72 



MERRILL AND DUCE 



CLIMATE AND WEATHER OF 

 ENEWETAK ATOLL 



Temperature and Humidity 



It is obvious that high surface temperature and hu- 

 midity are to be expected on tropical islands. It is less 

 obvious, but well documented, that it is difficult to obtain 

 accurate temperature measurements in an op)erational pro- 

 gram in such environments because of such factors as radi- 

 ational heating of the shelter in daylight. Thus it is likely 

 that the air temperature range rep>orted below is exag- 

 gerated by about 0.5°C (see Lavoie, 1963, for a discus- 

 sion). This is a small enough error for temperature, but it 

 significantly degrades the accuracy of the relative humidity. 

 Nevertheless, we can see that there is relatively little 

 change in these quantities through the year and that a reg- 

 ular diurnal cycle is evident. The temperature and hu- 

 midity both respond noticeably and regularly to rain 

 showers, but in the data presented here the nearly random 

 occurrence of rain with time has smoothed out this effect; 

 in fact, even hourly data do not show the full effect of 

 short-lived rain events. 



The temperature and humidity data shown in Fig. la-c 

 are from the U. S. Air Force measurements now archived 

 by the National Climatic Center. Made at hourly intervals 

 between 1945 and 1969 (with irregular breaks), the obser- 

 vations correspond to 14.1 years of uninterrupted mea- 

 surement. This record is sufficiently long that the overall 

 pattern and its variability can be perceived. The data aver- 

 aged over 3-hour periods are displayed as a function of the 

 hour and of the month; the draft plot is extended beyond 

 the borders shown so that edge effects are minimized. 



That the temperature depends very little upon the time 

 of year can be seen in the mostly horizontal contours 

 shown in Fig. la. Also note that the highest daily tempera- 

 ture is recorded between 12 and 15 hours local standard 

 time and lies between 28.5 and 30°C; the lower 

 temperature is observed in the dry season and the higher 

 during the wet season. In the morning and in the evening, 

 the temperature depends even less upon the time of year, 

 with values increasing and decreasing daily through the 

 upper 20s. In the hours after midnight, the decrease of 

 temperature slows in the dry season and ceases in the wet 

 season, with the lowest average value reaching 26 to 

 27°C. 



Now these are monthly and 3-hour averages over years 

 of data, and even though these patterns are generally 

 valid, there are fluctuations. The representativeness of this 

 pattern can be seen in the small variances: just over 0.5°C 

 at night to a maximum of under 1.5°C in the afternoon in 

 October. (These are variances of hourly data averaged 

 over 3-hour periods for each month, i.e., variances about 

 the mean shown in Fig. la.) This method of averaging 

 does not accurately record the average maximum and 

 average mmimum hourly temperatures for each day. These 

 are given in Table 1 for each month of the year. The 

 range of temperature is greater here, as expected. The 



average minimum temperature for each month is nearly 

 independent of month at about 23°C, whereas the average 

 maximum exceeds 32°C in August and September and is 

 30°C during the dry season. 



(c) 



M J J A 



MONTH 



Fig. 1 Temperature and humidity data for Enewetak. (a) 

 Dry bulb temperature, °C; (b) Relative humidity in percent: (c) 

 Dew point tempicrature, °C. These are three hour averages for 

 each month. Contour interval is 0.5°C for temperatures. 2.5% 

 for humidity. 



