532 PROTECTION AGAINST WINDS. 



which give's it a twist to the right in the northerly hemisphere. 

 The strength of the wind varies with the barometric gradient, 

 or difference in atmospheric pressure at places distant one 

 geographical mile normal to the isobars. The closer, there- 

 fore, the isobars are for any difference in the height of mer- 

 curial column, the greater is the gradient and the stronger the 

 wind. 



The isobars become crowded together wherever the pressure 

 is lowest, and this fact, combined with the twist to the right 

 of the winds rushing in from all directions to fill a depression, 

 causes the revolving storms also termed cyclones, the absolute 

 axes of which are more or less calm. Thus on the southern 

 side of a depression, the wind blows from the S.W., on its 

 western side from N.W., on its northern side from N.E., and 

 on its eastern side from S.E. 



The isobars are closest together on the western sides of 

 depressions, so that the strongest storms come from a westerly 

 direction (S.W. to N.W.). The depressions usually pass to 

 the north of Central Europe, and traverse the continent from 

 west to east, so that storms usually begin blowing from S.E. 

 and gradually change to S., S.W., W., and N.W. Most of 

 these storms travel across the British Isles, having originated 

 in the Atlantic ocean or Gulf of Mexico, but the south-easterly 

 direction of the wind before a cyclone is not very noticeable here. 



Powerful storms therefore depend on the existence of 

 barometric depressions, which may be only partial or irregular 

 interruptions of an isobar, in which case the storm extends 

 over a limited area only. Violent storms of limited extent 

 but withs teep gradients that do considerable damage over a 

 narrow zone of country, are termed tornados. 



The direction of winds may be considerably modified in 

 mountainous countries, by the spurs of the mountains as well 

 as the directions of the valleys. Thus, a west wind may be 

 converted into a north or south wind during its jprogress 

 through a valley. 



It is a still more frequent case for a south-west wind to 

 become a south wind, and a north-west wind, west. 



These local wind-directions must be considered whenever a 

 working-plan is prepared. 



