46 
of the data; for each rain day selected there has been 
chosen a fair day within the same region and with sim- 
ilar air temperature. 
The results give a mean value of 21.20 mm for rain 
days, and one of 19.19 mm for fair days, showing that 
precipitation has a significant effect on vapor pressure. 
Variation of Vapor Pressure 
with Air Temperature 
It has been shown that the quantity of water vapor on 
rain days depends to a considerable extent on precipita- 
tion; on rainless days it must depend largely on airtem- 
perature. The curve shown in figure 34 was construct- 
ed using vapor-pressure and air-temperature data for 
approximately half the days of the cruise (150 days), 
and represents vapor pressure plotted as a function of 
air temperature. As we should expect, the profile of 
the curve is similar to that of the saturation curve of 
vapor pressure. It appears to depart more widely from 
the saturation curve at the intermediate temperatures 
(15° to 25°). This is not surprising when it is consid- 
ered that these temperatures were obtained largely 
within the subtropical belts, where humidities are low 
with respect to air temperatures. 
The curve in figure 34 may be quite closely repre- 
sented by the empirical equation: 
e = 0.03t2 - 0.27t + 7.6 
where e is vapor pressure expressed in millimeters of 
mercury, and t is air temperature (3.6 meters above 
the sea surface) expressed in degrees centigrade. 
Vapor pressure was not plotted against sea-surface 
METEOROLOGICAL RESULTS OF LAST CRUISE OF CARNEGIE 
temperature since it seems obvious that a similar 
curve would be represented. 
DISCUSSION OF RELATIVE HUMIDITY 
Mean Relative Humidities for Groups 
The mean hourly values of relative humidity for the 
various Groups are presented in table 54. It appears 
that the values are highest in the Groups in equatorial 
regions and in higher latitudes, and lowest in the Groups 
in the subtropical belts. This result is what would be 
expected when it is considered that relative humidity is 
a function of both vapor pressure (specific humidity) and 
air temperature. Thus, whereas vapor pressure depends 
to a great extent on air temperature, relative humidity 
is a function of the differences between these two ele- 
ments. These differences appear to be greatest within 
the subtropical or trade-wind belts, where air moving 
toward the equator is being rapidly heated but is in- 
creasing its moisture content very slowly. 
Maxima and Minima of Relative Humidity 
The absolute maximum and minimum relative hu- 
midities for the various Groups are presented in table 
55. As has just been indicated, the extreme maximum 
values are found in the higher latitudes and in equatorial 
regions. Relative humidities of 100 per cent were re- 
corded on four days during the cruise for a total of six 
hours. These all occurred during July 1929, in the Alas- 
kan Peninsula Group. 
The absolute minimum relative humidity (53 per 
cent) was recorded at 16h, November 14, 1929, in lati- 
Table 54. Mean hourly values of relative humidity inper cent for groups, Carnegie, 1928-29 
(Corrected for noncyclic change) 
No. | ~~ Means ._ Local mean hours 
Group Dates 
a days [Latitude [Longitude | 0 | 1 | 2 
1928 C 
I July 29-Aug. 6 9 56.3N 40.7 W 86.8 87.4 86.2 
ll Aug. 7-10 4 42.8N 47.8 W 80.1 80.6 80.1 
Il Aug. 11-23 13 29.0N 42.0 W 82.4 82.2 81.9 
IV Aug. 24-Sep. 152 21 11.8N 43.0 W 81.2 81.3 81.6 
Vv Oct. 2-10 9 13.8N 71.0 W 80.0 79.8 80.6 
aNd Oct. 26-Nov. 6 12 4.0N 81.0 W 86.5 86.8 86.7 
(a) Nov. 7-Dec. 21) 35 16.58 104.3 W 81.4 81.6 82.0 
b Feb. 22-28, 1929 7 1351S 119.4 W 74.7 74.5 74.9 
Vin Dec. a c 8 S21S 96.7 W 87.9 88.6 88.1 
Ix Jan 1-14 14 24.78 83.3 W 77.4 77.0 75.6 
x Feb. 6-17 12 12I3IS 88.2 W 78.4 79.1 78.9 
XI Mar. 1-314 21 16.88 147.9 W 76.8 17.4 17.3 
a Apr. 22-May 31€ 32 9.7N 168.7 E 81.0 80.4 80.1 
(a) June 1-30! 13 34.3.N 143.1 E 88.0 88.7 88.2 
(b) July 1-3 3 39.6 N 149.4E 81.5 81.2 80.6 
XIV July 4-218 19 47.7 N 179.5 W 95.3 95.5 96.1 
XV July 22-28 7 41.5N 131.8 W 86.0 84.9 85.3 
XVI Sep. 4-8 5 34.1N 126.3 W 81.2 80.8 78.7 
XVII 
(a Sep. 9-16 8 27.8N 136.6 W ThlAl 70.6 71.0 
tb Sep. 17-Oct. 74 8 27.0 N 155.1 W 76.0 76.5 75.0 
c Oct. 11-251 14 25.2 N 140.7 W 80.4 80.8 81.1 
XVIII Oct. 26-Nov. 14 20 0.18 150.5 W 79.3 79.3 79.4 

