HUMIDITY 45 
venient to make simple readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as 
these hours afford much evidence in regard to the daily range. A good time 
also is that at which the temperature maximum occurs each day, but this is 
movable and in the press of field work can rarely be taken advantage of. A 
very fair idea of the daily mean humidity is obtainable by averaging the 
readings made at the hours already indicated. The comparison of single 
readings with the psychrograph record should not be made at a time when 
a rapid change is occurring, as the automatic instrument does not respond 
immediately. Such a condition is usually represented by a sudden rain, and 
is naturally not a satisfactory time for single readings in any event. 
71. Place and height. As stated above, the psy- 
chrograph is placed three feet above the surface of 
the ground in making readings for the comparison 
of stations. In low, herbaceous formations, the in- 
strument is usually placed within a few inches of the 
soil in order to record the humidity of the air in 
which the plants are growing. In forest formations, 
the moisture often varies considerably in the differ- 
ent layers. This variation is easily determined by 
simultaneous psychrometer readings in the several 
layers, or, if occasion warrants, a series of psychro- 
graphs may be used. In field work the rule has been 
to make observations with the psychrometer at 6 
feet, 3 feet, and the surface of the soil, but the read- 
ing at the height of 3 feet is ordinarily sufficient. 
Humidity varies so easily that several readings in 
different parts of one formation are often desirable. 
In comparing different formations, the readings 
should be made in corresponding situations, for ex- 
ample, in the densest portion of each. 
72. Check instruments. Humidity is so readily 
affected by temperature, wind, and pressure, that 
a knowledge of these factors is essential to an un- 
derstanding of its fluctuations. Pressure, disregarding daily variation, 
is taken account or in the tables for ascertaining relative humidity, 
and is determined once for all when the altitude of a station has 
been carefully established. The temperature is obtained directly from the 
dry bulb-reading. Its value is fundamental, as the amount of moisture in 
a-given space is directly affected by it; like pressure, it also is taken account 
of in the formula. The movement of the air has an immediate influence 
Fig.9. Atmometer. 
