where x^ is the monthly mean of observations at a given hour, j, and 

 X is the mean of x-: . Although the data were taken up to six times 

 daily, only the observations made at 0800, 1200, 1600, and 2000 hours 

 were used in order to reduce problems with night observations. Daylight 

 was probably sufficient to make accurate observations at 2000 hours 

 during the summer months. 



Because the wave height is estimated only to the nearest foot by the 

 observers, the accuracy of changes in averages expressed to the tenths 

 or hundredths of a foot may be suspected. In general, the standard devia- 

 tions of the Xj are significantly less than 1.0 foot. To make the 

 averages more meaningful, only those stations were selected where the Xj 

 was the average of at least 100 observations. There were 17 COSOP star 

 tions that met this condition, and only the averages from these stations 

 are discussed in this study. The absolute diurnal variation is defined 

 as : 



Ax. = X. - X. . (2) 



] J max jmm 



where Xj^^^^ is maximum value of xj for the given month and given station, 

 ■'^imin ^^ minimum value of x^ for the given month and given station. 

 From equations (1) and (2), the relative diurnal variation is defined as: 



AD = Ax./x . (3) 



Equations [2) and (3) both measure the importance of the diurnal effect. 

 Equation (2) is the maximum within-day variation shown by the monthly 

 averages for the four observation times of this study. Equation (3) 

 expresses this variation in dimensionless terms, relative to the average. 

 For example, if the monthly averages at 0800, 1200, 1600, and 2000 hours 

 were, respectively, 2.2, 2.5, 2.6, and 2.3 feet, then 



X. =2.6 feet (j = 1600 hours), 

 J max 



X. . =2.2 feet (j = 0800 hours), 

 jmm -^ 



X = 2.4 feet (average of all four), 

 Ax. = 0.4 foot (from eq. 2) , 

 AD = 0.17 (from eq. 3) . 



Figure 2 shows AD averaged by region for the 10-year period of this 

 study. Generally, there was a larger range of AD values during the 

 early summer months ;. Pacific stations showed a maximum in spring; gulf 

 stations showed a maximum in summer. All stations had a minimum in AD 

 values during the winter months. The summer months show more variation 

 in wave height probably because sea breeze generally occurs more fre- 

 quently in summer than at other times of the year, and because the mean 

 values of wave height (x) are lower in summer. 



8 



