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 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. 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 Sunda Strait On August 27, 1883, following several months to western Europe and also to explain the irregular of minor explosions, the volcano on the Island of rate of spread of the cloud poleward as described Krakatoa (Sunda Strait, between Sumatra and below. Java, 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 Unauthenticated | Downloaded 09/27/21 09:15 PM UTC VOL. 32, No. 2, FEBRUARY, 1951 49 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 Unauthenticated | Downloaded 09/27/21 09:15 PM UTC 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.