FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. Ill 



a west exposure is frequently prevented from reaching a high surface 

 temperature; but, if sunshine continues all day, its temperature, as 

 a natural consequence, may be very high after the air nas become 

 well warmed. 



SUNSHINE. 



Since the control station was established in 1910 the duration of 

 sunshine has been measured there by means of the standard Weather 

 Bureau electric-thermometric recorder. At stations in the various 

 forest types it has not been measured long enough to give records 

 of much value. Hence it is believed that, in comparing the types, 

 the indirect evidences of insolation may perhaps be more useful, 

 though it is not desirable to reach any such decision without ex- 

 haustive trial. In short, then, the available record is only for the 

 control station, which gives a measure of sunshine values for the 

 general locality at a middle elevation, and for a similar period at 

 Wagon Wheel Gap. These data are valuable in the general study 

 of the climate, and currently in studying such other phenomena as 

 soil temperatures. They may prevent reaching wrong conclusions as 

 to the import of other factors. 



Sunshine at the control station. — This record is made up wholly from 

 the graph written by the pen of the triple register, with almost 

 no interpolation, and without the additions which are termed " twi- 

 light corrections," and which are sometimes employed to compensate 

 for the lag in the response of the recorder — or, perhaps it might 

 better be said, its lack of response — when the sun is very close to 

 the horizon. The study has not been intensive enough to permit 

 the application of such corrections, although it may be roughly 

 estimated that they would add, on the average, 30 minutes each 

 day to the duration of sunshine, increasing the yearly average by 

 11,000 minutes or about 5 per cent of the possible. Because of the 

 failure to apply these corrections, the record is not strictly com- 

 parable with the Wagon Wheel Gap record or with Weather Bureau 

 records in general. 



The possible duration of sunshine at the point where the recorder 

 has been located was determined by observations of sunrise and 

 sunset at that point on almost every clear day during 1910 and 1911. 

 By plotting these hours, the length of each clay, decade, and month 

 was computed. The day at this location is considerably shorter than 

 it would be on a plain in the same latitude, as the sun sets at an 

 elevation of nearly 10°, and on the longest days also appears first 

 at a slight elevation. 



The basis at Wagon Wheel Gap is the same, namely, short days 

 caused by a high horizon to the west, which on the average shortens 

 the day by more than two hours. Here, however, closer observation 

 has permitted twilight corrections. 



In Table 32, the actual sunshine for each decade of record is 

 shown, together with the mean for each decade and the percentage 

 relation of this mean to the possible. It is desirable to call attention 

 to the fact that the records of the last two years have very materially 

 altered the averages, especially for May and June. 



