CHAPTER X 



OROGRAPHIC FACTORS 



Climatic and soil factors are variously affected by the contours of 

 the land. Local or microclimate, in contrast to general cHmate, is 

 largely controlled by orographic factors: mountains, plateaus, plains, 

 exposure, and slope of the surface. Even the soil conditions and soil 

 development are influenced by relief. The effectiveness of relief 

 factors in the northern hemi- ^Janfeb.MarJprMavJuneJulyAugSepOciNovDec. 

 sphere seems to increase from 

 south to north and from the foot 

 of mountains to their summits. 



Altitude. — In extra-tropical 

 mountains, according to Hann, 

 the mean temperature for the 

 year decreases 0.55°C. per 100 m. 

 elevation. This fall in tempera- 

 ture brings about a corresponding 

 shortening of the vegetative sea- 

 son. Sixteen years of observa- 

 tion in the northern Alps showed 

 that the average snowless vegeta- 

 tive period at an elevation of 

 1,000 m., on level ground, lasted 

 8 to 9 months; at 1,500 m., 7 to 

 8 months; at 1,800 m., 6 months; fig. 130.— Duration 



nnHnt^/innm a bm 1 1 ."^l^^ months ^"^ *^^ ^^^^^ ^"^ ^"^"^^ ^^^^^ °^ ^^"^ 

 anaatZ,4UUm.,aDOUt^/2moninS. y^^^y Mean of 1 6 years' record. Tem- 



FigUre 130 shows the progress of perature of atmosphere at time of melting 



disappearance and appearance of ^^ ^'^^^^ ^^^^'^ ^'"'"''■^ 

 snow at various altitudes on the shaded and exposed sides of the 

 Tirolese Inn Valley. The shortening of the vegetative period often 

 causes an acceleration of the life processes, especially of the flowering 

 and fruiting of plants. In high mountains, on the border of eternal 

 snow, we have the same phenomenon of hastened flowering and fruit as 

 in the almost rainless deserts and steppes. In spite of the brief 

 vegetative period, the seeds of the high-alpine plants of the Salicion 

 herbaceae germinate very well; in Veronica alpina 80 per cent, Gnap- 

 halium supinum 73 per cent, and in Soldanella pusilla 86 per cent. 



261 



snow cover 



