48 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

Spread of the Krakatoa Volcanic Dust Cloud as Related to the High-Level Circulation

H. WEXLER

U. S. Weather Bureau, Washington, D. C.

ABSTRACT

The spread of volcanic dust from the of Krakatoa is described. An explanation of the initial rapid lateral spread poleward in the Northern Hemisphere, the much slower spread in the second month, and the accelerated spread in the third and fourth months is attempted in terms of the normal monthly circulations at 19 km.

HE recent spread of smoke from forest week in November 1883, the solar radiation values fires in the Canadian Northwest to eastern at Montpellier Observatory, France, decreased by TUnited States and western Europe 1 has 25%, and remained below normal for three years. awakened interest in the spread of smoke palls. Both optical and radiation observations therefore One of the most spectacular of these cases was the agree in placing the appearance of Krakatoa cloud world-wide spread of dust from the Krakatoa vol- in Europe some three months after the explosion. canic explosion. The purpose of this note is to It is of interest to speculate as to why it took as review this phenomenon and to attempt an ex- long as three months for a portion of the main planation of the observed spread in light of what cloud of Krakatoa effluent which moved into the is known of the circulation at high altitudes. Northern Hemisphere to travel from On August 27, 1883, following several months to western Europe and also to explain the irregular of minor , the volcano on the Island of rate of spread of the cloud poleward as described Krakatoa (Sunda Strait, between and below. , 6° 9' S, 105° 22' E) blew up and ejected into Here are the known facts as to the spread of the the atmosphere an estimated 13 cubic miles of lava, cloud as deduced by optical observations and sum- ash, and mud. About one-third of the material fell marized from material presented in "Eruption of within 30 miles, covering some places 25 miles Krakatoa" [6]. distant with deposits to a depth of one foot. An- other third, composed of fine dust, fell within 2,000 1. Apart from off-shoots towards Japan and South Africa immediately after the explosion, the miles, while the remainder, consisting mostly of main body of the cloud moved from east to west very fine pumiceous bubble plates settled out at an average speed of 73 miles per hour, com- slowly from the atmosphere for several years and pleting at least two circuits of the earth in equa- produced unusual optical effects, such as the re- torial latitudes. markable twilight glows, colored suns and moons, 2. The cloud in making these circuits passed and the "Bishop's Ring." A committee appointed over most places in three or four days which, com- by the Royal Society of London studied various bined with the speed of travel of the leading edge, aspects of the explosion and summarized their indicates that the cloud was drawn out to a length findings in the classic "Eruption of Krakatoa" [6]. of 5,000 to 7,000 miles, presumably by the vertical From their analysis of hundreds of observations shear in the equatorial easterlies. they were able to plot roughly the spread of the 3. Excluding sporadic twilight glows, due prob- volcanic cloud in the northern and southern hemi- ably to small, broken-off masses of the cloud, the spheres. One of their results showed that it took northern extreme limit observed at the end of the approximately three months for the cloud to first circuit (Sept. 9) was 22° N (Honolulu) and travel to western Europe in concentrations large the southern extreme limit 33° S at Santiago, and persistent enough to produce the unusual and Chile. The average limits were 16° N and 22° S. prolonged optical effects observed. It was pointed 4. At the end of the second circuit (Sept. 22) out in a previous paper by the present writer [7] the average cloud limits extended roughly from that coincident with the appearance of the optical 24° N to 40° S. phenomena in western Europe during the last 5. North of latitude 30° N there was no further indication of spread of the cloud from east to west. 1A preliminary report on this appeared in the De- cember 1950 issue of Weatherwise. In October when the cloud material had reached

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30° N there were fewer accounts of its having tober, and November normal charts at 19 km (the travelled to new places than before or after that highest level available) will be used as a guide in date, and during that month it spread only slightly explaining the observed travel of the northern in latitude. hemispheric portion of the main cloud whose top 6. The twilight glows spread gradually north- was computed from optical effects and rate-of-fall ward and southward, but up to about November 23 formulae [4] to have decreased in height from 32 the glows seen north of about 32° to 36° N were km in August 1883 to 17 km in January 1884. An for the most part sporadic, apparently caused by earlier attempt was made by C. E. P. Brooks [2] detached portions from the main cloud. to relate the motion of the Krakatoa cloud in the 7. On November 23 a remarkable movement Northern Hemisphere with a much lower level, took place in such a manner that by November 27 namely the average cirrus motion (8 to 11 km) the twilight glows were generally observed over during the months of October to December. the United States and Europe; they are believed None of the normal monthly charts for 19 km to have spread to these regions from the mid- will be reproduced here because they are generally Pacific and mid-Atlantic oceans respectively. available. However, normal pressure profiles for 8. After December 1883 it was not possible to each month from August to December are shown follow the main cloud as a distinct entity. in FIGURE 1, from which the average zonal winds Thus, from the close agreement of visual ob- for each latitude from 10° to 80° N can be deduced. servations of the incidence of the twilight glow The 19 km normal chart for August shows a and from the solar radiation observations at zonal flow from east to west from the equator to Montpellier, there is strong evidence that the edge latitude 20°-25° N. To the north, large anti- of a main cloud mass moved from west to east cyclonic cells are located over the northern United over western Europe, beginning November 23. States and northern Europe. (Direct observations Referring to the northern limits of the cloud as of winds at and above 17 km over the United indicated in FIGURE 1, the question is why, after States and England have since verified the ex- spreading over one-half of the earth's surface in istence of summer easterlies.) The westerlies are one month, did it take two more months for the found only in a small area over the Arctic Ocean cloud to cover an additional 40% of the earth's and Greenland. The 19 km normal August pres- surface ? sure profile shows the broad latitudinal extent of In absence of current upper air charts in 1883 the easterlies and the narrow belt of westerlies to the proposed explanation will be based on the nor- the north (FIG. 1). mal monthly upper air charts for the Northern At 19 km in September, the zonal easterlies are Hemisphere [1]. The August, September, Oc- compressed into a narrower equatorial band ex- tending to 20° latitude or less, while the anti- cyclonic cells over the continents are displaced southward to the latitude belt 30°-50°. Whereas in August there existed practically no points of egress of air northward from the equatorial easter- lies, in September there are two such points: one off the west coasts of Mexico and the United States, and the other in the western Pacific. There is perhaps a third opening in northeastern Africa. The westerlies now cover a much larger area, and extend as far south as latitude 50° in the United States and latitude 60° in Siberia. The September normal pressure profiles (FIG. 1) illustrate the growth of the westerlies at the expense of the east- erlies. In October there is some evidence that the zonal equatorial easterlies may exist in a narrow strip within 10° of the equator. The circulation pattern is markedly more cellular in lower latitudes than FIG. 1. Normal monthly pressure profiles at 19 km. was the case in August and September. Main Arrows refer to approximate northernmost limits of spread points of egress of equatorial air to the higher lati- of Krakatoa dust. Abscissa is sine of the latitude. tudes are the Caribbean Sea, SW No. Atlantic

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Ocean, off the coast of northwest Africa, West of the Hawaiian Islands, and southeastern Asia. The westerlies now are found as far south as 25° latitude. In November there is further development of the changes noted in October: disappearance of the zonal equatorial easterlies from the field of data, greater role of the subtropic anticyclones in transporting air to and from the equator, and further penetration southward of the westerlies to latitude 10° N. In December the sinusoidal westerlies dominate the Northern Hemisphere to the limit of data at 10° N. Taking these normal maps as typical of the cir- culation patterns to which the Krakatoa dust cloud was subjected from August to December of 1883, we can throw some light on the slow prog- ress of the cloud to the United States and Europe after its first month of travel. During this first month (September) the main cloud was mainly confined to the broad zonal easterlies character- FIG. 2. Average speed of spread northward of edge of istic of equatorial latitudes at that season. Rapid Krakatoa dust veil (horizontal lines) as compared to normal meridional wind speed profiles (curved lines). lateral spread poleward in this current occurred as eddy diffusion carried portions of the cloud to higher latitudes. The eddies mainly responsible tudes 10, 20, 30, 40, 50, 60, and 70° N for the for the lateral spread of the dust cloud may be September, October, and November 19 km normal similar to those observed by Riehl [5] in the up- charts. These averages were then converted into per troposphere (8 to 16 km) over the tropical west average meridional geostrophic wind speeds as Pacific Ocean in September 1945; these eddies shown in FIGURE 2. The average meridional were imbedded between the trade-wind easterlies speeds are a maximum at low latitudes, decrease below and the high-speed stratospheric easterlies to a minimum at middle latitudes, and then increase above. at higher latitudes. On the same FIGURE are As the Northern Hemisphere cold season ad- plotted horizontal lines showing, for the indicated vanced and the equatorial easterlies diminished in periods and zones, the average speed of spread lateral extent, the dust cloud came more and northward of the northern limit of the dust cloud. more under the influence of the subtropic cellular Realizing that "normal" conditions at 19 km might circulations. In October the northern edge of the not have prevailed in the autumn of 1883 and that cloud appears to have become coincident with the the dust might have spread with different speeds northern limits of these cells, which apparently at levels either above or below 19 km, the agree- accounts for the virtual cessation of cloud motion ment between the average meridional wind speeds westward and the very slow advance northward. determined from the normal charts, and the aver- The northern portion of the main cloud in the age spread of the volcanic dust northward, is Northern Hemisphere may have been broken into quite striking, both with regard to numerical val- several separate masses by these cells. ues and dependency on latitude. In November as the current systems were dis- The average meridional wind speeds for the re- placed farther south, the dust was enabled to maining months have been computed, but are not spread rapidly with the sinusoidal westerlies from shown here. They exhibit a clear-cut annual cycle west to east and from south to north over higher whereby August has the lowest speed averaged latitudes in the Northern Hemisphere. from 10° to 70°N, 0.8 m/sec, and March and To illustrate the relation between meridional cir- April, the highest, 1.2 m/sec. Thus if the Kraka- culation intensity and poleward spread of the dust, toa explosion had occurred in winter or spring, the zonal pressure differences found from readings it appears likely that the spread of the cloud in at meridians 10° apart were averaged (regardless the Northern Hemisphere would have been much of sign) around the Northern Hemisphere at lati- more rapid. As additional support for this state-

Unauthenticated | Downloaded 09/27/21 09:15 PM UTC VOL. 32, No. 2, FEBRUARY, 1951 51 ment, it should be pointed out that the explosion marks the base of the very strong temperature actually occurred in the Southern Hemisphere increase with altitude characteristic of the ozono- winter, and it is quite apparent from the charts sphere observed over White Sands, New Mexico presented in Plate XXXVII of "Eruption of Kra- [3]. It appears as if the Krakatoa blast had katoa" that the spread of the dust in the Southern enough energy to push up through the equatorial Hemisphere was much faster than in the Northern tropopause at 18 km, but could not penetrate far Hemisphere. For example, from the distribution into the upper strong inversion beginning at about of sky phenomena during September 22 to October 30 km. 10, 1883, the southern edge of the main cloud was near 50° S latitude while the northern edge REFERENCES was near 30° N. The fact that the cloud origi- [1] (Anonymous) Normal Weather Maps, Northern nated at 6° S latitude could hardly account for the Hemisphere, Upper Level, U. S. Weather Bureau, cloud in the Southern Hemisphere being 20° Washington, D. C., 1945. nearer the pole than in the Northern Hemisphere. [2] Brooks, C. E. P., The movement of Volcanic Ash over the Globe, Met. Mag., 67, 81, 1932. It is therefore likely that the Southern Hemisphere [3] Newell, H. E., Jr., Upper Air Research by Rockets, winter circulation at 19 km possesses the same cold Trans. Am. Geophy. Union, 31, 1, pp. 25-33, 1950. season properties as the circulation at 19 km in the [4] Pernter, J. M., Der Krakatau-Ausbruch und seine Folge-Erscheinungen, Met. Zeit., 6, pp. 329, 409, Northern Hemisphere, namely, disappearance or 447, 1889. very narrow lateral extent of the zonal equatorial [5] Riehl, H., On the Formation of Typhoons, Jn. of Met., easterlies, maximum displacement equator ward V. 6, p. 247, 1948. [6] Symons, G. J. (Editor), The Eruption of Krakatoa and intensification of the westerlies. and Subsequent Phenomena, Report of the Krakatoa Regarding the maximum height (32 km) of the Committee of the Royal Society, London, 1888. [7] Wexler, H., On the Effects of Volcanic Dust on In- volcanic dust cloud, computed from optical effects solation and Weather (I), Bull. Amer. Meteor. [4], it is of interest to point out that this level Soc., V. 32, No. 1, Jan. 1950, pp. 10-15.

second at the — 10° C level. Again, too few data are REVIEWS presented to support this conclusion although the argu- ments are quite resonable. Most of the second half of the book is devoted to light-

(Continued from page 46) ning. Wichmann favors Toeppler's theory to explain the by the technique suggested by P. Raethjen. The latter, lightning discharge mechanism. Lightning structure is using parcel-method reasoning, has suggested that the illustrated by the photographic methods of B. F. J. Schon- vertical velocity within the thundercloud, at any level, is land and B. Walter, and the electrical methods of H. proportional to the difference between the ambient air Norinder, W. Watt, and A. Mathias. Several sample and the virtual temperature indicated by the wet adiabat traces are presented showing the electrical field varia- tions with time as measured at the earth's surface. These through the convective condensation level. The observed variations are analyzed in terms of the findings of the and calculated velocities compare fairly well, but, here above investigators and the Wichmann-Findeisen theory. again, the data are too few to prove conclusions. The author concludes that the relatively rapid electric Studies relating a similar temperature difference with field fluctuations are caused by cloud to ground lightning turbulence intensity—made by J. J. George and the U. S. while the slower variations are caused by cloud to cloud Thunderstorm Project—have shown only a fair relation- lightning. The book closes with a chapter on the diurnal ship. However, all of these studies present evidence that and seasonal variations of the lightning frequency over there is a relationship between such a temperature param- the globe and the role of the lightning discharges on the eter as obtained from a recent sounding, and the degree maintenance of the earth's electric field. of vertical motion in the atmosphere during convective "Grundprobleme der Physik des Gewitters" may be activity. recommended as worthwhile reading for both the mete- One chapter is devoted to a discussion of the influence orology student and the practicing meteorologist. A valu- of the freezing-level height upon thunderstorm develop- able feature of this monograph is that the work of many ment. It is concluded that thunderstorm activity will investigators of the thunderstorm is gathered into and take place if: (1) the freezing level is at least 1,000 coordinated in one volume. Although at times the reader meters above the condensation level and at least 1,500 may feel that the discussions are out of date, since the meters below the top of the cloud, and (2) the vertical latest American work on the subject was apparently un- velocity in the updraft attains a magnitude of 8-10 meters available to the author, there is more than enough to this per second at the freezing level and 15-17 meters per book to make its reading profitable.—Harry Moses.

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