212 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. SI 



? 



§29. Ocean wind and land wind 



We consider the ocean winds and the land winds as variable sys- 

 tems of parallel winds of the second order. The ocean winds be- 

 long to an ascending system of parallels and the land winds to a 



descending system of 

 p' parallels. During the 



day the land becomes 

 much warmer than the 

 sea and consequently the 

 pressure p at the upper 

 Po end of the column of air 



(see fig. 30) increases and 

 exceeds the pressure p'\ 

 fig. 30 the air at the upper end 



leaves the column and at 

 the same time the pressure p diminishes, because the weight of 

 the column diminishes, and thus produces a horizontal current which 

 is the ocean wind. Approximately we can neglect the time neces- 

 sary to fill the column of air from the ocean and Ave can consider the 

 depression p ' — p as a function of the temperature. 



During the night the land becomes much cooler than the ocean, 

 the pressure p diminishes at the same time that the weight of the 

 column of air increases and a descending system of parallels obtains 

 with a depression p — p '. 



The barometric depression which depends on the unequal heat- 

 ing of the ocean and the land is a function of the time and of the 

 place, and must be determined by observations. This depression 

 produces a horizontal current which commences with a velocity 

 equal to zero; the depression gradually increases, the velocity in- 

 creases and the current extends more and more up to the moment 

 when the depression attains its maximum value. Then the depres- 

 sion and the velocity of the current decrease simultaneously up to 

 the moment when the current ceases. 



Consider the horizontal current at any time and denote its max- 

 imum velocity which occurs near the coast, by U , its length along 

 the gradient by x, and the depression in millimeters by D ; it is 

 evident that D is a function of U , of x and of the time. 

 We approximately assume 



10 333 

 760 



D = p U > 



