74 



THE HABITAT 



a ready clue to the relative amount of run-off in each fall of rain. The 

 measurement of snowfall is often made merely by determining its depth. 

 For comparison with rainfall, the rain gauge with receiver and tube with- 

 drawn is used. The snow which falls is melted, poured into the measur- 

 ing tube, and measured in the ordinary way. The U. S. Weather Bureau 

 standard rain gauge, with measuring stick, may be obtained of H. J. Green, 

 or of J. P. Friez for $5.25. 



107. Precipitation records. From the periodic character of precipitation, 

 rainfall sums, means, and curves have little importance in the careful study 

 of the habitat. The rainfall curve for the growing season is an aid in ex- 

 plaining the curve of water-content, and the mean rainfall of a region gives 

 some idea of its vegetation, though even here the matter of its distribution 

 is of primary importance. The rain and snow charts published by the U. 

 S. Weather Bureau furnish data of some importance for the general study 

 of vegetation, but it is evident that they can play little part in a system 

 which is founded upon the habitat. Precipitation records, for reasons of 

 brevity and convenience, are united with wind records, and the form will 

 be found under the discussion of this factor. 



WIND 



108. Value of read- 

 ings. On account of its 

 direct effect upon hu- 

 midity, and its conse- 

 quent influence -upon 

 water-content, the part 

 which wind plays in a 

 habitat can not be 

 ignored in a thorough 

 investigation. It is an 

 important element in ex- 

 posure, and accordingly 

 has a marked mechani- 

 cal effect upon the -vege- 

 tation of exposed habi- 

 tats, alpine slopes, sea- 

 coasts, plains, etc. 

 Owing to its inconstancy and its extreme variation in velocity, single wind 

 readings are absolutely without value. When read in series, anemometers 

 give some information upon the comparative air movement in different hab- 



Fig. 23. Simple anemometer. 



