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Updated: May 24, 2025
The dust-showers take place most frequently in spring and autumn; that is, 'after the equinoxes, but at intervals varying from thirty to fifty days; the cause being, that the equatorial calms, at the time of the vernal equinox, extend to four degrees on either side the equator; and as the rainy season then prevails between those limits, no dust can consequently be taken up in those latitudes.
The conclusion here arrived at was still further verified by another sirocco-storm in May 1846, which extended to Genoa, and bore with it a dust that 'covered the roofs of the city in great abundance. This, as was clearly ascertained, contained formations identical with those which had been collected off the Cape de Verd; and it was shewn that the dust-showers of the Atlantic, and those of Malta and Genoa, were 'always of a yellow ochre-like colour not gray, like those of the kamsin, in North Africa. The peculiar colour of the dust was found to be caused by iron-oxide; and from one-sixth to one-third of the whole proved to consist 'of determinable organic parts. In the following year, 1847, Ehrenberg had another opportunity of testing his conclusions, in specimens of dust which had fallen in Italy and Sicily in 1802 and 1813; the same result came out on examination; 'several species peculiar to South America, and none peculiar to Africa.
Dr M'Gowan states, that the fall amounted to ten grains per square foot, but without specifying whether this quantity includes the whole duration of the shower. During calms, it remains suspended. The dust thus raised from the Mongolian steppes gives the peculiar tinge to the Yellow Sea. Notwithstanding the annoyance of these dust-showers, they have a valuable compensation.
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