Volcanoes Suwanose, Sakurajima, which may effect on the quantity of soil par- Aso, and Mihara erupted frequently and were ticles blown upwards by the wind. very active in this period. Fig. 6 shows the ii) The increase in the concentration of ice locations of these volcanoes. nuclei in a periods when volcanoes erupted On 19th June 1958, Volcano Sakurajima frequently exploded seven times. Dust clouds of ash In a period from 19th June to the end of reached to 2000 m above the crater. Before

July in 1958, measurement was made to get these explosions, Sakurajima continued to

Table 3. Concentration of atmospheric ice nuclei measured in Tokyo (June 19-Aug. 8 in 1959). Colour of smoke, height reached above the crater are indicated.

Vol. 37, No. 6, 1959 222 K. Isono, M. Komabayasi and A. Ono

Fig. 6. Active volcanoes in Japan of which ash samples were tested in a cold-box.

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 223

Table 4.

* For the sake of reference , the source area of loess cloud is added. blow up grayish and white smoke for a few in Tokyo reached very high values. There- days. On the same day, Volcano Mihara has fore, the high concentrations described above been continuing to blow grayish smoke to are considered to be due to the volcanic er- 1000 m above the crater. The concentracions ruptions. of -15•Ž and -20•Ž nuclei were observed to be very high. Hereafter, both volcanoes showed successive eruptions. On 24th June 1958, Volcano Aso had a great explosion at 2215 JST, and began to blow smoke higher than usual. Volcano Suwanose also erupted and continued to blow smoke through the month so actively that sometimes sounds of explosions were reported to be heard at the distant spots. Thus, these volcanoes were in their active period at the same time. No data of their eruptive activities could be obtained for the later half of the period in which the measurement of ice nuclei was made, because of the continuous cloud coverage onto the volcanoes after the end of June. Fig. 7. Concentration of ice nuclei measured The concentrations of -15•Ž and -20•Ž in Tokyo. Four volcanoes in Japan ex- nuclei were very high and even -13•Ž nuclei ploded continuously in this period. June- were detected in the period, although the air- July 1959. mass in the period was one which had come It is difficult to make each peak of the ob- from the Pacific ocean. Usually, as described served concentration of ice nuclei correspond in the preceding section, the air from the to each eruption in this period. The effect Pacific ocean contained ice nuclei in low con- of each volcano is considered to overlap one centration. On 21st and 22nd June when another. Mihara emitted white smoke, that is, cloud A clear correspondence between the peak of condensed water vapour alone and free of concentration of ice nuclei and the volcanic from ash, from its crater, the low concen- eruption was observed in the next period of trations of ice nuclei were observed in Tokyo ; measurement from 6th November to 28th nuclei of -13•Ž and -15•Ž were not detected, November 1958. Volcano Asama began in- and the concentration of -20•Ž nuclei were tense eruptive activity in this period. Among below 0.3/l. its eruptions, the explosion at 2250 Japan On the following day, 23rd June, Mihara Standard Time on 10th November was the blew grey smoke, dust cloud of ash, and the greatest. Dust clouds reached to the level of concentration began to increase. Further, Aso 8 km and volcanic bombs were ejected up to and Sakurajima began successively to blow the height of 5 km above the crater. This is ash. The concentration of ice nuclei observed the most violent explosion in the past twenty

Vol. 37, No. 6, 1959 224 K. Isono, M. Komabayasi and A. Ono

years. After this greatest explosion, very explosion could not be made on that day. frequent explosions took place for three days; The concentration of ice nuclei was observed 15 explosions were observed on 11th and also to be at the maximum on 11th and 12th No- the same number of explosions were observed vember. The data of the concentration and before 1600 on 12th. After this time, cloud the explosions of Asama are tabulated in covered the volcano and visual observation of Table 5.

Table 5. Concentration of atmospheric ice nuclei measured in Tokyo (November 6-28 in 1958). Frequency of explosions, height above the crater reached by emitted dust cloud and the direction to which erupted dust cloud flowed are added.

* Measurement was made from 11 .h a. m. to 4 p. m, on each day.

As other volcanoes were rather quiet and wind in upper levels was predomi- nantly from west in this period, the correspondence between the peaks of concentration of ice nuclei and the ex-

plosions of Asama can be seen clearly as is shown in Fig. 8. The highest

peak concentration was observed on 12th Nov., being 4.2 nuclei per litre for -13•Ž nuclei , 9.7/1 for -15•Ž nuclei and 30/1 for -20•Ž nuclei. It is to be 3 noticed that the ratio of concentration Fig. 8. Concentration of ice nuclei. Volcano Asama of -20•Ž nuclei to that of -13•Ž nuclei exploded many times in this period (after the greatest explosion at 22.50 Nov. 10th 1958). is smaller in the case of the volcanic

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 225

Table 6. Concentration of atmospheric ice nuclei measured in Tokyo (April 16-May 18 in 1959).

Fig. 9. Concentration of ice nuclei. April-May 1959.

Vol. 37, No. 6, 1959 226 K. Isono, M. Komabayasi and A. Ono eruptions than in the case of the arrival of concentration of ice nuclei in the air is found clouds of Loess particles. to be associated with the arrival of loess iii) Variation of ice nuclei concentration in particles from northern China and volcanic spring in 1959 ash emitted by eruptions of active volcanoes The concentration of ice nuclei in spring in Japan. In order to ascertain the nature season was measured from 16th April to 18th and origins of the ice nuclei, the authors May 1959. In the same period in 1958, the made experiments on the effectiveness of dust cloud of loess particles from North China volcanic ash particles of various volcanoes, arrived in Japan several times, but in 1959 loess, a meteorite and other mineral particles only one arrival of loess cloud was observed as ice nuclei. in the period of measurement. Dust storms (i) Samples tested were reported to occur at both area in the Volcanoes of which volcanic ash or pro- north and the south of the Yellow River in jectiles were tested are as follows ; Asama, North China on 16th April. On the following Aso, Komagatake, Kusatsu-Shirane, Mihara, day, 17th April, surface wind became calm at Miyakejima, Niijima, Sakurajima, Suwanose the area in North China and no dust storm and Yakeyama. Among them, Asama, Aso, was reported to occur. On the other hand, Mihara, Sakurajima and Suwanose have a dust cloud of loess particles arrived in Japan erupted very frequently. For example, Asa- with a cold front on that day. The concen- ma has had 112 times of explosions per month tration of ice nuclei reached a maximum in at its most active period. On the other hand, Tokyo, the value being 1.0, 5.6 and 71 nuclei Komagatake (in Hokkaido district), and Ku- per litre for -13•Ž, -15•Ž and -20•Ž nuclei satsu-shirane have had much fewer eruptions. respectively. Hereafter the pressure pattern Miyakejima and Yakeyama (in Niigata Pre- around Japan took an aspect like one of fecture) have had only rare eruptions. Nii- summer season. Typhoon, 985 mb at her jima has had no eruption in this century. center, stayed at a location off the southern Samples of loess are those obtained from coast of Japan Islands and maritime air which the ground soil of Northern China. One came from south-east, south or south-west sample of loess was obtained from the snow covered Japan. Neither -13•Ž nuclei nor in northern Japan, Hokkaido district. It fell -15•Ž ones were observed . The wind at as yellow snow on Mt. Daisetsu in Hokkaido upper levels and sometimes the air at the on 26th February 1958, and was collected by surface came from the continent, where the Okita. surface wind was rather weak so that no For reference, samples of stone meteorite, dust storm was considered to have developed. volcanic hair of Hawaii, surface soil of ground Small peaks of the concentration of -20•Ž in the campus of Tokyo University where nuclei corresponds to the airmass which came the measurement of ice nuclei has been made, from the middle part of continent, and no and of quartz were tested. very large concentration was observed. The stone meteorite was the one which In May, the pattern of pressure became fell in Gifu Prefecture, in the central part of like that of " Baiu ", and a persistent front Japan Islands, in 1909. A piece of the me- was on the south side of Japan. A family teorite in its inner part was cracked out and of cyclones moved along this front which tested. This part was not affected by oxi- moved sometimes northwards and sometimes dation and fusion due to heating in its fall. southwards. Though small variation of the It is composed of enstatite and chrysolite. concentration of ice nuclei occurred everyday The volcanic hair, hairlike structure ejected in this period, generally the concentration from basaltic volcano, was obtained at Puna was low. near Mauna-Loa. The surface soil of the ground in the campus was chosen because sometimes strong wind blew up dust from 4. Ability of volcanic ash, loess and other the ground onto the roof of the adjacent mineral particles as ice nuclei building on which the ice nucleus counter As described above, the increase in the was set.

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 227

Quartz was tested as one of the main Table 8. mineral components of rocks and soil. Other It can be seen that ash of Asama, Kusatsu- rock-forming minerals of igneous rocks and shirane, volcanic sands of Niijima and Miya- clay minerals were also tested. Among them, kejima contain minerals very effective as ice magnetite, hypersthene and kaolinite are nuclei ; were detected to be form- found to be the best ice nuclei. Detailed ed even at -8•Ž. On the other hand, bomb data concerning ice forming ability of rock- of Sakurajima, volcanic hair of Hawaii, sur- forming minerals will be reported later in face soil in the campus of Tokyo' University another article. and the quarts have the lowest ability as ice nuclei among samples tested. Bombs of Mi- (ii) Method of test harayama, ash of Sakurajima fallen at city Each sample was crashed and ground with a of Kagoshima and loess are, though not the pestle and a mortar. The diameters of parti- best, very effective ice nuclei. Ash of Aso cles thus prepared were about 1~l0 microns. 10 milligrams of crashed samples were seeded and stone meteorite are of medium ability as into the cold chamber filled with supercooled ice nuclei. fog through a glass pipe at the top. The fog was made by supplying water vapour through another glass pipe before the seeding. Num- ber of particles actually introduced into the clouds was about 106. Ice-crystals formed in the chamber were counted with the method of sugar solution. Seeding were made at temper- atures -8•‹, -10•‹, -12•‹, -13•‹, -14.5•‹, -16•‹, and -20•Ž respectively. The chamber is a cylinder of 10 cm in diameter, 40 cm in depth and was cooled with ethylene grycol and dry- ice. The temperature of the air in the cham- ber was measured by a thermocouple inserted in the central part of the chamber. Sugar solution was set at the bottom of the cham- ber. Weight percentage of sugar to water in the solution used for the test are tabulated in Table 7. The temperature of sugar solution was maintained at the values indicated in Fig. 10. Temperature dependence of ice Table 7 by use of ice and table salt mixture. nucleating ability of volcanic ashes and other substances. Table 7. Concentration of sugar solution used for ice crystal detection. Some of the results obtained by Isono and Ikebe (1959) for rock-forming minerals are shown in Table 9. As is seen in the table magnetite and hypersthene are very effective as ice nuclei. Some of oxides of iron also have been found to be effective. The stronger

Table 9. Number of ice crystals produced by seeding of 106 particles of mineral.

(iii) Result The numbers of ice crystals produced by seeding of the samples are tabulated in

Vol. 37, No. 6, 1959 228 K. Isono, M. Komabayasi and A. Ono

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 229

dependence of ability of ash and soil on temper- 500 in 106 above -14.5•Ž. But this cannot ature in comparison with the component be considered to mean that some kind of minerals is considered to be due to successive minerals contained in volcanic ejectg of very participation of several constituting minerals small quantity act as ice nuclei and their in ice formation with decreasing temperature. main constituting minerals do not. In the Isono and Komabayasi (1954) reported that ash test of rock forming minerals made by Isono particles of Asama had the threshold temper- and Ikebe (1959), relatively very small number ature between -12.5•Ž and -13.5•Ž. This of particles act as ice nuclei also in the case was the highest temperature at which glitter- of pure minerals. The nucleation is of statis- ing of ice crystals could be observed visually tical nature, so the relative number is largely through a glass window in supercooled fogs affected by the time interval of suspension in a cold chamber when samples were seeded. of minerals in the supercooled cloud. If In the present test, ice crystals were detect- minerals were kept suspended in supercooled ed above -12•Ž when ash of Asama was clouds for longer time interval, the larger seeded. This may be due to higher detect- number of mineral particles would act as ice ability of the method of sugar solution than nuclei. visual observation of glittering. The abilities of ashes emitted by different Discussion eruptions of the same volcano were found As described in Section 3, the following to be not always the same. Seven samples characteristic features of the variation of of ash of Asama were tested (see Table 8). the concentration of atmospheric ice nuclei The most effective one of them is the sample have been revealed. obtained at the fringe of the crater. This (a) The ice nuclei concentration in the difference in the ability of ice formation may maritime airmass over the Pacific Ocean are be partly due to the difference in constitut- very low, usually below 0.1 nucleus per litre ing minerals, and partly due to the degree even for -20•Ž nuclei. of weathering which alter the minerals of (b) The arrival of clouds of loess particles igneous rocks to clay and oxides of metals. originating from the arid area in North China

When explosion takes place, a part of fresh gives rise to a sudden increase in the ice magma may be blown up to the atmosphere, nuclei concentration in the air over Japan while fragment rocks of wall of the crater, and furthermore it is expected that this area sand and ash of the past eruptions filled in the may be important in the world-wide budget crater may be blown upwards at the same of atmospheric ice nuclei. time. Volcanic ash, therefore, is considered (c) By eruptions of volcanoes, the atmo- to contain some clay minerals. The crushed sphere is supplied with effective ice nuclei. bomb of Sakurajima was found to have much. It is to be noticed that nuclei effective at lower ability of ice formation as compared high temperature, -13•Ž nuclei, increase by with ash of the same volcano. This may be eruption of Volcano Asama. due to difference in degree of weathering. (d) The velocity and direction of local Ashes of Asama, and Komagatake are known wind and dust from the ground in Tokyo to be easily altered to kaolinite by weather- have no marked relationship to the concent- ing. The rocks of Kusatsu-shirane is strong- ration of ice nuclei. This fact indicates that ly weathered and this volcano does not eject neither the combustion products from the fresh lava. It should be noted that ashes of industrial region nor ground soils in and near these three volcanoes all serve as very ef- Tokyo are the main origin of ice nuclei in fective ice nuclei. the air over Tokyo. Another main feature of the result is that The typical values of concentration of ice the numbers of particles which are activated nuclei over Tokyo in various situation are as ice nuclei are very small compared with summarized in Table 10. the total number of particles seeded. The The fact that the concentration of ice nuclei ratio of the number of ice crystals detected is very low in maritime airmass compared to the total number seeded was 10 in 106', with continental airmass may be considered

Vol. 37, No. 6, 1959 230 K. Isono, M. Komabayasi and A . Ono

Table 10. Typical concentration of ice nuclei. that particles of iron meteorites which come (number per litre) from the upper atmosphere act as ice nuclei and stimulate precipitation. As continental airmasses contains relatively many ice nuclei originating from the ground, it seems difficult to distinguish meteoric dust particle among them. In order to study the validity of the meteoric dust theory, it is desirable to make measurement of ice nucleus- concentration in maritime air masses or in air over ocean, and, if possible, in air in the stratosphere. The present result concerning the ice nucleus concentration in maritime air seems, at first sight, to disagree with Soulage's findings that high concentration occurred at the coast of Basque especially when wind blew from sea. But there remain possible to show that the main components of atmos- causes of such a high concentration other pheric ice nuclei are of ground origin, includ- than the cause proposed by Soulage who ing volcanic origin. This view point agrees suspected that some effective ice nuclei such with the results of Schaefer (1951), Mason as nitrate would be produced as the result of (1953), Kumai (1951), Isono (1955), Isono and the reaction of salts originating from sea Komabayasi (1954) and others. On the other water with some materials of continental on hand, some workers obtained results which gin. As was mentioned in the previous sec• might be, according to their opinion, incom- tions, it is suspected that the airmass from patible with the ground origin theory. The the sea might have been supplied with dust increases of the concentration of ice nuclei from the ground when it passed over some arid with altitude or their higher concentration region or by a volcanic eruption, or oxides of above temperature inversion than at the iron, which are effective as ice nuclei, from an ground level found by Smith and Heff ernan industrial area, if any, might be responsible (1954) and Murgatroyd and Garrod (1957) for the high counts. Another possible cause may not necessarily lead to the conclusion which may give rise to high nucleus concentra- that their origin is in the upper atmosphere, tion at the coast is the presence of particles but shows that the nuclei are not originating of magnetite or hypersthenes in sand on the from the ground near the site where the ob- seashore. As mentioned above, particles of servation is made, for the airmass above the magnetite and hypersthenes are very effective inversion may have come from a source region as ice nuclei, but their concentrations in the of ice nuclei far from the site. When the atmosphere are considered to be very low, origin of nuclei is sought, we should investi- for their size and their weight is large. gate the history of the airmass in which the Therefore, such particles blown up from the nuclei are contained, for the concentration ground will fall out in a short time. When of nuclei in air is closely related to the large a measurement of ice nucleus concentration scale synoptic situation, as was shown in the is made near seashore especially near the previous paragraph. The present authors, ground, particles of magnetite or hypers- however, do not intend to oppose to the thenes of the scud may be counted. If it meteoric dust theory proposed by Bowen. were the case, the concentration of ice nuclei Although the stone meteorite tested does not thus counted might be of only local signifi- serve as very effective ice nuclei, magnetite cance. has been found to be very effective or the There is another possibility that ice nuclei most effective among all the minerals we are produced from the sea salt through the tested. Magnetite is known to be contained following process. The result of an experi- in iron meteorites. It is, therefore, possible mental study (Isono and Ono 1959) shows

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 231 that potassium sulfate and calcium carbonate whose threshold temperature is above -20•Ž, particles produced by drying out a drop of and this agrees with the fact that -13•Ž sea water or aqueous solution of the respective nuclei increased when Volcano Asama erupt- salts on a platinum wire heated with electric ed (see Fig. 8) . In the case of the eruption current act as effective ice nuclei, while so- of Volcano Mihara, the increase in -13•Ž dium chloride particles produced from its nucleus concentration is smaller (see Table aqueous solution in the same manner are not 10). This agrees with the result of the ex- effective. It is, therefore, probable that ice periment with ash from the volcano (see nuclei may be produced when maritime air Table 8). The feature of the increase of which contains sea salt particles is desiccated ice nucleus concentration at the arrival of through some process, for instance, the loess particles from North China also agrees migration of maritime air over an arid region with the result of the experiment in laborato- or its ascension to a dry high level. How- ry. The clouds of loess particles contain a ever, as the composition ratios of potassium large number of -20•Ž nuclei, but compara- sulfate and calcium carbonate are small, the tively small number of -13•Ž nuclei. The number of ice nuclei thus produced may not analysis of trajectories of airmasses shows be large in the ordinary maritime air. that the nucleus concentrations in the air- It is noticeable that values of the concent- masses were reduced to very low levels after ration on days of the arrival of Kosa (loess) a week travel over the ocean as a result of in Japan agree with one another as described falling out. On the other hand, volcanic ash in the preceding section and shown in Tables or dust ejected high up into the upper tropos-

1 and 6. This fact seems to indicate that phere or the stratosphere will remain much the particles contained in Kosa from the longer time in the atmosphere and will be North China arrived on different days and carried around the Earth. It is, therefore, effective as ice nuclei have nearly the same conceivable that the influence of a violent compositions. On the other hand, ice nucleus volcanic eruption is more persistent and ex- spectra obtained on days of volcanic dusts tends to more distant regions from the source differ from each other according to volcanoes than in the case of dust from arid regions. from which the dusts had originated as is Although many works have been done about shown in Table 10. As the duration of emis- the effect of volcanic eruption on weather sion of ash is comparatively short and a and climate through depletion of solar radia- volcano may be considered as a point source, tion by dust, little attention has been given one may often miss the detection of increase to the effect of volcanic eruptions on the of ice nuclei in the atmosphere due to erup- glaciation of super-cooled clouds. Isono and tions, when observation is made at a single Komabayasi (1954) found that the amount site and no more than a few times a day. of rainfall in the leeward regions of volcano The increase may be detected when wind increased in years with frequent occurrence direction is favourable for the transport of of its eruptions, and also on days with erup- ash from the volcano to the site of observa- tions. Komabayasi (1957, 1959) stated that tion and when the observation is made at the the occurrence of thunderstorm was strongly time of arrival of the ash. This may be the surpressed by the eruption of Asama. wex- reason why one to one correspondence of ler (1956) suggested a possible mechanism of volcanic eruption and the increase of ice climatic variation caused by the effect of nucleus concentration does not always exist. volcanic dust on precipitation. As volcanic Taking into account these circumstances, the dust serves as efficient ice nuclei as described correspondence described in the preceding sec- above and there are many active volcanoes in tion may be considered to be remarkable. The the world, it is very probable that the activi- ice nucleating abilities of ashes of volcanoes ty of the volcanoes may have influence on in Japan and some minerals tested in the the amount of precipitation and on other me- laboratory are shown in Tables 8 and 9 and teorological phenomena in world wide scale described in the preceding section. Ash from and may be a factor responsible for the cli- Volcano Asama contains very effective nuclei matic change.

Vol. 37, No. 6, 1959 232 K. Isono, M. Komabayasi and A. Ono

Summary and Conclusion Tokyo Kyoiku University and Prof . T. Okita, The results of the analyses of ice nucleus Hokkaido Gakugei University who kindly gave concentrations in air over Tokyo show that them samples of loess. Sincere thanks are the concentration is usually low in maritime due to Prof. H. Kuno, Tokyo University for airmasses and high in continental air masses, his helpful suggestion about loess. especially in air which comes from arid and The authors are grateful to Messrs A . Suwa, semi-arid regions of North China and carries Y. Tanaka and I. Takeyama, the Japan Mete- loess particles, and that volcanic eruptions orological Agency, who informed the authors give rise to marked increase in ice nucleus of many unpublished observational materials concentration. The temperature spectra of about volcanic activities and gave them ice nuclei observed have been compared with samples of volcanic ash. those of various rock-forming minerals, They are indebted to Mr. S. Oki, Chief of soil particles, volcanic ashes and particles of Yokohama Meteorological Observatory who meteorite determined experimentally in the sent them samples of ash of Volcano Asama laboratory. Based on these studies, it is found emitted in various eruptions during his form- that the main sources of ice nuclei in air over er term at Maebashi Meteorological Observ- Japan are the arid and semi-arid region of atory, and indebted to members of meteoro- the continent of Asia, especially in North logical section, Japan Air Lines, for their China, and active volcanoes in Japan. A part courtesy in use of flight log on 26th and 27th of nucleus content in the maritime airmass April in 1958. may be particles of K2SO4 and CaCO3 separat- ed from sea salt particles by desiccation. References As some volcanoes emit a large amount of Bigg, E.K. (1956): Counts of atmosoheric freez- dust particles which are very effective as ice ing nuclei at Carnarvon, Western Australia, nuclei, the eruptive activity of volcanoes in January 1956. Austral. J. Phys. 9, 561-565. the world might have some influence on the - (1958): A long period fluctuation in climate. freezing nucleus concentration . J. Met. 15, 561 It may be concluded that soil dust particles -562. originated from arid and semi-arid regions Bowen, E. G. (1953): The influence of meteoritic and volcanic dust from volcanoes on the earth dust on rainfall. Austral. J. Phys . 6, 490-497. are important in the budget of ice-nucleus Bracewell, R. N. (1956): Counts of atmospheric content in the atmosphere, and play an im- freezing nuclei at Palo Alto, California , Janu- ary 1956. Austral. J. Phys. 9, 566-568 portant part in meteorogical phenomena in . both small and large scale. Heffernan, K.J. and R.N. Bracewell (1959): Com- As the data available at present about the parison of Florida and California freezing nucleus measurements, January 1957. J. Met. nature and the distribution of the atmos- 16, 337-339. pheric ice nuclei are insufficient for the study Georgii, H.W. (1957): Relation between ice nuclei on meteorological significance of the ice and particles of atmospheric aerosols. Tech . nuclei, it is urgently needed to establish the Note, Contract AF 61 (514)-927,Johan Wolfgang world-wide network of observation of the Goethe Universitat, Institut fur Meteorologie atmospheric ice nuclei. and Geophysik, Frankfurt/Main. Isono, K. (1955): On ice-crystal nuclei and other Acknowledgement substances found in snow crystals. J. Met . 12, 456-462. The authors wish to express their cordial -(1959): Microphysical processes in pre- thanks to Drs. H. Ozaki, S. Murayama, K. cipitation mechanism. Japanese J. Geophysics 2, 1-57. Murauchi, T. Asanuma, I. Fujiyama and M. - and M. Komabayasi (1954): The in- Hashimoto, the National Science Museum, fluence of volcanic dust on precipitation . J. who kindly allowed the authors to use vari- Meteor. Soc. Japan, Ser II. 32, 345-353. ous minerals, volcanic ash and meteorite for M. Komabayasi and A. -Ono (1958): their experiment. Note on the daily variation of concentration They also wish to thank Prof. H. Fujimoto, of atmospheric ice nuclei over Japan and their

Journ. Met. Soc. Japan The Nature and the Origin of Ice Nuclei in the Atmosphere 233

origin (in Japanese). Tenki 5, 273-276. Soc. 83, 528-533. M. Komabayasi and A. -Ono (1959) : Nemoto, S., Y. Takahashi, K. Soma and T. Kudo Volcanoes as a source of atmospheric ice nuclei. (1957) : On the observation of the number of Nature 183, 317-318. ice crystal nuclei in the air near the ground and Y. Ikebe- (1959) : On the ice-nucleat- at Tokyo and the influence of the cloud seed- ing ability of rock-forming minerals, Paper ing with silver iodide at the top of Mt. Fuji. presented at the meeting of the Meteorological J. Meteor. Soc. Japan, Ser II, 35, 137-149. Society of Japan held in May 1959 (to be Schaefer, V. J. (1951) : Snow and its relationship published). to experimental meteorology. Compendium of - and A. Ono (1959) : Note on atmospheric Met. pp. 221-234. Amer. Met. Soc. Boston. ice nuclei of sea origin, Paper presented at (1954) : The concentration - of ice the meeting of the Meteorological Society of nuclei in air passing the summit of Mt. Wash- Japan held in May 1959 (to be published). ington. Bull. Amer. Meteor. Soc. 35, 310-314. Kline, D. B, and G. W. Brier (1958) : A note on Smith, E. J, and K. J. Heffernan (1954) : Airborne freezing nuclei anomalies. Month. Weath. Rev. measurements of the concentration of natural 86, 329-333. and artificial freezing nuclei. Q.J. Roy. Met. Komabayasi, M. (1957) : The suppresion of thun- Soc. 80, 182-197. dercloud occurrence by frequent volcanic erup- A.R. - Kassander and S. Twomey (1956) : tions. J. Meteor. Soc. Japan Ser II, the 75th Measurements of natural freezing nuclei at Anniversary Volume 25-30. high altitudes. Nature 177, 57-69. (1959) : The - suppression of thun- Soulage, G. (1958) : Sur une origine cotiere de dercloud occurrence by frequent volcanic erup- certains noyaux glacogene. Compt. Ren. Acad. tions (II). J. Meteor. Soc. Japan Ser II, 37, Sci. 246, 143-145. 79-82. Warner, J. (1957) : An instrument for the measure- Kumai, M. (1951) : Electron microscope study of ment of freezing nucleus concentration. Bull. snow crystal nuclei. J. Met. 11, 104-120. Obs. de Puy de Dome 2, 33-46. Mason, B.J. (1950) : The nature of ice form- Wexler, H. (1956) : Variations in insolation, gener- ing nuclei in the atmosphere. Q. J. Roy. Met. al circulation and climate. Tellus 8, 480-494. Soc. 76, 59-74. -andJ. Maybank (1958) : Ice-nucleating Sources of Data - properties of some natural mineral dust. Q. J. Roy. Met. Soc. 84, 235-276. The data of volcanic activities are taken from Murgatroyd, R. J, and M.P. Garrod (1957) : Some Seismological Bulletin, the Japan Meteorological recent airbone measurements of freezing Agency. nuclei over Southern England. Q. J. Roy. Met.

大 気 中 の 氷 晶 核 の 物 質 と 発 源 地

磯野 謙治･駒 林 誠･小 野 晃 (東京大学地球物理学教室)

東 京 で氷 晶核 の 日 日観 測 を 行 い,一13℃,一15℃,一20℃ の 各 温度 で 働 く氷 晶 核 数 を 測定 した 結果,核 数 の 日 日 変 動 が 大 変 大 きい こ と及 び大 陸 の空 気 が 日本 に 来 た 場 合 に は核 数 が 多 く,太 平 洋 の 空 気 が 来 た場 合 に は核 数 が 少 い こ とが見 出 さ れ た｡ 特 に 中 国 の 北 部 及 び モ ソ ゴル の乾 燥 地 帯 で 風 塵 を 経 験 した 空 気 が 寒 冷 前 線 に 伴 つ て 日太 に来 た場 合 に は著 し く大 き い核 数 が 観 測 され た｡一 方 海 洋 性 気 団 の中 の核 数 は 少 い が,し ば しば大 きい 核 数 が 観 測 され,こ れ は 日本 の 火山 爆 発 に 伴 つ てい る こ とが 見 出 され た｡ 室 内 実 験 で 日本 の10個 の活 火山 の 火山 灰,中 国 の北 部 の黄 土,北 海 道 の雪 に 混 つ て降 つ た黄 土,観 測 点附 近 の土 壌,及 び 石 質 阻 石 の氷 晶核 と して の 能 力 を 測 定 した｡ これ らの 結 果 か ら大 気 中 の主 な氷 晶核 は乾 燥 地 帯 か ら吹 き上 げ られ る土 壌 鉱 物(特 に粘 土鉱 物)及 び 火 山灰 で あ る こ とが 結 論 され る｡ 尚 火 山 灰等 の 氷 晶 核 と して の 能 力 を 論ず る際 に は各 種 造 岩 鉱 物 及 び 海 水 成 分 塩 類 の氷 晶 核 と し て の 能 力 を 測定 し た 結果 を一 部 加 えて 比 較 す る と共 に,氷 晶核 の発 源 地 に 関 す る今 迄 の い くつ か の 説 に つ い て は気 団 の移 動 そ の 他 の 点 で 問題 が あ る こ とを 指摘 した｡

Vol. 37, No. 6, 1959