213 



ZONES AND REGIONS 



[Pt. Ill, Sect. I 



The relative humidity of the air usually corresponds to the amount of 

 rainfall, and is naturally greater in the rainy than in the dry season. 

 Again, districts on the coast, islands, and mountainous countries are 

 characterized by a high degree of relative humidity, by which we under- 

 stand a mean annual relative humidity of not less than 8o°/ o . The 

 relative humidity rises at night and in the early morning hours up to 

 saturation, but falls during the day, in sunny weather, low enough at 

 65-70 c / to exert considerable desiccating effects on vegetation. In 

 districts with marked dry seasons, the relative atmospheric humidity 

 during these descends on the average to $$-6$ °/ o , and much lower in 

 desert districts. Many districts with dry seasons have during the nights 

 in these seasons a heavy formation of dew, which is important to the 

 vegetation. 



In many districts during the rainy season constant clouds prevail, so 

 that, according to Harm, a heavy, dark cloudy sky persists for months. 

 This, however, is by no means the case everywhere, and does not agree 

 with my own experience of tropical rainy seasons in Trinidad and Java, 

 during which most days included several sunny hours ; although com- 

 pletely rainy days were not wanting, yet, on the other hand, bright days 

 were not less frequent. In Buitenzorg during the rainy season the sky 

 is usually quite bright before midday, and the bursts of rain, which cause 

 the high annual rainfall of about 4! meters, for the most part fall only 

 during some hours after midday, though with a violence unknown in 

 Central Europe. The dry season in many districts is characterized by 

 a continuously cloudless sky, whereas in some the dry season exhibits 

 a cloudiness hardly or not at all less marked than does the rainy season. 



The following table, taken from Hann and compiled by J. Murray and 

 S. Arrhenius, represents the mean distribution of the atmospheric precipi- 

 tations in the tropical zone and in the neighbouring belts of the temperate 

 zones l : — 



TABLE OF MEAN DISTRIBUTION OF ATMOSPHERIC PRECIPITATION 

 IN TROPICS AND ADJACENT BELTS (after Hann). 



1 Hann, op. cit., Bd. II, p. 37. 



