20 WINDBREAKS. 



(4) The air is heated or cooled in direct proportion to the length of 

 time that it remains in contact with warmer or colder objects, or, 

 in other words, in inverse proportion to its rate of movement over 

 such objects. 



(5) The air moves in a horizontal direction over the ground unless 

 so heated and expanded that it is caused to rise. At night there is 

 very little vertical movement of the air currents, but when their 

 horizontal movement is rapid there is a constant mixing of the upper 

 and lower strata of air. 



When the earth is radiating heat any object which, like a wind- 

 break, checks air currents raises the temperature of the surface 

 layers of air by increasing the length of their contact with the soil. 

 The presence of this warm blanket of air over the ground increases in 

 turn the temperature of the soil. In like manner a windbreak cools the 

 air by holding it in contact with the ground when late at night or 

 in cloudy weather the earth has practically ceased to radiate heat. 

 Such arrest of air currents, commonly called "stagnation," is known 

 to have an appreciable influence on day temperatures and on the 

 liability to frosts at night. Its effects may be felt by anyone who 

 passes quickly into the zone protected by a windbreak, where in the 

 daytime the air feels more sultry and at night more chilly than in the 

 open. 



These influences of windbreaks upon temperatures have been 

 studied by means of simple thermometers located with respect to the 

 windbreak as were the evaporometers. In every case the normal 

 temperature was obtained outside the windbreak's zone of influence, 

 but in other respects the control thermometer has been exposed to 

 the same local influences as were the thermometers on both sides of 

 the windbreak. 



To measure air temperatures during the day, thermometers at 4 

 feet above the ground were read at each hour between sunrise and 

 sunset. At night the minimum temperature at the same points 

 was obtained by a minimum-registering thermometer. Soil tem- 

 peratures were obtained at the same points by inserting a very deli- 

 cate thermometer into a small hole bored to a depth of 1 or 2 feet 

 and kept covered between successive readings. 



HUMIDITY. 



It has been assumed that the greatest effect of a windbreak upon 

 the humidity of the air over grain fields arises not so much from the 

 possibility of the trees adding to or detracting from the moisture con- 

 tent of the air as from their disturbing influence upon the movement 

 and direction of air currents. To determine how great this effect 

 might be the amount of moisture in the air at several points on both 

 sides of the windbreak and at a height of 4 feet from the ground was 



