The Science of the Weather 783 



ologist. Bjerknes, is that in a cyclonic depression there are twc 

 main streams of air that converge upon each other. One of these 

 is a southerly current, warm and moisture-laden from its passage 

 over the Atlantic; the other is a cold dry polar current from the 

 Arctic regions. They meet, and the warm air, being lighter than 

 the cold, is forced to rise over the latter, and in doing so carries 

 up with it the water-vapour it contains. The warm air, being 

 thus forced up to a higher level, has its pressure decreased, and 

 therefore becomes cooler (as has already been described), and its 

 water-vapour is condensed into cloud, and finally falls as rain, 

 because cold air cannot contain as much water-vapour as warm air 

 can. 



If the sequence of weather-changes occurs in the order just 

 described, an observer may know that the centre of the depression 

 has passed to the north of his station; but should the cyclone 

 centre be to the south instead, the observer would experience a less 

 marked difference in temperature, he might probably have more 

 rain, and the wind would shift in the other direction, becoming 

 first easterly, then gradually northerly, and, finally, north- 

 westerly. 



Our Wet West Coasts 



The mountain chains in our Islands form a barrier to the wind 

 and cause a forced rising of the air, which is attended likewise by 

 the formation of cloud and rain. That is why our west coasts 

 are wetter than our east coasts ; firstly, they are more mountainous, 

 and secondly, they are exposed to the moist warm ocean winds 

 from the westward and south-westward. 



9 

 Clouds 



The water-vapour that is evaporated into the atmosphere 

 manifests itself in many forms other than rain. All the clouds are 

 due to it, for they are nothing but aggregations of excessively 



