MEASURING FACTORS. 19 



perfectly free, so that it responds readily to changes of temperature 

 and to changes in wind velocity. 



In this case, as in measuring wind, the normal evaporation for 

 the period in question has always been obtained by exposing an 

 evaporometer at a point where it could not be affected by the wind- 

 break. Other evaporometers were similarly exposed on both sides 

 of the windbreak at distances from the windbreak which were one, 

 two, and five tunes its height, and, in addition, on the leeward side 

 at ten and twenty times the tree height. These distances practi- 

 cally cover the entire range of the windbreak's influence upon evapo- 

 ration. The instruments show the quantity of water evaporated, 

 and the amount evaporated in the open is taken as the normal, or 

 100 per cent. Amounts evaporated at other points are reduced to 

 percentages of the normal. 



HEAT. 



While the windbreak affects evaporation by checking the move- 

 ment of the surface air currents, it may also affect the temperature 

 of the air. 



A body of trees modifies the temperature of the air within it. In 

 the daytime direct insolation is prevented and the air is cooler than 

 it is outside. At night the heat waves radiated from the ground are 

 intercepted and the air under the trees is warmer than outside. 

 Comparative uniformity of temperature is further secured by the 

 fact that the boles and branches of the trees store certain quantities 

 of heat and that this heat is only very slowly radiated. These 

 effects of forest cover are, however, confined to the forested area and 

 its immediate neighborhood. Windbreaks are much more important 

 for their effects at a distance. The following general principles 

 should be borne in mind in considering what effects windbreaks may 

 have upon temperatures and in comprehending the methods used 

 to measure these effects: 



(1) The earth is warmed daily by insolation and the heat so 

 received is dissipated very soon after the heat of the sun is withdrawn. 



(2) The lower strata of soil are heated by conduction of the warmth 

 from above. Conduction is increased by a moderate amount of 

 moisture in the soil, yet evaporation of moisture may reduce the sur- 

 face temperature and thus reduce also the amount of heat to be 

 conducted downward. 



(3) The ah* is heated only very slightly by the rays of the sun 

 passing through it. As it comes in contact with warm or cold objects, 

 however, it is heated or cooled. By convection or circulation of the 

 air thus heated or cooled there is a constant tendency to reestablish 

 uniformity of air temperature. 



