CLIMATIC FACTORS 



133 



Table 10. — Decrease in Relative Humidity and Vapor Pressure in More 



Continental Localities (48.3 to 38.6° N. Lat.) 



(After Hann) 



The general rule is often modified by local influences. For exam- 

 ple, Miilhausen, in the rain shadow of the Vosges, has only 67 per cent 

 annual relative humidity, whereas Warsaw has 79 per cent. 



There is no immediate connection between humidity and total 

 precipitation. Scanty humidity cannot be inferred from low precipita- 

 tion. Kola, in Russian Lapland, has but little rain (18 cm. per year) 

 but a mean relative humidity of 80 per cent. The whole southwestern 

 coast of Morocco, the climax area of the Argania forest, has an annual 

 rainfall mostly less than 30 cm., but the relative humidity runs between 

 80 and 90 per cent. On the other hand, the rainy Monte Generoso in 

 southern Ticino (176 cm. rainfall) has a mean relative humidity of 

 only 64 per cent. 



Pgg — Yog observations give important information about humidity 

 of the air. The cloud belt of our medium and high mountains is 

 characterized by luxuriant epiphyte communities (mosses and hchens). 

 The lichen association of the Lobarietum pulmonariae, best developed 

 in the Atlantic province, is wholly confined, in central and southern 

 Europe, to the cloud levels of 1,200 to 1,600 m. (Ochsner). Cooper 

 (1919) has demonstrated that the eastern distribution of Sequoia 

 sernpervirens is determined by the extent of the landward movement of 

 the fog of the Pacific coast of North America. 



The cloud belt corresponds to the beech level in southern Europe, to 

 the Quercus mirheckii-Q. maroccana forest in north Africa, and to the 

 laurel forest in the Canaries. 



Relative Humidihj in Small A /-eas.— Humidity of the air is subject to 

 sudden and considerable variations, caused by rain, clouds, fog, local 

 winds, and exposure. 



Stocker (1923) measured humidity in natural plant communities. 

 He found astonishingly great differences, not only between different 

 habitats but also between the different layers of vegetation in the same 

 habitat. On a quiet sunny day in July in a pasture community of 

 grass, Trifolium and Lysimachia nummularia near Freiburg, he found 

 the relative humidity and saturation deficit to be respectively 90 per 



