232 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



Chapter VIII 



SYSTEMS OF WIND IN NATURE 



§34. Influence of the surface of the earth 



In nature the systems of wind show many deviations from the 

 ideal systems that we have considered. It is especially the sur- 

 face of the earth with its irregularities that produces the greatest 

 disturbances. To take an extreme case, let us consider a valley, 

 that is to say an uncovered channel in the crust of the earth, it is 

 evident that the wind follows the direction of the channel, whatever 

 may be the direction of the gradient in the strata above the vallej^. 



At meteorological stations situated at the surface of the 

 earth, we must expect that the angle <p between the gradient and 

 the direction of the wind will generally differ from the theoreti- 

 cal angle, because the value of the friction depends on the irregu- 

 larities of the surrounding land. We must therefore add a local 

 correction J i[> which is determined by observations. We believe 

 that the determination of this correction is of great importance 

 for the prediction of the movement of systems of wind. 



When a system of wind extends over a great part of the surface 

 of the earth, the variation of the latitude produces disturbances of 

 the normal angle, and a disturbance of this angle acts also on the 

 system of isobars. 



When the surface of the earth over which the system of wind 

 occurs offers irregularities, the coefficient of friction varies. The 

 variation of the coefficient of friction from one point to another 

 produces disturbances of the angle between the gradient and the 

 wind and consequently a deformation of the system of isobars. 



Example. Let us consider a cyclone of which one half is over 

 the land and the other half over the ocean. The equation of con- 

 tinuity is independent of the coefficient of the friction and of the 

 latitude, and for the exterior portion where the current is considered 

 as horizontal it is necessary to have 



U x r x cos a x = U 2 r 2 cos cc 2 



Assume that the radius of the ascending part be 7°and calculate 

 the different curves as we have shown them in §12 and § 14; and 

 we shall find for the two portions of the cyclone; 



