FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 53 



200 feet higher on the south, at an angle slightly less. Toward the head 

 of the more open of the three stream channels, in a northwesterly direc- 

 tion, the elevation is somewhat less than 200 feet, and the angle is 7 de- 

 grees. Down the valley the immediate negative elevation is still less. 



The control station is thus appreciably affected only by air cur- 

 rents paralleling the main axis of the valley, namely those from the 

 southeast or the northwest. Only rarely do currents from the south- 

 west or south reach the station through the valley having its origin 

 in that direction, as the station is somewhat west of the axis of that 

 valley. Due to this influence of local topography there is every 

 reason to believe that the full velocity of the prevailing winds is not 

 recorded. The prevailing wind direction is always southeast during 

 the periods when valley breezes constitute the chief element of air 

 circulation, that is, during the summer; and always northwest or west, 

 the focmer predominating, when mountain breezes or anticyclonic 

 winds dominate the atmosphere. Cyclonic winds, which are usually 

 of northeasterly origin, are rarely recorded as such at this station, 

 and may appear as east or southeasterly winds. It must, therefore, 

 be recognized that winds from the northeast or north, and to a less 

 extent those from the south and southwest, must conform to the con- 

 figuration of the valley to become effective at this station. 



Further evidence of local influences is given in the gusty character 

 of winds, whenever the velocity is at all high. This is not shown by 

 records, since the gusts of wind are usually so short that any individual 

 mile recorded may be made up of several squalls and intermediate 

 calms. From extended observation the writer has estimated that 

 the momentary velocity of the wind gusts is fully twice the rate of 

 movement indicated by the anemometer records. In any study of 

 the mechanical effects of wind this discrepancy would be very impor- 

 tant, and only the pressure-tube anemometer or some similar con- 

 trivance could possibly depict the actual conditions. For the pur- 

 poses of this study, on the other hand, momentary velocities are of 

 no importance, and, it may as well be stated at this point, the mechan- 

 ical effects of wind in the region under study are quite negligible. 

 The soil is of such hrm texture and the trees are so short-boled as to 

 preclude windfalls — except, of course, when trees have become badly 

 decayed — and even the breaking of limbs is extremely rare. On 

 one occasion in eight years, at a time when intense cold was accom- 

 panied by high wind, considerable snapping of twigs was noted. 

 Even this was confined to the especially long side branches whose 

 rapid, slender development subjected them to unusual exposure. 



At timberline, of course, the mechanical effects of wind are always 

 apparent in the distortion of limbs and the one-sided development 

 of* trees as a whole. At lower elevations that which appears to be a 

 similar mechanical distortion of form has been traced by the writer (6) 

 to winter-killing of limbs on the most exposed portions of the trees. 



With these facts in mind, the recorded wind movements at the 

 control station, as shown in Table 12, may be considered. As this 

 record is an almost perfect one, it can be quite positively stated that 

 apparent discrepancies between succeeding decades or corresponding 

 periods of different years are true to the facts. The credit Delongs 

 to the designer of the thoroughly practical Robinson anemometer, 

 and to the fact that the daily dial readings are compared with an 

 almost unbroken automatic record. 



