REPORT OF THE SECRETARY. 41 



regions and the highest in the middle latitudes. Furthermore, its 

 temperature, while practically constant from season to season, varies 

 greatly from place to place and from day to day. European investi- 

 gations seem to show that the beginning of the upper inversion is 

 found at a lower altitude over cyclonic than over anticyclonic areas, 

 and that it is higher in summer than in winter. Observations in 

 this country coincide with those in Europe as to the winter and sum- 

 mer heights, but are inconclusive respecting the supposed relation 

 of its altitude to areas of differing atmospheric pressure. 



Sounding balloon ascensions have added much to our knowledge 

 of the temperature of the atmosphere up to heights of 9 miles, and 

 even higher, but the number of ascensions above that altitude is yet 

 small. The lowest temperature recorded in any of the Weather 

 Bureau's series of observations is —92° F. at Huron, S. Dak., in 

 September, 1910. The vertical distribution of temperature in dif- 

 ferent sections of cyclones and anticyclones presents at times unusual 

 features, the importance of which will be realized when it is remem- 

 bered that forecasts of temperature changes are at present based en- 

 tirely upon prevailing surface temperatures without taking into 

 account the possible modifying effects that unusual temperature con- 

 ditions above may introduce later. 



Equally interesting are the facts regarding wind direction and 

 velocity in the upper atmosphere. Observations show that while 

 the lower limit of the upper inversion is not sharply defined, the 

 air motion in the explored part of that region partakes of and is to 

 some extent controlled by that of the lower atmosphere on which it 

 rests. At the same time it also appears that the gyratory motion 

 of the air characteristic of cyclones at the earth's surface and for 

 some distance above does not extend far upward. The general con- 

 clusions as to the winds in the upper inversion in their relation to 

 those of the lower layers are that the air currents are from some 

 northerly direction on the east side of anticyclones and from some 

 southerly direction on the west side, and that under practically all 

 other conditions the drift of the air at very high levels is from west 

 to east. 



The observations taken at mountain stations in Colorado show 

 that variations in temperature at the summit and base stations are 

 nearly coincident in point of time and that they are generally simi- 

 larly directed, but that a fall in temperature occasionally sets in 

 on the plains while the temperature on the mountain tops is still 

 rising. At other times the weather conditions on the mountain sum- 

 mits have been controlled by causes that are not operative on the 

 plains to the eastward. These studies have increased our knowledge 

 of the effect of local topography in the warming and cooling of the 

 air that is trapped between the mountain ranges. 



