A popular formula of evaporation suggested by Trabert is of the form
in which
Q is the depth of the evaporated water expressed in mm., E the maximum vapour tension at the air temperature,
e the actual tension,
W the wind velocity in metre per second,
P the observed pressure,
T the temperature in absolute unit and the suffix represents the standard value.
We examined the variation of the coefficient
C with the data during 6 months in 1921, obtained at the Central Meteorological Observatory in Tôkyô, and the remarkable parallelism, the positive under free exposure and the negative in shade, was found between the monthly me_??_n of
C and the monthly mean air temperature (
t). Of which, a linear relation
C=C' (1-
kt),
k: constant, was satisfactorily assumed, especially in the case of evaporation in shade. As to the evaporation under free exposure, however, this relation could not be found in the rainy season, May and June. But generally these
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