WIND TABLES. XXV11 



Example : 



tan a = ^-^. 



Table 41 gives 90 - a = 32 



a = S 58 W. 



Note. — If the numerator and denominator both exceed 150 or if either exceeds 200, 

 the fraction must be divided by some number which will bring them within the limits of 



the table. The larger the values, provided they are within these limits, the easier and more 



.0 



accurate will be the computation. For example, let tan a = . The top argument is 



14 



not given for 18, but if we multiply by 5 or 10 and obtain or , the table gives, 



70 140 



without interpolation, 90 — a = 38° and a = N 52 W. 



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 frictionless 

 flow the winds will blow exactly parallel to the isobars, — that is, perpen- 

 dicular to the gradient which produces and sustains the motion. Such 

 winds are called gradient winds. The anomalous condition of flow of terres- 

 trial winds perpendicular to the moving force is the result of* the modifica- 

 tions 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 cen- 

 trifugal 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 



V = r 

 For Anticyclones 



V = r 



co 2 sin 2 d> -f — co sin (6 (1) 



pr J 



• j. J 2 • t* AP 



co sin cf> — \ co- sin- 4> 



pr J 



(2) 



In C. G. S. Units, V = 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; co = angular velocity of the earth's rotation 



