140 AIR 



is saturated and there is standing water on the surface for too long 

 a time. This often happens to such crop plants as corn and wheat. 

 These facts are extremely important in irrigation projects. Common 

 practice in irrigated regions has often involved the use of more water 

 than is best for crop production. The plants need both air and . 

 water and in general the amount of air increases as the water is de- 

 creased. Therefore the most economical practice in irrigation is one 

 that strikes a compromise between these two needs of the plant. 

 The water should be applied in such a way as to interfere as little as 

 possible with the aeration of the soil since otherwise an actual reduc- 

 tion in yield per acre is likely to result. 



89. Atmospheric Humidity.— The amount of moisture present in 

 the atmosphere, as water vapor, varies greatly from time to time 

 although there is always some present. When the air becomes com- 

 pletely saturated with water condensation begins and some of the 

 water is precipitated as rain or snow. When the atmosphere is 

 saturated with moisture the loss of water from plants by transpira- 

 tion is negligible and the drier the atmosphere the more rapid is 

 transpiration. The amount of water that the air can hold at any 

 given atmospheric pressure, however, varies directly with the tem- 

 perature. Therefore, from the ecological point of view, the absolute 

 amount of moisture in the air is not nearly so important as the dif- 

 ference between the amount present and the amount necessary to 

 completely saturate it. This difference is called the saturation 

 deficit. It is a measure of the capacity of the atmosphere for taking 

 up water and thus, indirectly, of the rate at which evaporation or 

 transpiration may take place. The amount of moisture present in 

 the air is expressed, not as an absolute amount, but as a percentage 

 of the amount necessary for saturation at a given temperature. This 

 is called the relative humidity. Every change of temperature, of 

 course, changes the relative humidity, without changing the ab- 

 solute humidity. The saturation deficit, on the other hand, is 

 expressed as weight per cubic foot of air or, more often, as pressure, 

 in inches or centimeters of mercury. 



In the measurement of atmospheric humidity use is made of a 

 psychrometer. This instrument consists of two thermometers 

 mounted in such a way that they can readily be whirled to facilitate 

 evaporation. The mercury bulb of one thermometer is covered with 

 cloth which is saturated with water. The evaporation of the water 

 cools the thermometer and the difference between the readings of the 

 two thermometers is called the depression of the wet bulb. Tables 



