4 8 BULLETIN A M E R I C A N METEOROLOGICAL SOCIETY

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

H . W E X L E R

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

ABSTRACT

The spread of volcanic dust from the explosion 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.

THE recent spread of smoke from forest fires in the Canadian Northwest to eastern United States and western Europe 1 has

awakened interest in the spread of smoke palls. One of the most spectacular of these cases was the world-wide spread of dust from the Krakatoa vol-canic explosion. The purpose of this note is to review this phenomenon and to attempt an ex-planation of the observed spread in light of what is known of the circulation at high altitudes.

On August 27, 1883, following several months of minor explosions, the volcano on the Island of Krakatoa (Sunda Strait, between Sumatra and Java, 6° 9' S, 105° 22' E ) blew up and ejected into the atmosphere an estimated 13 cubic miles of lava, ash, and mud. About one-third of the material fell within 30 miles, covering some places 25 miles distant with deposits to a depth of one foot. An-other third, composed of fine dust, fell within 2,000 miles, while the remainder, consisting mostly of very fine pumiceous bubble plates settled out slowly from the atmosphere for several years and produced unusual optical effects, such as the re-markable twilight glows, colored suns and moons, and the "Bishop's Ring." A committee appointed by the Royal Society of London studied various aspects of the explosion and summarized their findings in the classic "Eruption of Krakatoa" [6]. From their analysis of hundreds of observations they were able to plot roughly the spread of the volcanic cloud in the northern and southern hemi-spheres. One of their results showed that it took approximately three months for the cloud to travel to western Europe in concentrations large and persistent enough to produce the unusual and prolonged optical effects observed. It was pointed out in a previous paper by the present writer [7] that coincident with the appearance of the optical phenomena in western Europe during the last

1 A preliminary report on this appeared in the De-cember 1950 issue of Weatherwise.

week in November 1883, the solar radiation values at Montpellier Observatory, France, decreased by 25%, and remained below normal for three years. Both optical and radiation observations therefore agree in placing the appearance of Krakatoa cloud in Europe some three months after the explosion.

It is of interest to speculate as to why it took as long as three months for a portion of the main cloud of Krakatoa effluent which moved into the Northern Hemisphere to travel from Sunda Strait to western Europe and also to explain the irregular rate of spread of the cloud poleward as described below.

Here are the known facts as to the spread of the cloud as deduced by optical observations and sum-marized from material presented in "Eruption of Krakatoa" [6].

1. Apart from off-shoots towards Japan and South Africa immediately after the explosion, the main body of the cloud moved from east to west at an average speed of 73 miles per hour, com-pleting at least two circuits of the earth in equa-torial latitudes.

2. The cloud in making these circuits passed over most places in three or four days which, com-bined with the speed of travel of the leading edge, indicates that the cloud was drawn out to a length of 5,000 to 7,000 miles, presumably by the vertical shear in the equatorial easterlies.

3. Excluding sporadic twilight glows, due prob-ably to small, broken-off masses of the cloud, the northern extreme limit observed at the end of the first circuit (Sept. 9) was 22° N (Honolulu) and the southern extreme limit 33° S at Santiago, Chile. The average limits were 16° N and 22° S.

4. At the end of the second circuit (Sept. 22) the average cloud limits extended roughly from 24° N to 40° S.

5. North of latitude 30° N there was no further indication of spread of the cloud from east to west. In October when the cloud material had reached

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VOL. 32, N o . 2, FEBRUARY, 1951 49

30° N there were fewer accounts of its having travelled to new places than before or after that date, and during that month it spread only slightly in latitude.

6. The twilight glows spread gradually north-ward and southward, but up to about November 23 the glows seen north of about 32° to 36° N were for the most part sporadic, apparently caused by detached portions from the main cloud.

7. On November 23 a remarkable movement took place in such a manner that by November 27 the twilight glows were generally observed over the United States and Europe; they are believed to have spread to these regions from the mid-Pacific and mid-Atlantic oceans respectively.

8. After December 1883 it was not possible to follow the main cloud as a distinct entity.

Thus, from the close agreement of visual ob-servations of the incidence of the twilight glow and from the solar radiation observations at Montpellier, there is strong evidence that the edge of a main cloud mass moved from west to east over western Europe, beginning November 23. Referring to the northern limits of the cloud as indicated in FIGURE 1, the question is why, after spreading over one-half of the earth's surface in one month, did it take two more months for the cloud to cover an additional 40% of the earth's surface ?

In absence of current upper air charts in 1883 the proposed explanation will be based on the nor-mal monthly upper air charts for the Northern Hemisphere [1]. The August, September, Oc-

FIG. 1. Normal monthly pressure profiles at 19 km. Arrows refer to approximate northernmost limits of spread of Krakatoa dust. Abscissa is sine of the latitude.

tober, and November normal charts at 19 km (the highest level available) will be used as a guide in explaining the observed travel of the northern hemispheric portion of the main cloud whose top was computed from optical effects and rate-of-fall formulae [4] to have decreased in height from 32 km in August 1883 to 17 km in January 1884. An earlier attempt was made by C. E. P. Brooks [2] to relate the motion of the Krakatoa cloud in the Northern Hemisphere with a much lower level, namely the average cirrus motion (8 to 11 km) during the months of October to December.

None of the normal monthly charts for 19 km will be reproduced here because they are generally available. However, normal pressure profiles for each month from August to December are shown in FIGURE 1, from which the average zonal winds for each latitude from 10° to 80° N can be deduced.

The 19 km normal chart for August shows a zonal flow from east to west from the equator to latitude 20°-25° N. To the north, large anti-cyclonic cells are located over the northern United States and northern Europe. (Direct observations of winds at and above 17 km over the United States and England have since verified the ex-istence of summer easterlies.) The westerlies are found only in a small area over the Arctic Ocean and Greenland. The 19 km normal August pres-sure profile shows the broad latitudinal extent of the easterlies and the narrow belt of westerlies to the north (FIG. 1).

At 19 km in September, the zonal easterlies are 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 was the case in August and September. Main points of egress of equatorial air to the higher lati-tudes are the Caribbean Sea, SW No. Atlantic

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50 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

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-istic of equatorial latitudes at that season. Rapid lateral spread poleward in this current occurred as eddy diffusion carried portions of the cloud to higher latitudes. The eddies mainly responsible for the lateral spread of the dust cloud may be similar to those observed by Riehl [5] in the up-per troposphere (8 to 16 km) over the tropical west Pacific Ocean in September 1945; these eddies were imbedded between the trade-wind easterlies below and the high-speed stratospheric easterlies above.

As the Northern Hemisphere cold season ad-vanced and the equatorial easterlies diminished in lateral extent, the dust cloud came more and more under the influence of the subtropic cellular circulations. In October the northern edge of the cloud appears to have become coincident with the northern limits of these cells, which apparently accounts for the virtual cessation of cloud motion westward and the very slow advance northward. The northern portion of the main cloud in the Northern Hemisphere may have been broken into several separate masses by these cells.

In November as the current systems were dis-placed farther south, the dust was enabled to spread rapidly with the sinusoidal westerlies from west to east and from south to north over higher latitudes in the Northern Hemisphere.

To illustrate the relation between meridional cir-culation intensity and poleward spread of the dust, the zonal pressure differences found from readings at meridians 10° apart were averaged (regardless of sign) around the Northern Hemisphere at lati-

FIG. 2. Average speed of spread northward of edge of Krakatoa dust veil (horizontal lines) as compared to normal meridional wind speed profiles (curved lines).

tudes 10, 20, 30, 40, 50, 60, and 70° N for the September, October, and November 19 km normal charts. These averages were then converted into average meridional geostrophic wind speeds as shown in FIGURE 2. The average meridional speeds are a maximum at low latitudes, decrease to a minimum at middle latitudes, and then increase at higher latitudes. On the same FIGURE are plotted horizontal lines showing, for the indicated periods and zones, the average speed of spread northward of the northern limit of the dust cloud. Realizing that "normal" conditions at 19 km might not have prevailed in the autumn of 1883 and that the dust might have spread with different speeds at levels either above or below 19 km, the agree-ment between the average meridional wind speeds determined from the normal charts, and the aver-age spread of the volcanic dust northward, is quite striking, both with regard to numerical val-ues and dependency on latitude.

The average meridional wind speeds for the re-maining months have been computed, but are not shown here. They exhibit a clear-cut annual cycle whereby August has the lowest speed averaged from 10° to 70°N, 0.8 m/sec, and March and April, the highest, 1.2 m/sec. Thus if the Kraka-toa explosion had occurred in winter or spring, it appears likely that the spread of the cloud in the Northern Hemisphere would have been much more rapid. As additional support for this state-

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V O L . 3 2 , N o . 2 , F E B R U A R Y , 1 9 5 1 5 1

ment, it should be pointed out that the explosion actually occurred in the Southern Hemisphere winter, and it is quite apparent from the charts presented in Plate X X X V I I of "Eruption of Kra-katoa" that the spread of the dust in the Southern Hemisphere was much faster than in the Northern Hemisphere. For example, from the distribution of sky phenomena during September 22 to October 10, 1883, the southern edge of the main cloud was near 50° S latitude while the northern edge was near 30° N. The fact that the cloud origi-nated at 6° S latitude could hardly account for the cloud in the Southern Hemisphere being 20° nearer the pole than in the Northern Hemisphere. It is therefore likely that the Southern Hemisphere winter circulation at 19 km possesses the same cold season properties as the circulation at 19 km in the Northern Hemisphere, namely, disappearance or very narrow lateral extent of the zonal equatorial easterlies, maximum displacement equator ward and intensification of the westerlies.

Regarding the maximum height (32 km) of the volcanic dust cloud, computed from optical effects [4], it is of interest to point out that this level

marks the base of the very strong temperature increase with altitude characteristic of the ozono-sphere observed over White Sands, New Mexico [3]. It appears as if the Krakatoa blast had enough energy to push up through the equatorial tropopause at 18 km, but could not penetrate far into the upper strong inversion beginning at about 30 km.

REFERENCES

[1] (Anonymous) Normal Weather Maps, Northern Hemisphere, Upper Level, U. S. Weather Bureau, Washington, D. C., 1945.

[2] Brooks, C. E. P., The movement of Volcanic Ash over the Globe, Met. Mag., 67, 81, 1932.

[3] Newell, H. E., Jr., Upper Air Research by Rockets, Trans. Am. Geophy. Union, 31, 1, pp. 25-33, 1950.

[4] Pernter, J. M., Der Krakatau-Ausbruch und seine Folge-Erscheinungen, Met. Zeit., 6, pp. 329, 409, 447, 1889.

[5] Riehl, H., On the Formation of Typhoons, Jn. of Met., V. 6, p. 247, 1948.

[6] Symons, G. J. (Editor), The Eruption of Krakatoa and Subsequent Phenomena, Report of the Krakatoa Committee of the Royal Society, London, 1888.

[7] Wexler, H., On the Effects of Volcanic Dust on In-solation and Weather ( I ) , Bull. Amer. Meteor. Soc., V. 32, No. 1, Jan. 1950, pp. 10-15.

R E V I E W S

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by the technique suggested by P. Raethjen. The latter, using parcel-method reasoning, has suggested that the vertical velocity within the thundercloud, at any level, is proportional to the difference between the ambient air and the virtual temperature indicated by the wet adiabat through the convective condensation level. The observed and calculated velocities compare fairly well, but, here again, the data are too few to prove conclusions.

Studies relating a similar temperature difference with turbulence intensity—made by J. J. George and the U. S. Thunderstorm Project—have shown only a fair relation-ship. However, all of these studies present evidence that there is a relationship between such a temperature param-eter as obtained from a recent sounding, and the degree of vertical motion in the atmosphere during convective activity.

One chapter is devoted to a discussion of the influence of the freezing-level height upon thunderstorm develop-ment. It is concluded that thunderstorm activity will take place if : (1) the freezing level is at least 1,000 meters above the condensation level and at least 1,500 meters below the top of the cloud, and (2) the vertical velocity in the updraft attains a magnitude of 8-10 meters per second at the freezing level and 15-17 meters per

second at the — 10° C level. Again, too few data are presented to support this conclusion although the argu-ments are quite resonable.

Most of the second half of the book is devoted to light-ning. Wichmann favors Toeppler's theory to explain the lightning discharge mechanism. Lightning structure is illustrated by the photographic methods of B. F. J. Schon-land and B. Walter, and the electrical methods of H. Norinder, W. Watt, and A. Mathias. Several sample traces are presented showing the electrical field varia-tions with time as measured at the earth's surface. These variations are analyzed in terms of the findings of the above investigators and the Wichmann-Findeisen theory. The author concludes that the relatively rapid electric field fluctuations are caused by cloud to ground lightning while the slower variations are caused by cloud to cloud lightning. The book closes with a chapter on the diurnal and seasonal variations of the lightning frequency over the globe and the role of the lightning discharges on the maintenance of the earth's electric field.

"Grundprobleme der Physik des Gewitters" may be recommended as worthwhile reading for both the mete-orology student and the practicing meteorologist. A valu-able feature of this monograph is that the work of many investigators of the thunderstorm is gathered into and coordinated in one volume. Although at times the reader may feel that the discussions are out of date, since the latest American work on the subject was apparently un-available to the author, there is more than enough to this book to make its reading profitable.—Harry Moses.

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