and with most rational modifications thereof, a hurricane (or similar 

 disturbance), once started, will continue to exist as long as sufficient 

 water vapour is available (unless it is disrupted by a superior mechanical 

 disturbance) ; the intensity of the hurricane being an approximate 

 function of the specific humidity of the available air. 



Because a tropical cyclone, both theoretically and actually, " mines 

 out " the water vapour in a given area in a relatively short time, and, bj' 

 shading the same area, effectively inhibits evaporation there, a stationary 

 tropical cyclone, lasting for more than a few hours, is not only practically 

 unheard of, but is physically improbable under conditions normally 

 found in Nature. In consequence, a lasting tropical cyclone, or hurricane, 

 must have a trajectory. 



Any object on earth, once started in motion parallel to the surface, 

 will tend to pursue a great circle trajectory unless and until it is acted 

 upon by other forces. Because of the rotation of the earth, any object 

 moving away from the Equator, and parallel to the surface, is deflected 

 eastward. This effect of terrestrial rotation, commonly described as the 

 " deflecting force " or " Coriolis Force," increases in magnitude, for a 

 given latitudinal motion, as the latitude increases. In the area under 

 consideration this " deflecting force " becomes more important north- 

 ward. 



Partially opposing this deflecting force are the trade winds, which 

 blow from north-east to south-west (in the Northern Hemisphere) between 

 the Horse Latitudes and the Doldrums, and have maximum intensity, 

 during the time of hurricane maximum, in the area concerned, between 

 latitudes 15° N. and 20° N., becoming definitely weaker and less constant 

 as the subtropical belt of atmospheric subsidence (the Horse Latitudes, 

 about 30° N.) is approached. 



Computation of hypothetical hurricane trajectories, considering only 

 the deflecting force due to terrestrial rotation and the effects of the 

 north-east trade winds, shows clearly that a recurved trajectory, starting 

 toward the north-west, and terminating in a north-easterly tangent, is to 

 be expected. Such hurricane trajectories are standard in this area 



(Fig. 1). 



Theory and observations are in such close accord for the first part of 

 most hurricane trajectories that influences other than the trade winds and 

 terrestrial rotation are of minor magnitude here. However, neither 

 terrestrial rotation nor trade wind influence will explain the sharp land- 

 ward kinking of many hurricane trajectories, nor will they account for the 

 apparent sudden dissipation of other hurricanes at sea, usually north-west 

 of the Revilla Gigedo Islands. 



Reference to the pattern of winds, pressures, and ocean currents over 

 and in the north-eastern Pacific during the late spring, summer, and early 

 fall discloses that conditions north-west of Todos Santos (Fig. 3) become 

 increasingly unfavourable to hurricane travel in that direction. A 

 hurricane travelling far offshore in this area encounters opposing winds, 

 which are usually relatively dry, and tend to be or become convectionally 

 stable from contact with a cool sea. In consequence, not only is hurricane 

 travel north-westward opposed mechanically, but the amount of water 

 vapour available for condensation decreases markedly north-westward. 



Hurricanes travelling close to shore encounter the cold California 

 current not far north of lat. 25°, and here, almost literally, they " run out 

 of steam " and dissipate. 



26 



