301 
Marsh , and its Relations to Evaporation , &c. 
of the vegetation subjected, during the summer months, to far greater 
humidity variations than the lower, but also to a considerably greater 
diurnal range of temperature. 
Effect of Temperature, etc., on Evaporation. 
We are now in a position to inquire into the effect of the various 
meteorological factors on evaporation. This effect can be very well seen 
by reference to Tables I and II, and the curves in Text-figs. 5, 7 and 8. 
But the more important factors may be referred to individually. 
(1) Temperature. The curves show at a glance the connexion 
between evaporation and temperature. In a general way, the evaporation 
rises and falls, pari passu with the rise of temperature by day and its fall by 
night. 1 But that temperature is not the only factor concerned is shown by 
the fact that the highest temperature is recorded at B, but the maximum 
evaporation at A. Again, the curves of evaporation diverge much more 
widely during the daytime than do the temperature curves (Cf. Text-figs. 
7 and 8). 
(2) Relative Humidity is, after all, the most important factor ; 
for most of the others act by influencing this. Relative humidity and 
temperature are intimately connected, the former reaching a maximum 
during the coldest, and a minimum during the warmest, hours of the day. 2 
High evaporation with low relative humidity (in the morning) was recorded 
on July 11 and August 15, 1907, and from July 31 to August 4, 1908 (see 
Tables I and II). 
(3) Wind. The effect of wind is well seen on August 1 5, 1907 (Text- 
fig. 5)* The breezes, though fairly light, materially assisted the evaporation 
from July 31 to August 3, 1908. Thus, on August 3, the early morning 
was calm, and, in spite of the high temperature, the evaporation between 
8 and 10 a.m. was lower than on several of the preceding days. A light 
breeze sprang up soon after 10 a.m., and increased in strength till about 
1 p.m. The effect of this breeze on the rate of evaporation was at once 
apparent (see Text-fig. 7). 3 
In general (though they can, of course, only act by influencing the local 
relative humidity), it can scarcely be doubted that air-currents are the chief 
factor in determining the high rate of evaporation in the exposed position A. 
Conversely, the stagnation of the air in the lower layers of the vegetation, 
and its consequent high relative humidity, even on dry days, no doubt 
account to a large extent for the flatness of the C evaporation curve. 4 
1 Cf. Livingston (’ 06 ), p. 32, and the curves on p. 30. 
2 Sprung (’ 85 ), p. 352. 
3 Cf. also Livingston, 1 . c., pp. 32 and 33. 
4 Of course much of the actual air-moisture in the vegetation is that transpired by the plants ; 
but some, no doubt, is due to direct evaporation from the damp soil. 
