Dew as a Source of Plant Moisture 
Luna B. Leopold ^ 
Records of the frequency of occurrence 
or of the total amounts of dew are scarce. 
This is surprising in view of the fact that dew 
has been used for water supply in certain 
Mediterranean countries for many centuries. 
The lack of records can undoubtedly be at- 
tributed to the difficulty of making quanti- 
tative measurements. 
Dew is the moisture which condenses from 
the atmosphere on plants, soil, or other sur- 
faces near the ground. Common experience 
tells us that dew forms primarily during the 
early morning hours when the temperature 
approaches its minimum diurnal value. It 
often forms in the early evening also, and 
in such cases probably continues to accumu- 
late slowly throughout the night. However, 
the rate of formation must decrease owing to 
the fact that the layer of air closest to the 
ground becomes saturated. The vapor pres- 
sure gradient at the immediate surface of 
ground becomes nearly zero and probably 
stays near zero because of lack of mixing in 
the lowest layers. 
Temperature in a standard instrument 
shelter may give little clue to the probability 
of dew formation at the ground surface. The 
temperature gradient increases so rapidly in 
the lowest few feet near the ground that 
temperature at grass level is considerably 
lower than at the height of the instrument 
shelter. Radiational cooling is, of course, the 
cause of the low temperatures at ground level. 
It follows that dew will be heaviest on sur- 
^ 7305 Gloster Road, Washington 16, D. C. Manu- 
script received December 18, 1951. 
faces which are the best radiators, that is, 
dark and opaque surfaces such as green 
grass^ For the same reason, dew will be 
heaviest when the wind is light or absent 
because the lack of mixing allows the tem- 
perature inversion near the ground to be 
maintained and to intensify. 
Only plants which are shallow rooted 
could make use of dew because the moisture 
penetrates only a thin layer of soil and evapor- 
ates quickly when the sun begins to warm 
the surface. Pineapple is an example of a 
plant whose trough-shaped leaves allow dew 
to run down to the plant base. This tends to 
concentrate the dew water near the roots, 
which, being shallow, can utilize the dew 
with some efficiency. Dr. Gordon Nightin- 
gale measured with a pipette appreciable 
amounts of dew water which collected at the 
axil of pineapple leaves (personal communi- 
cation). I have frequently observed the 
ground at the base of the pineapple stem to 
be noticeably moist as a result of dew col- 
lected by the leaves. The shade cast on the 
plant base by the leaf mass enhances the 
probability of intake by roots inasmuch as 
the shade reduces the evaporation rate. 
My own experience in the semiarid south- 
western United States supports the view that 
dew is sufficiently frequent to constitute, in 
all probability, an important source of mois- 
ture for some plants. Measurements of the 
frequency of dew in Hawaii tend to confirm 
this conjecture. Because data on dew fre- 
quency are uncommon, the record of measure- 
ment made during 2 years in Honolulu is of 
some interest. 
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