METEOROLOGY 9 



Exner which adversely criticises this theory, in so far as it 

 postulates a continuous impenetrable line separating the warm 

 westerly winds of middle latitudes from the cold circumpolar 

 circulation of easterly winds in the northern hemisphere. 

 He points out that the idea of a polar front is not new, but that 

 it has never before been so strongly insisted upon in meteoro- 

 logical circles as recently by Professor V. Bjerknes. Margules, 

 in his well-known work on the energy of storms, has deduced 

 the conditions required for stability at the surface of separation 

 of warm and cold currents. Bjerknes takes the stability for 

 granted in the case of the cold and warm winds mentioned 

 above, and suggests that all cyclones are merely waves at the 

 surface of separation of the two. This very attractive theory 

 is rejected by Exner, who points out that the changes which 

 take place in a depression are not reversible, and therefore 

 cannot be part of a wave motion. In particular Margules has 

 shown that the winds in cyclonic depressions can only have 

 their high velocity explained as a result of the vertical displace- 

 ment of masses of air of unequal density, in the course of which 

 the centre of gravity of the system is lowered. This is an 

 irreversible process. Moreover, friction at the earth's surface, 

 mixing, and eddy-motion are also irreversible. A depression 

 cannot therefore be part of a reversible oscillation. Looking 

 at the matter from yet another point of view, the motive power 

 of the atmospheric movements being due to unequal heating 

 at the poles and the equator, there must be a circulation of 

 air between middle latitudes and the polar basin ; we there- 

 fore cannot suppose that the cold polar air is enclosed, as it were, 

 with an impenetrable membrane. Exner regards the " cold 

 burst " behind the depression as a " break-through " of polar 

 air. He had some years previously advanced the view that this 

 phenomenon is due to the checking of the easterly wind at a 

 particular point in the circumpolar circulation, which accords 

 with Helmholtz's views on the subject. 



The Louth " Cloudburst " of May 29, 1920. — ^The disastrous 

 floods that wrecked numerous houses in Louth (Lincolnshire) 

 and drowned more than twenty of the inhabitants has been 

 the subject of an investigation by the Meteorological Office 

 [Professional Note, No. 17, by E. V. Newnham]. The flooding 

 began so suddenly and assumed such extraordinary proportions 

 that it is difficult to believe that mere rainfall, even of the 

 heavy kind associated with thunderstorms, could have been its 

 cause, yet this appears to have been the case. The rainfall 

 records showed that a considerable area of the Wolds to the 

 west of Louth experienced over 100 mm. of rain in about three 

 hours, and a large proportion of this had to pass through Louth 

 along the course of the Lud (normally a mere brook) before it 



