Atmospheric Effects and the Andean Eruptions

Abstract

WHEN a solar halo and sun pillar were observed in London a few days after the recent volcanic outbreaks in the Andes, the question of a connexion between the two phenomena was raised. There was no serious case to be made out for any such connexion, in view of the enormous distance that separates Great Britain from the scene of the eruption, and the incredible wind velocity that would have been necessary to bring volcanic dust in such a short time. Moreover, optical phenomena of this kind are known to be due to ice crystals at great heights and not to dust. The more rational question of ultimate meteorological effects in the northern hemisphere is discussed in a recent bulletin from Science Service, Washington, D.C. We are reminded that after the explosion of the East Indian volcano Krakatoa in 1883, volcanic dust drifted entirely round the earth, and that the remarkable red sunsets that occurred for several years after can reasonably be attributed to the presence of this dust, for the selective absorption effects of dust upon the constituent colours of the light of the sun is proved at almost every fine sunset in industrial regions. Prof. W. J. Humphreys, of the U.S.A. Weather Bureau, has expressed the opinion that it is only on the supposition that the ash has ascended in large quantities above the level of the highest clouds that it may be expected to cross over to the northern hemisphere; if confined to lower layers, it may perform long journeys in the atmospheric circulation of the southern hemisphere but will tend to be entangled by condensing cloud particles and be brought to the earth by rain. Apart from sunset effects, the probable influence of dust at very high levels upon the weather through weakening of the solar radiation is difficult to estimate, owing to the complexity of the reactions likely to be set up.