8 SCIENCE PROGRESS 



winds, whereas those of polar origin very commonly follow a 

 curved path and arrive as S.W. winds. It is for these reasons 

 that there is such a small connection between the temperature 

 and the direction of the wind at any one time. The matter is 

 not one that lends itself very well to treatment by the method 

 of correlation, and the most convincing arguments are those 

 based on a consideration of the extreme values of observed 

 temperature in relation to the weather maps of the previous few 

 days. The notably low temperatures were all with a current 

 which evidently came from regions north of the Arctic Circle ; 

 the highest temperatures were with winds from between south 

 and west (occasionally from N.W.) when there was an anticyclone 

 over France. 



The author passes on to consider what bearing his observa- 

 tions have on Bjerknes's theories about cyclones and anti- 

 cyclones. According to Bjerknes there is in every cyclone a 

 warm sector, where a current of air of equatorial origin forms a 

 sloping surface of discontinuity with the surrounding air 

 (which is of polar origin), this surface meeting the ground so as 

 to form two lines of discontinuity in front of and behind the 

 warm sector, which he designates the " steering " and " squall " 

 lines respectively. Discontinuities such as those postulated 

 were actually met with in the course of flying, at various heights 

 and on several occasions. The very strong winds from S. or 

 S.W. often observed at great heights above the " squall line " 

 of a depression are strong confirmation of the existence of a 

 warm sector of considerable height, for the contrast of tempera- 

 ture would produce a strong pressure gradient high up, due 

 to the more rapid falling-off of pressure with height in the cold 

 dense air behind the warm sector. In front of a depression 

 very strong currents from a northerly direction are well 

 known, and are the counterpart of those already mentioned. 

 They are due to the opposite kind of pressure gradient found 

 immediately in front of the warm sector. The author mentions 

 three points in the paper which he considers to be of special 

 importance, namely : 



(i) Both troughs of low pressure and wedges of high 

 pressure normally lie farther west in the upper air than at the 

 surface. 



(2) The pressure in the upper air may be regarded as being 

 partly a consequence, and not purely a cause, of the tempera- 

 ture of the underlying column. 



(3) Very powerful wind currents are observed at great heights 

 between the polar and equatorial air at those levels. 



Bjerknes's Theory of the Polar-Front. — ^The Meteorologische 

 Zeitung of January 192 1 contains a short article by Dr. F. M. 



