72 EEPORT— 1891. 



4. The Origin and Development of the North-west Wi7ids 

 of Netu Zealand. 



By John T. Meeson, B.A. 



The nor'westers of Canterbury — though they seem, and in 

 some respects are, so exceptional in character — in point of fact 

 are nothing but the winds common to the whole of New 

 Zealand, and, indeed, common to our latitudes in the Southern 

 Hemisphere, right round the earth — winds blowing towards the 

 area of abnormally low pressure (29° to 29-3°) which prevails 

 generally about lat. 56° S., but modified very materially in 

 character by their passage across the Southern Alps, which, 

 though not much higher than two miles even in their topmost 

 peaks, have power to affect the currents of air shown by cirrus 

 clouds to prevail to the height of five miles. These winds are, 

 in fact, the Beturn Trades, and would be north winds if they 

 followed the direction which they first take after forming at 

 the Zone of Calms ; but, having come from parts of the earth 

 where the rotatory motion is great to places where it is less, 

 they have acquired, for the opposite reason to that which 

 Hadley gives to explain the western tendency of the Trades, an 

 eastern proclivity : and as they proceed to higher latitudes 

 after becoming lower surface winds at the Tropic of Capricorn, 

 they become more and more westerly until they merge into 

 the " roaring forties " with which all voyagers to this part of 

 the world have a somewhat intimate acquaintance. Dove 

 remarks that "all winds are liars," and it must never be for- 

 gotten that the apparent horizontal direction of a wind, on 

 account of the shape and aerial motion of the earth, is never 

 or hardly ever the real one. 



These roaring westerly winds show frequently unmistakable 

 cyclonic disturbance, which, by applying Buys Ballot's, or, 

 as it should perhaps be called, Galton's law, can be roughly 

 located. That law recognises the fact that the wind blows 

 along isobars with less pressure on the left in the Northern 

 Hemisphere, but on the right in the Southern : in other words, 

 if you stand with your back to the prevailing wind, in the 

 Southern Hemisphere the lowest depi'ession of the barometer 

 will b3 on your right hand — i.e., in the case of the nor'westers, 

 to the south-west. If we knew in New Zealand, as meteorolo- 

 gists in the Old World do know, the exact measurement of the 

 acute angle between the direction of the wand and the lie of 

 the isobar — that is, the inclination of the wind to the isobar — 

 we might fix the position of the cyclonic centre much more 

 precisely. But the narrowness of New Zealand, the few 



