366 SCIENCE PROGRESS 



step towards its goal, the pole. The cold air, which is pressed 

 hard, escapes to the west in order suddenly to make a sharp 

 turn towards the south, and attack the warm air in the flank ; 

 it penetrates under it as a cold W. wind. The centre of the 

 cyclone is thereby displaced to the east, along the southern 

 edge of the broad rain-band. This edge shows thus which way 

 the cyclone will take. We have called it the * steering-line ' 

 of the cyclone," 



The second part of the paper deals with the local showers 

 that often occur in summer afternoons in otherwise bright 

 sunny weather in Norway, when there is not much wind. At 

 first sight the appearance of these appears to be extremely 

 capricious. That they are the result of local heating is evident, 

 but this was found to be only one condition for their forma- 

 tion. The other is that the inflowing air shall be not merely 

 dry inland air, but fresh vapour-laden air from the sea. The 

 detailed study of a number of cases showed clearly that this 

 was the case, and observations of humidity at a centre of con- 

 vergence that for several days produced no rain showed the 

 appearance of rain to be correlated with a rapid increase of 

 moisture in the inflowing surface winds. During the period 

 referred to it was found that sea-breezes were the cause of a 

 gradual penetration of damp air into the inland districts. 



Thunderstorms. — In Professional Note No. 8, published by 

 the Meteorological Office, Air Ministry, Capt. C. K. M. Douglas 

 summarises many observations of temperature and humidity 

 in the upper air made when flying in France. These cannot 

 well be summarised in a short space, but his classification of 

 conditions favourable for thunderstorm development is worth 

 mentioning. He finds that many summer storms can be placed 

 in one of the following classes : 



A. Those due to heated surface air in sunny weather. 



B. Those associated with powerful upper currents from 

 south-west, with light variable or south-easterly surface winds. 



C. Those with very low upper air temperatures in the south- 

 westerly or north-westerly currents of depressions. 



Class A. — Conditions are best in early summer, when the 

 upper air is still cold. The wind is generally light, and the 

 storm seldom travels far before dying out. 



Class B. — Instability is due to the relative coolness of the 

 strong south-westerly current high up, and not to heating of 

 the surface layers. They therefore occur as often by night as 

 by day, and they move rapidly with upper current from south- 

 west. The most violent summer line squalls are included in 

 this class. 



Class C. — ^These are often preceded by storms of Class B, 

 and differ from them chiefly in having the westerly wind at 



