METEOROLOGY AND ATMOSPHERIC CHEMISTRY 



75 



The fraction of the sky covered by clouds exceeded 75, 

 50, and 25% of the time as shown in Fig. 3, which is 

 based on the tabulated three hourly frequencies of cloud 

 cover in tenths. 



The cloudiness is more variable in the dry season than 

 the wet; indeed, the daytime sky is covered Vio or more 

 75% of the time in July through October. More than 7,0 

 cover is common (^25%) in April through November, 

 and this frequency does not depend u|x>n the time of day. 

 The expected cloudiness (i.e., the 50% value) varies from 

 about Yio, higher in daylight and lower at night, in the dry 

 season to about '/lo, with somewhat less diurnal variation 

 in the wet season. The sky is seldom clear, even in the dry 

 season. Two-tenths of the sky is covered more than 75% 

 of the time (although this may be biased at night). 



A different measure of cloud cover is obtained from 

 satellite observations. Images obtained routinely from geo- 

 stationary platforms show the aerial extent and temporal 

 evolution of cloud systems. In addition, radiometric mea- 

 surements of the cloud top temperature yield good esti- 

 mates of the cloud top height. Accurate measurements 

 below obscuring layers of high clouds are not yet obtain- 

 able routinely. 



We are not aware of any published solar radiation data 

 for the Marshall Islands area, but there are data for certain 

 times in 1977 to 1979 (see "Sources of Additional Data"). 

 It is obvious that the typical partial cloudiness and the high 

 moisture content of the near-surface air significantly dimin- 

 ish the incident sunlight. Working against this, however. Is 

 the long (and unvarying) day. The interval between sunrise 

 and sunset varies from 11 hours 29 minutes to 12 hours 

 46 minutes. The available data show these effects, with 

 the average value exceeding 21 X 10^ j m~^ d~' in 



I 



z J*^' f'EB MAR APR MAY JUN 



2 ° 10 10 10 10 10 



•" '■ " I' M T I I, I 



01 ■ ' 



04 

 _l 07 



5 '° 



O 13 



—I 



16 



19 



22 



AUG 



SEP 



OCT 



NOV 



DEC 



Fig. 3 Cloud cover data for Enewetak. For each month the 

 three measures are the fraction of the sky obscured, in tenths, 

 at least 75% (left), 50%, and 25% (right) of the time for each 

 three hour period of the day. 



many months; this value corresponds to 500 cal cm~^ 

 d , a typical maximum total at mid-latitudes. Neverthe- 

 less, during disturbances the flux can be reduced for 

 periods of several days, and the value can drop below V^ 

 of this figure for a day or two at a time. 



Surface Wind 



The surface wind data are shown in Fig. 4 as wind 

 roses for each month (a) and for the entire year (b). As 

 indicated in the key in Fig. 4b, in each rose the bar indi- 

 cates the frequency of winds coming from that direction 

 for each range of speed above calm. The numerical values 

 beside each bar are the frequency, in percent, for wind 

 from that direction and for that range of speed. The fre- 

 quency of calms, to which no direction is assigned, is 

 shown in the center of the circle. The frequency of 

 occurrence of wind in each range of speed for all directions 

 is shown, in percent, in the line below each rose. The wind 



JAN 



42 



37 



1 1 



Fig. 4 Surface wind data for Enewetak. Wind roses for each 

 month (a) and for the year (b). Frequency of wind for each 

 directon and range of speed is shown by the printed figure 

 beside the bar. which shows the frequency of winds for that 

 direction for all speeds above calm. Frequency of calms is 

 shown in the center of the circle. Frequency of wind speeds 

 for all directions shown below each rose. 



(Fig. 4a cont'd on next page) 



