The Christchurch Star PUBLISHED BY New Zealand Newspapers Ltd. WEDNESDAY, APRIL 19, 1933. CANTERBURY’S NOR’-WESTER

Star (Christchurch), Volume LXIV, Issue 740, 19 April 1933, Page 6

The Christchurch Star PUBLISHED BY New Zealand Newspapers Ltd. WEDNESDAY, APRIL 19, 1933. CANTERBURY’S NOR’-WESTER

qpHE NOR’-WESTER that struck -*• Christchurch with such force last night, tearing up trees and tilling the air with the dust of the parched plains, was of an intensity fortunately rare, although nor'westers generally are frequent enough to have developed in Canterbury residents a philosophical appreciation of the effect on climate and even upon personal temperament, of this strange wind. The Canterbury nor’-wester, indeed, is not a simple phenomenon, and meteorologists admit that it is not yet fully charted. The most detailed description of its causes and variations has been supplied by Dr Kidson, who differentiates between the cold nor’-wester that comes down the valleys and the true nor’wester or the hot wind of the plains. The former is caused by gravity, whereas the wind that strikes Christchurch so frequently is characterised by a horizontal variation of pressure. Schoolboys know tbe simple theory of the westerly depression advancing across the Tasman, expelling the moisture from the atmosphere on the western slopes of the Alps and blowing on as a dry wind; but the turbulent nature of the wind and its velocity vary in different localities. The most severe nor’-wester strikes the Alps head on. Meeting an obstruction, it is retarded and banked up, as a line of motor cars piles up in a traffic jam. Some of the air flows round the Alps to windy Wellington, but that which climbs over the ridge moves quickly when it crosses the divide, expands suddenly, and being warmer flows over the lower layers of cool air instead of descending at once into the valleys. On the leeward side of the mountains blustery eddies are set up, which mix the cold and warm air, and with the fall of pressure and the consequent increase of velocity the higher currents descend rapidly to the plain, blowing with the strongest force, however, near the foothills. But owing to the cooling of the air by radiation the daytime downward currents are usually modified, and only a wind of exceptional velocity creates the nocturnal disturbances that gave so many people unease last night.