TEMPERATURE FACTS 



207 



233. Lag of the Seasons. For a similar reason the warmest 

 part of our summer usually comes in July, or in August, in- 

 stead of at June 21 when the sun is farthest north, and the 

 coldest part of winter in January instead of at December 22, 

 when the sun is farthest south. A corresponding lag is found 

 in the southern hemisphere where summer occurs during our 

 winter. 



234. Night-time Cooling and Radiation. The cooling at 

 night is caused chiefly by radiation, and is therefore greatest 



FIG. 142. A lath screen. These are usually arranged to roll or slide aside 

 to admit sunshine. Night temperatures under this screen averaged 4 degrees 

 higher than in an unprotected orchard nearby. To left of center, in fore- 

 ground, is seen one type of firepot used in warming the ground air. 

 (Illustration by F. A. Carpenter, in Monthly Weather Review.) 



where radiation is greatest. On mountains and plateaus, the 

 air is thinner, owing to elevation, and also, usually, contains less 

 dust and smoke. This permits freer radiation and causes cool 

 nights even in midsummer; night frosts occur there in portions 

 of the year when the days are warm. 



235. Radiation and Frost. In any locality radiation is 

 most rapid when the sky is clear; hence clear nights are coolest, 

 and most likely to give frost in spring and autumn. 



