Solar Radiation and Sun-Spots

Abstract

SINCE I communicated to NATURE the first results (vol. xii. p. 147) of an examination of the Indian registers of solar-radiation temperatures, I have examined some other registers, all of which confirm the conclusion adumbrated in my former note. Among these the most interesting and striking is the hill station Darjiling, in Sikkim, nearly 7,000 feet above the sea. The place is very cloudy, being on the outer Himalayan range, and much exposed to the moist southerly winds, but it has two advantages over the stations in the plains, viz., that there are nearly 7,000 feet less atmosphere above it, and it is free from the dust haze, so prevalent on the plains, which perhaps more than water vapour (if not thickly condensed) stops a large part of the solar radiation. On clear days and in intervals between the clouds, the sun's heat is sometimes very intense. The table that follows has been compiled in a different manner from that which I communicated a fortnight since. Instead of picking out days with little or no cloud (which are sure enough during the greater part of the year), I have taken the three highest recorded sun-temperatures in each half-month, and from these have deducted the maximum air-temperatures recorded on the same days; the mean of the six observations being taken to represent the month. The same instrument has been in use since the observations were commenced in April 1870. I must leave it to meteorologists at home to compare these temperatures with the recorded sun-spot areas, which I am unable to ascertain. But the maximum radiation temperature evidently falls in 1871, the year of maximum spots, and the increase on that of the imperfect year 1870, and the fall in the subsequent years, at least up to the end of 1874, are very marked.