XXX INTRODUCTION. 



In the Quarterly Journal of the Royal Meteorological Society, volume 

 XXX, No. 132, October, 1904, Prof. A. Lawrence Rotch has described an 

 instrument for obtaining the true direction and velocity of the wind at sea 

 aboard a moving vessel. If a line A B represents the wind due to the motion 

 of a steamer in an opposite direction, and A C the direction of the wind rela- 

 tive to the vessel as shown by the drift of its smoke, then, by measuring the 

 angle DBA that the true wind makes with the vessel — which is easily done 

 by watching the w^ave crests as they approach it — we obtain the third side, 

 B C, of the triangle. This represents, in direction and also in length, on the 

 scale used in setting off the speed of the ship, the true direction of the wind 

 relative to the vessel and also its true velocity. The method fails when the 

 wind direction coincides with the ship's course and becomes inaccurate when 

 the angle between them is small. 



GRADIENT WINDS. 



When the motions of the atmosphere attain a state of complete equilib- 

 rium of flow under definite systems of pressure gradients, the winds blow 

 across the isobars at small angles of inclination depending upon the retard- 

 ing effects of friction. At the surface of the earth friction is considerable and 

 the angle across the isobars is often great. In the free air, however, the 

 friction is small, and for some purposes may be disregarded entirely. Un- 

 der an assumption of complete equilibrium of motion and f rictionless flow the 

 winds will blow exactly parallel to the isobars — that is, perpendicular to the 

 gradient which produces and sustains the motion. Such winds are called 

 gradient winds. The anomalous condition of flow of terrestrial winds per- 

 pendicular to the moving force is the result of the modifications of atmospheric 

 motions due to the deflective influence of the earth's rotation, and to that other 

 influence due to the inertia reaction of matter when it is constrained to move 

 in a curved path, and commonly called centrifugal force. The equations for 

 gradient wind motions have long been known to meteorologists from the work 

 of Ferrel and others, and may be written in the following form : 



For Cyclones 



F = r V co2 sin2 (p + - CO sin </) 



For Anticyclones 



CO sin (^ — \ 



AP 



V = r\ CO sin (^ — 'V '•^^ sin- </> 



pr J 



(I) 



(.2) 



In C. G. S. Units, F = velocity of the gradient wind in centimeters per 

 second; r= radius of curvature of isobars in centimeters; AP = pressure 

 gradient in dynes per square centimeter per centimeter; p= density of air 

 in grams per cubic centimeter; w = angular velocity of the earth's rotation 



