58 BULLETIN 1333, V. S. DEPARTMENT OF AGRICULTURE. 



sufficient evaporation to cause any apparent injury to trees. It is" 

 the continuity of these winds that makes them dangerous, and prob-j 

 ably the decade is the shortest period that need be considered in this 

 connection. 



Only one additional feature of the record need be noted. Although 

 there is a general decline in the average velocities from the end ol 

 April to August, there is a somewhat sharp rise at the end of May] 

 continuing into June, which is nearly always associated with lack of 

 precipitation. Whether the dry winds occurring at this season are 

 actually injurious or not depends, no doubt, primarily on the reserve 

 moisture of the soil. On the other hand, if the dryness occurs when 

 the season, so far as temperature is concerned, is well advanced, as in 

 1917, a day or two of even moderately high winds with a high evapor- 

 ating rate may have a veiy noticeable influence. On June 29, 1917, 

 wilting of herbaceous vegetation, in general, and of some new shoots 

 on conifers, was noted, although the soil was by no means dry, and 

 the recorded velocity for the day was only 6.2 miles per hour. 



This late spring dry period, with moderately high winds, is especially 

 injurious to newly planted trees, and it may well be concluded that it 

 has an indirect relation to forest types in so far as it tends io preclude 

 the germination of seeds early in the season. It can not be said 1 hat 

 any difference in this respect has been noted between different sites 

 and exposures. When the atmosphere is lacking in moisture and is 

 freely circulating, even the most protected situations seem to suffer 

 superficial drying so quickly that germination is postponed until 

 the beginning of the frequent summer rains, usually about July 1 •">. 

 This fact may create special problems in comparing the species in the 

 Pikes Peak region, and it may be said that spring dryness has a 

 bearing on the selection of the species which shall survive through the 

 autumn drought. 



Wind movement in the various forest types. — In Table 13 all the 

 available data on wind movements are given. As the stations havina 

 this record are relatively few in number, it has not been thought 

 necessary to consider the local stations apart from those lying at a 

 greater distance from the control station, and hence subject to 

 altogether different influences. Neither is it necessary to separate 

 those stations at which the anemometer has been placed a consider- 

 able distance above the ground from those at which for a special 

 reason the surface circulation has been measured, although in com- 

 paring the results this factor should be kept in mind. Inasmuch as 

 the period of observation is very different for different stations, and 

 at most of them is not over 2 or 3 years in length, the method has 

 been followed throughout of giving the actual mean monthly move- 

 ment for each station, and the percentage relation which this bears to 

 the movement at the control station for the same period. These 

 percentages might, then be related to the "'average" movement for the 

 control station, as shown in Table 12, but it is doubted whether : 

 computation would have any additional value. As the matter of 

 primarv interest is not wind, but rather its influence on evaporation, 

 the main intent at this stage of the discus-ion is to show relative 

 wind movements in a rough way only. 



