The Spectroscope and the Weather

Abstract

WE were visited here, on the 11th inst., by a very severe thunderstorm, beginning a little before noon and lasting for about an hour and a half. Anxious to confirm some observations made recently in the West Indies, in which I got from lightning a continuous spectrum, I took out my pocket spectroscope, and on looking through it was at once struck by the peculiarity of the spectrum. The band noticed by Prot. P. Smyth (vol. xii. pp. 231, 252) on the less refrangible side of D was very distinct, while the band (W.L.L. 5830–5680) on the more refrangible side of D was also very, though not nearly so dark, leaving the appearance of a bright yellow band over the part of the spectrum W.L.L. 5880–5830, as in the sunset spectrum, only much more marked. The A, B, and C lines were all visible; E and b were very sharp, b being easily separable into three lines; while there was also a dark band (W.L. 5040?) between b and F, but no lines visible beyond F. The most peculiar point, however, was the rapidity with which the spectrum varied, for, keeping the instrument pointed in one direction, each different cloud that passed differed in the intensity of the darkness of the band W.L.L. 5970–5900, which, sometimes could be distinctly separated from D, while at other times it appeared quite continuous with it. The darkest bands were given by the lurid purple and pillared white-grey clouds. During all this time the heat had been intense, and the thunder was accompanied by light gusts of wind varying as much as 90° in direction, but about 1 o'clock rain began to fall and the abnormal bands to disappear. By 4 P.M. the band W.L.L. 5830–5680 was almost quite gone, and the band W.L.L. 5970–5900 had also become faint, appearing like a shadow cast by D, which was sharp and clear except in the light reflected from a few of the heaviest clouds. On the 12th the sky was still very much overcast and the spectrum again slightly abnormal, but not more so than I have noticed it in a thick “Scotch mist.” To-day, with sky still completely overcast, the spectrum is quite normal.