[Letters to Editor]

Abstract

IF all balloon ascents had been made by day, I confess that I should be inclined to agree with Mr. Hughes and think that the recorded temperatures were due to radiation, but that idea is disposed of, to my mind, by the fact that the isothermal column of air shows just as plainly in ascents made after sunset as in those made in the day. At night the thermograph must receive some heat by radiation from the earth, and lose some by radiation into space, but both amounts must be infinitesimal in comparison with that which would be given to it by the sun. If, then, exposure to the sun does not seriously alter the temperature, and it does not do so even at the greatest height provided there is a moderate amount of vertical motion, the effect of the radiation after sunset must be utterly insignificant. That solar radiation in the ordinary conditions is not important is proved by the fact that if the balloon bursts, and therefore does not float, it is not possible to say from the trace alone if the ascent was by night or by day. There have been cases in which the balloon did not burst, and the temperature at the top reached the freezing point of water. If I asserted that the rate of ascent does not matter, I must plead guilty to a mistake, but I think I said “apart from radiation,” and I still believe that radiation at night to and from the bright metal of the thermograph is so trifling that the rate of ascent is of no consequence. There is also the fact that the uptrace, where the motion is comparatively slow, is identical with the down-trace where the motion is rapid.