148 The Realm of Nature CHAP. 



a strength proportional to the gradient. The circulation of 

 winds in a cyclone is following Ferrel's or Buys Ballot's 

 law in the direction opposite to the movement of watch- 

 hands in the northern hemisphere, and in the same direc- 

 tion as the hands of a watch in the southern. As the 

 centre of a cyclone remains at the lowest pressure in spite 

 of surface winds pouring in from every side, the air must 

 rise in the centre and flow out above. A cyclone is thus 

 an inward and upward whirl or eddy of the atmosphere. 

 The air above has its normal pressure raised by the upflow, 

 and consequently gives rise to outward-flowing upper winds ; 

 the cyclone in fact is necessarily crowned by an anticyclone. 

 The cyclone is not a stationary eddy, such as is represented 

 by the low-pressure areas on the charts of average atmo- 

 spheric conditions, but is always moving. In the northern 

 hemisphere the direction of its motion is westward near 

 the equator, gradually turning to the right and becoming 

 north-eastward in high latitudes. In the southern hemi- 

 sphere it is westward near the equator, turning toward the 

 left and becoming south-eastward in high latitudes. The 

 rate of motion of the centre is from 15 to 30 miles an hour 

 in the temperate zones, but only from 3 to 8 miles an hour 

 in the tropics. The actual particles of air do not move 

 forward, but the diminution of pressure is passed on like 

 a wave ( 57) through the air. Cyclones usually form 

 on the edge of the permanent regions of high pressure, 

 and travel away along their margins. Professor Hann, the 

 great German meteorologist, has concluded that the cyclones 

 and anticyclones of the temperate zone are true eddies and 

 bankings-up formed in the great streams of air which set 

 poleward from the equator. 3 



207. Cyclonic Weather. There are certain changes 

 of weather associated with a cyclone which result from the 

 fact that it is an eddy of ascending surface air. The air 

 on rising near the centre is cooled by expansion, and the 

 vapour condenses into cloud, and ultimately falls as rain. 

 Hence, when the cyclone is approaching an observer and 

 condensation has just begun to take place in the upper 

 regions, a halo produced by reflection from the condensed 



