20 
The Colorado Experiment Station 
TABLE VIII.—SHOWING AT WHAT TEMPERATURES SMUDGING IS NECESSARY IN 
THE VARIOUS STAGES OF DEVELOPMENT OF APPLE AND PEAR BUDS. 
Degrees 
Calyx closed . 30 
Flowers gone, calyx closing... 30 
Petals dropping . 31 
Fruit forming. 30 
Full bloom . 30 
Buds in pink. 25 
Buds separating . 20 
Buds swelling. 15 
From the two tables above, it is evident that a temperature of 
32°, which may be accompanied by the formation of frost, is not 
low enough to cause injury to our fruits in any stages of their de¬ 
velopment. It very frequently happens that the temperature just 
touches 32°, not going lower, and after remaining there for a short 
time, becomes higher. 
Average Date of Last Spring Frost. —Fig. 7 shows the aver¬ 
age date of the last spring frost in the different sections of Colo¬ 
rado. In a few localities the average date is before May 1. Over 
the greater part of the Great Plains it is between May 10 and May 
20. Throughout the entire mountainous parts of the state above 
about 7,000 feet, and also at lower altitudes in the northwest coun¬ 
ties, the average date is after June 1. 
Inversion of Temperature.—As a rule, temperature decreases 
with an increase in altitude. Occasionally, however, as a result of 
air drainage, the valleys and canyon bottoms are cooler than the 
adjacent hillsides; in other words, temperature may increase lo¬ 
cally with altitude. Temperature inversion is a common phenom¬ 
enon in mountainous regions, and is particularly noticeable in the 
spring and autumn. 
A comparative study* of temperatures on two mesas near 
Boulder, with a difference in elevation of 415 feet, showed for 
May, 1908, the mean monthly temperature to be 2.6° higher on the 
upper mesa than on the lower. Furthermore, the last frost in the 
spring was 16 days later on the lower mesa than on the upper one. 
This inversion is undoubtedly present all along the foothills and 
in many other sections of the state, not occurring every year, but 
to be expected. 
At certain periods, inversion of temperature of more than local 
influence occurs along the eastern edge of the foothills, ivhich up 
to a considerable altitude experience a higher temperature than 
the plains. Loudj* points out that on the eastern slope of the 
*RamaIey, Francis. Climatology of the mesas near Boulder, Colo., in Studies on 
mesa and foothill vegetation. Univ. Colo. Studies 6 :ll-49. 1909. 
fLoud, F. H., The Colorado Sky 1:1-9. 1908. 
