S On Travelling Atmospheric Disturbances. 



dimensions of an ordinary cyclone, as has already been 

 mentioned. Thus i£ the pressure were distributed in the 

 simple way here supposed, the velocity of translation would 

 be much greater than the general velocity of the winds 

 around the cyclone. This does not appear to be the case ; 

 for it would imply that the depression moved at such a rate 

 that no part of the surrounding air could keep pace with it, and 

 probably none of the air it itself contains. This contradicts 

 the fact of the existence of a tornndo centre *, or portion of 

 air within the cyclone moving with the cyclone as a whole. 

 The explanation seems to be that the actual effect of super- 

 posing a general pressure gradient on a mass of rotating air 

 is to cause a considerably smaller change in its symmetry 

 than would be expected, the reason being that some internal 

 compensation reduces the asymmetry. That this reduction 

 is not complete is, on the other hand, seen from the fact that 

 the cyclone does move, and usually does so in the direction 

 of the general gradient wind. 



Summary. 



The geostrophic relation between the wind and the surface 

 pressure gradient is incapable of accounting for any variation 

 whatever with time in the pressure distribution. All changes 

 in this arise from those terms in the equations of motion 

 that are neglected when the geostrophic relation is assumed. 

 When these terms, which depend on the squares and differ- 

 ential coefficients of the velocities, are taken into account, it 

 is round that an asymmetrical cyclone can move. It seems, 

 however, from the low speed of travel of these depressions, 

 that a simple superposition of a general pressure gradient on 

 a rotating system must be compensated internally in some 

 way, so as to reduce the asymmetry introduced. Thus the 

 remarkable circularity of the isobars in a cyclone is seen to 

 be a condition of its slow movement. It is indicated that 

 the cyclone itself is a very special type of disturbance, in 

 which the pressure, temperature, and velocity are so distri- 

 buted as to make the wave tending to readjust it travel with 

 extreme slowness; other types of disturbance spread out 

 much more rapidly (with velocities of the order of that of 

 sound) and are dissipated, and this fact is probably the 

 reason why of all the irregularities possible the cyclone is 

 the most conspicuous, other forms dissipating before they 

 can be observed. 



* Sir Napier Shaw, Geophysical Memoirs of the Meteorological Office, 

 No. 12. 



