METEOROLOGY AND ALLIED SUBJECTS. 28 T 



fore, not less than it is in Southern Europe during the rainiest half of 

 the year, and is, therefore, not at all to be compared with the drought of 

 the great continental deserts. These regions of small and great rainfall 

 vary their position and their extent in the course of the year. During 

 our northern summers the equatorial rainy region and the two adjacent 

 regions of light rainfall have a position ten or fifteen degrees more to the 

 north, and the northern extra-tropical rain-belt has a much smaller exten- 

 sion than during our winter season. The equatorial rain-belt coincides 

 with the belt of calms, and corresponding to this it lies during March 

 between 4P north and 4^ south latitude 5 but during July it lies between 

 C° north and 12^ north. The position at which the rain-belt is found at 

 the close of our winter is occupied in midsummer by the belt of least 

 rainfall in the region of southeast trades. The position of the calm-belt 

 in summer is occupied in the beginning of spring by the belt of least 

 rain lying in the region of the northeast trades. The region beyond 

 the northern limits of the Tropic of Capricorn, where rain falls on more 

 than half the days of the year, is confined in the summer time to a small 

 space in the center of the ocean between 42^ and 60*^ north, while in 

 winter time it extends from the neighborhood of the Tropic of Capri- 

 corn to beyond Iceland. The southern limit of the extra-tropical re- 

 gion of slight rainfall, on the other hand, experiences smaller annual 

 variations, and extends in general in the spring time and autumn 

 farthest towards the equator, while, on the other hand, in the southern 

 hemisphere it retreats the farthest southward. By these variations in 

 location and extent of the rain region a very different distribution of the 

 rain with respect to the seasons of the year is brought about in the 

 various portions of the ocean. {Z. 0. G. M., Vol. XV, 1880, p. 475.) 



Hann has investigated the rainfall of Austro-Hungary and attempted 

 the solution of several questions hitherto slightly touched upon, bas- 

 ing his studies upon monthly means for twenty years or more at ten 

 stations. Some of his results may be expressed as follows : The mean 

 departure of the rainfall for any month from the average rainftill for the 

 whole period is called the mean variability ; it increases with the mag- 

 nitude of the rainfall itself, so that in general places of greatest rainfall 

 have the greatest variability. From the mean variability of four stations 

 he computes the probable error of the mean of ten years as ± of 2 milli- 

 meters, whence it would require 840 years to reduce the probable error 

 to one millimeter, whence we see the absurdity of giving the monthly 

 sums of rainfall to tenths of milimeters. If we exijress the mean de- 

 partures in percentages of the total rainfall, we find the average varia- 

 bility for Austro-Hungary to be 40 or 50 per cent, of the total value, 

 and it requires from sixty to seventy years of observations to obtain a 

 monthly mean of rainfall whose probable error is 6 per cent, of its whole 

 value. The above demonstrates clearly how uncertain are the rainfaUs 

 deduced from observations for ten years or less, and how easily erron- 



