NOR'-WESTERS

Evening Post, Volume CXIV, Issue 122, 19 November 1932, Page 15

NOR'-WESTERS

CANTERBURY'S WINDS

METEOROLOGICAL RESEARCH

CAUSES ANALYSED

Xew Zealand is undoubtedly a windy country, owing to its isolation in watery latitudes where air circulation is strongly marked. Wellington of its' several cities has the TOputation of being, the windiest of all, but the "nor'-wester" so frequently-experienced in Canterbury is idally the Dominion's champion wind. This linpleasarit. draught has ibeen niado the subject of a special aieteoroiogical bulletin by Dr. B. Kidson, Director of Meteorological Services. His paper rofers especially to thd. north-westers of Canterbury, but though, the maximum, development is on the Canterbury Plains, winds of the Bame type are experienced to some extent onthe eastern-sides of most of the mountain ranges in New; Zealand, from Otagd and Southland to north, of Gisiborne and even on the eastern side of the Coromandel Peninsula. Parts of Marlboroughj for instance, are scarcely less subject to them than the Canteribury Plains. . The north-wester which. iblows;down the.valley of the Clarorice Biver is Worthy of especial mention, ho Bays, since it extends frequently for several niileS out to sea and hence is a serious; obstacle to air traffic between [Wellington and Christchurch.

The true "north-wester,": says Dr. Kidson, is a wind that has crossed over iiora the west coast and, in descending from the Southern Alps, has developed tho well-known . "Fohn" chafactor.istics. It is caused by a suitable distribution, of pressure, and only blows when the winds in the upper air are from some north-Westerly direction. It must-not be confused with tho "cold mOrth.-wester," which is entirely a local wind and which is'not produced by the ge-neral distribution of pressure. ■ -When the. true warm, and dry Fohn ■wind north-wester blows, tho winds at a high level are from a direction botween. north and west. It will be realised that there must be a pressure gradient in the Upper air from southwest to north-west, pressure being high in. the latter direction. Now, this is normally the case When a westerly depression, is advancing across the Tasittan Sea, And, since we generally have westerly depressions at the rato of about one per Week, it will be understood why north-Westerlies are so frequent. The pressure gradient at the surface, also, is from south-west to north-west as the depression' moves over Australia and acrossvthe Tasman Sea, "but on reaching 2^e-vv Zealand it becomes considerably anodified. '

We have, then, air currents moving across the Tasman. Sea from a- northwesterly direction. At sea level, owing $o friction, the direction may be from *he north, but at higher levels it Is jmor,6 Westerly. We: have now- to conEider what happens to this current on reaching the New Zealand coast. As is well known, it will usually have been (preceded by south-ivest winds Which will ihaye brought cold.air to New Zealand. It is clear that it will generally not be easy for thp north-wester to climb over !the Southern. Alps. Instead it ivill try togo; round. That some of it succeeds, we in, "Wellington know only too well. Bo also do the lightfceepers at Puysegur EoinVwherea northerly galo is a.comSnon feature of an advancing depression. But that a good'deal of air is forced over the Alps is proved by the iK{ayy\,rains .expe4en,eed on. ihe .west coast. . Its ascent, will,- in tho beginning, of ten. bo made easier by the,pi-e----6eri.ee of .the cold air mentioned above, which was brought by the south-wester-Hies. The' north-vveSterly current will be warm ." and moist and therefore ■. less densS than the cold air froni.tho southwest, and will tid§ easily over it, the cold air remaining trapped against the Alps. A similar process would take place naturally if the Alps were absent. The north-Westerlies' would ascend over the cooler south-westerlies. Bain is sometimes! produced in this way, and is what is called "warm front" rain, the warm front being-the boundary line at the surface between the cold and warm winds. .■-'■■• ■ ' .

On the leeward side of the Alps, owing to the teondensation of water vapour ana the givihg-up to the air of its latent heat, when the north-wester ■begins it- will almost "invariably be potentially warmer on reaching the divide than the air below it to leeward. It will consequently tend to flow over the lower layers instead of .descending. But as it comes over the Alps it Will be very turbulent and largo • eddies will be- formed on the leeward side-of the summit. The effect' of these eddies will be to pick up the air on the eastern side and gradually mix it with that coming over. Hence the frequent gusts of -warm and cold air which are experienced on the plains just prior to the onset of the north-wester. By the pressure gradients tending to make the lower layers move away eastward, the north-wester is enabled to descend farther and farther until finally it reaches the plains,-where it sets in as a gusty, squally, unstable Tvind, varying greatly from place to place. '■'.■■ ' The alternate heating of the ground by the sun and the cooling of it by nocturnal radiation causes north-westers to be much more .frequent and stronger from about 9 a.m. to 7 p.m. than in tho night and early morning. THE ONCOMING OUEKENT. .It is seldom that tho current approaching across the Tasman Sea is a homogeneous one. Tho air itself will differ in different cases in its conditions of stratification and humidity. The speed and direction of movement ■will vary from ono part to another of the current. It is clear, therefore, that thero is plenty of room for variation between one north-wester and another, and no two will be exactly alike. The Jndst Severe will occur when the pressure distribution is such that the oncoming air current across the Tasman Sea is directed exactly head on to tho ranges and when, of 'course, tho> gradients are steep and tho velocity consequently high. ■ Theoreticalconsiderations would lead tia to-expect the air to be checked in speed and deviated toward the south aC some distance westward of the ranges. At this point the pressure would be high. Further eastward tho pressure would fall somewhat at corresponding levels and tho wind would Cross the divide at a high speed. As the range is passed, tho air suddenly has to expand. Instead of being confined to levels above that of tho divide, it has to extend also down to the level of the plains if the north-wester is fully developed. This must lead to a further fall of pressure and a farther increase of wind velocity. During the stage in which the pressure is falling, there would be a deviation to the northwards, but it is not large, because the movement does not have time to become adjusted to the new conditions. Farther eastward again, pressure will begin to rise and the velocity to decrease. 'At this stage tho air in the north-wester usually meets air which has taken a roundabout course, or which has some other origin. This will usually be cooler than the north-west air, and the latter will, therefore, rise 1 above it. The cool surface air will move down the east coast from north to south with a component inland to-' ward the area of lowest pressure. Still farther eastward the north-west current will take up its regular movement and pressure distribution again. Tho ; Jine along jyhieh the north-west and

north-east currents meet will vary with the strength 6f the north-Wester and other-.conditions. Usually it is west of Christchurch. ;

Hoar the east coast the surface wind is frequently from the north-east, the north-wester Continuing above aa an upper current. Observations in the upper air at Christohurch show that it is generally not until a height of about 13,000 ieet is reached that the full force of tho north-wester is encountered. ' . -; TYPICAL SKY EFFECTS. As regards clouds, on the west coast alto-stratus; nimbo-Stratus, and nimbus will be tho prevailing types during a north-wester. Along tho east of the crests of the ranges will be a band of mainly clear sky which, viewed from the eastward, forms the well-known north-west arch. The sky in this region often has a peu'rly hue and the cloud fragments sometimes have a brownish cir smoky tingo. ' To the West will be seen in the highest levels cirrus with, below it, altd-eumulus kiid alto-sti'atus, .and the torn and nigged rain clouds. Sometimes, towering cumuliform clouds Will be seen over.the mountains. Anotho' common typo of cloud, especially when tho upper layers of the atmosphere are stable, will be lenticular alto-stratus the stationary" type of cloud with smooth contours and lenticular form which is due to the ascent of the air current as it crosses a mountain. These clouds will often be seen to be associated with certain definite peaks or ridges., !;■.-. .-■ ■ ' . ;_• . . L ...

To the east Of the band of clear sky alto-stratus and alto-cumulus will usually again form. .Tho under surface of this cloud layer has a pocky, ragged appearaueo, and brilliant colour effects are often, seen when it is illuminated by the setting sun. Still farther eastward the cloud system gradually takes cv the charastnristics of the General weather typo prevailing, the effect of the mountains disappearing by degrees.