Errors in the measurement of relative humidity as much as 20% were frequently recorded in a series of observations taken in closed tents with hair hygrographs. An investigation into the causes of these inaccuracies follows. It is shown that (1) because of the extremely high temperatures in the tents, both winter and summer, the air will be subjected to relative humidities so low as to cause the hair to change its zero calibration point and (2) because of the normal diurnal range of approximately 80% relative humidity, the amount of change in length of hair for a given increment of relative humidity will increase with time. Other lesser inaccuracies, some not necessarily restricted to operation in closed tents, are discussed.

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* Published with permission of the War Dept.