AIR CURRENTS IN RELATION TO AVIATION 349 



A diagram (10) representing actual winds in relation to 

 gradient winds will make clear the advantage of the gradient 

 wind as a standard of reference. A circle (fig. i) is taken to 

 represent by its radii the velocity of the gradient wind for any 

 point of the compass for the neighbourhood of Falmouth. 

 Corresponding thereto is represented also, by radii for the 

 several directions, the velocity of the actual wind as recorded at 

 the Falmouth Observatory and the actual velocity given by an 

 anemometer at Pendennis Castle, about two miles away. The 

 differences between the two records are due simply to exposure ; 

 it will be seen that they are very large. The Pendennis 

 exposure is much better than the Observatory exposure, that is 



Fig. i. — Velocity of the surface winds at Falmouth and Pendennis Castle in 

 relation to the velocity of the gradient wind. 



to say it gives a closer approximation to the gradient wind but 

 it is not perfect ; for westerly winds, the observed velocity is 

 only about half the gradient velocity. 



The determination of the barometric gradient, from which 

 the gradient velocity is computed, requires a map or at least 

 readings of the barometer at three points of a triangle surround- 

 ing the station. For the calculation of the gradient wind a 

 table of wind velocities corresponding with distances apart of 

 isobars on the map in nautical miles is the most convenient 

 plan. Such a table is given in a Memorandum on Wind Struc- 

 ture published in the Report of the Advisory Committee for 

 Aeronautics, 1910 (3). 



Since measurements at several stations are required, the 



