Dew as Plant Moisture — Leopold 
261 
maximum in late summer. The seasonal vari- 
ation of temperature is also small. The mean 
temperature at Honolulu varies from about 
71°F. in February to 78° in August (Palmer, 
1950 ). The modest decrease of temperature 
during the winter months overcompensates 
for the winter minimum of moisture and en- 
hances the possibility of reaching the dew 
point by radiational cooling at the ground 
surface. The annual march of dew frequency 
thus appears directly related to the annual 
march of minimum daily temperature. 
The importance of dew lies not in the total 
quantity but in the frequency of occurrence. 
At the Kahala station there were nearly as 
many days of dew as days of rain. Because of 
the frequent showers in the trade-wind belt, 
the number of days of rain is unusually high 
relative to the annual rainfall. 
Recent work has shown that radiational 
cooling of the ground surface causes moisture 
to condense within the pore spaces of the 
surface layer of soil. This moisture condenses 
out of the air which is in the soil pores. It has 
been demonstrated that this moisture is suf- 
ficient to affect soil moisture of road sub- 
grades (Winterkorn, 1944), and it is possible 
that the same process also can provide water 
to plants. 
Consider these facts in relation to the grass- 
lands of the world. Grass or desert shrub 
characterizes a very large geographic area 
where the annual rainfall is roughly between 
6 and 14 inches. Such areas are, by their 
meteorologic setting, characterized by a large 
number of days of no rainfall, and most of the 
annual rainfall occurs in relatively few days. 
It is reasonable to suppose, then, that in such 
an ecologic setting any factor which doubles 
the frequency of moisture availability, even 
though the moisture amounts be small, must 
materially affect the growing conditions of 
plants. It is suggested, therefore, that the oc- 
currence of dew is a factor of some import- 
ance in sustaining the plant associations in 
grasslands and some xerophytic habitats. 
REFERENCES 
Palmer, H. S. 1950. Annual march of daily 
mean temperatures at Honolulu. Pacific 
Sci. 4 (1): 50-54. 
Winterkorn, H. F. 1944. Climate and high- 
ways. Amer. Geophys. Union, Trans. 1944. 
Pt. Ill: 405-411. 
