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



KNOWLEDGE OF EXISTING WEATHER BUREAU STATIONS NECESSARY. 



Before attempting any meteorological observations the investi- 

 gator should visit the nearest permanent meteorological stations 

 and obtain a clear understanding of the manner in which the obser- 

 vations 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 re- 

 gion. This is essential to enable the investigator to decide whether 

 and to what extent he would be justified in connecting his special 

 meteorological observations with those of the permanent station. 

 Observations 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 their 

 permanent 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 weatherstation 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 particular plant 

 formation wdiich 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 wdiole region 

 under study. Later, for more exact comparison of the component 

 formations and after careful determination, the specific period^ of 

 plant activity may be employed in summarizing temperatures, etc. 



