12 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



fined. The use of these same data in strictly local studies will de- 

 pend entirely on the minute examination of the conditions surround- 

 ing the station. 



KNOWLEDGE OF EXISTING WEATHER BUREAU STATIONS NECESSARY. 



Before attempting any meterological observations the investigator 

 should visit the nearest permanent meteorological stations and ob- 

 tain a clear understanding of the manner in which the observations 

 are made, compare his own instruments with those of the station, 

 and ascertain the natural conditions in which the permanent station 

 is located and the extent to which they are typical of the region. 

 This is essential to enable the investigator to decide whether and to 

 what extent he would be justified in connecting his special meteoro- 

 logical observations with those of the permanent station. Observa- 

 tions at permanent, well-equipped Weather Bureau stations are not 

 always conducted in the way that meets the special needs of the 

 investigator. There may be observations essential to the forester 

 which are not being made at all. Furthermore, the data of the per- 

 manent station will not always enable one to judge of the effect of 

 the climatic conditions upon forest vegetation. For instance, the 

 measurements of the temperature of the air are always made at a 

 regular Weather Bureau station at some height above the ground 

 and in a more or less open place outside of the forest; while to the 

 forester, the temperature of that layer of the air in which most of 

 the forest vegetation is found has the greatest significance. Again, 

 while a very precise measure of precipitation may be of no use to 

 the investigator, the amount falling in single storms may vary so 

 greatly in short distances that a record obtained a few miles away 

 will be very misleading. It is thus evident that forest research has 

 special meteorological problems, and that usually the long-estab- 

 lished weather station may serve better as a control than as a definite 

 point for obtaining information about forest conditions. 



COMPUTATION OF ALL WEATHER DATA BY PERIODS OF GROWTH 



AND REST. 



One essential thing to be kept in mind is that plants may react 

 to the climatic conditions in altogether different ways during periods 

 of growth and rest. To analyze the reactions of plant life it is 

 usually desirable, therefore, to compute climatic data by such 

 periods. They may be based either on a knowledge of the particu- 

 lar plant formation which each observation-point represents, or on 

 the average period of the native vegetation of the locality. Usually 

 it will be preferable to adopt first a " growing season " for the whole 

 region under study. Later, for more exact comparison of the com- 

 ponent formations and after careful determination, the specific 



