DYNAMICS OF CYCLONES AND ANTICYCLONES. 153 



of air would have equal and opposite velocities, there would be no tendency for the 

 centre of the cyclone to travel. But a little consideration will show us that the air to 

 the north-western side of anticyclones, in our area, will generally have a greater velocity 

 than the air on any of the other sides. The high-pressure areas which regulate the 

 movements of cyclones over our area are situated to the south-west of Spain, and over 

 Siberia and northern Asia. Now, the air descending from high elevations over these 

 areas has, in all probability, come from the upper general air circulation, and was 

 previously moving from the equator towards the north pole. It will therefore have a 

 greater easterly rate of motion than the surface of the earth where it descends, on 

 account of it coming from a lower latitude, and further, it will also have a northerly 

 motion. The result of this is, that the air descending from the upper parts of the 

 atmosphere, while it tends to move spirally outwards all round over the high-pressure 

 area, yet owing to it having a northerly motion and a greater easterly motion than the 

 surface of the earth where it descends, the air moving to the northwards and eastwards 

 will have a much greater velocity relatively to the earth than the air which moves south- 

 wards and westwards. Further, over Europe the high-pressure area to the north is 

 always much weaker than the one to the south, and is sometimes absent, so that the 

 cyclone is always more influenced by the anticyclone to the south than the one to the 

 north, and it thus receives its north-easterly motion from the strong winds on the north- 

 west side of the southern anticyclone. These remarks only explain in detail how cyclones 

 are affected by the general easterly drift of the atmosphere over our area. 



An examination of the weather charts, however, shows that if a cyclone receives its 

 strongest winds from any side of an anticyclone, it will move in the direction of these 

 winds. If, for instance, it receives its strongest indraught from an easterly direction 

 on the south side of the anticyclone, it will move to the west, or it may move 

 northwards along the west side of the anticyclone, or southwards along its east side, 

 but as the winds on these sides are generally feeble, the cyclonic movements are 

 generally slow. 



There is another point to which reference should be made here, as it has lately 

 assumed considerable importance. For long it was held that cyclones were due to a 

 a low-pressure area being formed by a rising column of hot moist air towards which the 

 surrounding air flowed in spirally ; while lately it has been contended that cyclones are 

 only secondary effects due to the interaction of air currents, and are, in fact, eddies in 

 the atmosphere formed by the general circulation of the earth's atmosphere. These 

 two theories thus attribute the energy of cyclones to two different causes. According to 

 the first theory the cyclone receives its energy from the hot and moist air; whilst 

 according to the other it gets its energy from the general air circulation. The one 

 theory we might call the convectional theory, the other the dynamical or driven 

 theory. The question then comes to be, which of these two theories seems to be the 

 most probable. To many it may seem difficult to imagine how the dynamical theory 

 ever originated. The diameter of cyclones is so very great it is difficult to imagine any 



