CYCLONES 31 



Cyclones 



AYhen the barometric pressure over a raore or less circular 

 area of sea or land is less than the pressure over the surround- 

 ing sea or land, the pressure diminishing from the outside 

 towards the centre, and when the winds rotate about the 

 centre, the whole mechanism is called a cyclone ; and in 

 the northern hemisphere the motion of rotation is opposite 

 to that of the hands of a watch. 



The above definition includes tornadoes, or violent whirl- 

 winds, where the diameter of the disturbed area is compara- 

 tively very small ; but as we have no tornadoes in Jamaica, 

 we must confine our attention to cyclones properly so called, 

 with diameters of 100 or 1000 miles. 



At the centre of a cyclone there is a calm area generally 

 5 or 10 miles in diameter ; and while the winds rotate round 

 this calm area they are somewhat drawn in towards it ; and 

 the following is the rule to find approximately the direction in 

 which the centre lies : stand with your back to the wind, and 

 the centre will lie in a direction between your left hand and 

 your face. The rule respecting upper currents was given at 

 the end of the last section on clouds and cloud-drift. 



The whole cyclone may be stationary, or it may move on 

 its course over sea and land with a velocity more or less 

 uniform, seldom exceeding 10 or 15 miles per hour in the 

 West Indies. 



The fall of pressure at the calm centre may be very 

 small say one-tenth of an inch and then the rotating 

 winds will be very gentle ; or it may be large say one 

 inch and then the rotating winds may be violent, especially 

 towards the calm centre ; but the strength of the wind really 

 depends on the barometric gradient, or the fall in pressure 

 per mile of approach towards the centre. 



It will be convenient to call the former cyclonic depressions, 

 and to restrict the term " cyclones " to storms dangerous to life 

 and property.* 



The general principles given above apply to cyclones all 



* OtherwiBe called " liurricanes," the term applied to them before their 

 cyclonic nature was known. 



