260 
PACIFIC SCIENCE, VoL VI, July, 1952 
The record was made in Kahala, a section 
of the city of Honolulu just east of Diamond 
Head, where the mean annual rainfall is ap- 
proximately 20 inches. Every morning about 
7 o’clock I walked across the lawn to read the 
rain gauge and at that time observed the rela- 
tive amount of dew which collected on my 
shoes. The procedure has more an air of 
levity than of science but such a record is 
better than none. It was observed that this 
daily procedure provided enough experience 
to allow a consistent estimate of the relative 
amount of dew. The amount was recorded as 
one of four categories — none, light, moderate, 
or heavy. On days of rain, no record of dew 
was made because one could not tell whether 
the moisture was derived from dew or from 
precipitation. 
Deducting from each month the number 
of days of rain or no record, the frequency of 
dew can be expressed as percentage of days. 
This represents, in other words, the percent- 
age of possible days on which dew could 
occur in the absence of rain. The number of 
days, by months, is presented in Table 1. 
The annual march of dew occurrence is shown 
graphically in Figure 1 . 
TABLE 1 
Frequency of Occurrence of Dew at Honolulu 
DURING THE PERIOD OF OCTOBER, 1947, TO JULY, 1949 
MONTH 
TOTAL DAYS 
ON WHICH 
DEW WAS 
OBSERVED 
TOTAL 
DAYS OF 
NO RAIN 
PER- 
CENTAGE 
RAINLESS 
DAYS 
January 
23 
35 
66 
February 
27 
45 
60 
March 
16 
45 
35 
April 
12 
42 
29 
May 
19 
52 
37 
June 
12 
45 
27 
July 
2 
15 
13 
August 
13 
52 
25 
September. . . . 
15 
53 
28 
October 
25 
48 
52 
November. . . . 
27 
43 
64 
December 
7 
23 
30 
Total 
198 
498 
40 
Fig. 1. Frequency of dew at Honolulu (as per- 
centage of rainless days). 
Despite the rough manner of collecting 
the data, the relatively smooth annual march 
lends credence to the record. The December 
percentage is low. This can probably be at- 
tributed to the fact that one of the two 
Decembers of record was relatively windy as 
indicated by a concurrent anemometer record. 
For July only one-half month of record is 
available. 
Dew occurred on 198 days, which repre- 
sented 40 per cent of the total rainless days. 
At the same place and in the same period, 
there were 217 days of some rain. Traces of 
rain were not counted as rain days. Of the 217 
rain days, only 89 had amounts greater than 
0.1 inch. 
On many days of heavy dew, enough col- 
lected on the funnel of the rain gauge to drip 
into the collection tube where it could be 
measured as .01 inch. Assuming that the 
average amount of dew was .0075 inch in a 
day, which is indeed a guess but probably of 
the right magnitude, the dew contributed 
0.8 inch per year. Let us compare this with 
the moisture contributed by light rains or 
showers. The total rain contribution of those 
days on which .01 to .09 inch of rain oc- 
curred was 2,6 inches per year. Of the total 
annual rainfall, the dew moisture would con- 
stitute about 4 per cent, while rain in the 
category .01-. 09 inch in a day would con- 
stitute 13 per cent of the total. 
The frequency of dew at Honolulu is 
greatest in winter and least in summer. Mix- 
ing ratio (grams of water vapor per gram of 
dry air) has a small annual variation with a 
