MOUNTAIN RANGES

Evening Post, Volume CIX, Issue 75, 29 March 1930, Page 13

MOUNTAIN RANGES

EFFECTS OiY WEATHER

RAINFALL DISTRIBUTION

The control exercised on rainfall in New" Zealand by topography is very close," remarks Dr. E. Kidson, Director of Meteorological Services, in his bulletin on New Zealand's annual rainfall. The effect, he points out, is twofold: First, there is the effect of the aspect which the gfound surface presents to the rain-bearing winds, and the influence which the relief of the surface exerts on tlie'sc winds; and, second, the, direct effect of altitude. The effect of j mountains on wind and weather is complicated. First of all, it will check the speed of the wind, and, if rain is falling,' it is likely to be in excess on-the windward side for that reason. Tile air will, if possible, move round an obstacle rather than climb over it; consequently, there will bo a convergence of air at boinc places and a divergence in others. In the first the congestion will lead to uplift of some of the air, and ia the second there will be a tendency for subsidence. Tho first will be an aiei. of relatively heavy and tho second of relatively light rainfall. Some of the air, especially when it meets a long range, as the westerlies meet the Southern Alps, is forced up by the direct effect of the mountains, and the effect on the rainfall on the windward side is very marked. In all those cases, of course, where one area receives an excess of rain there is some corresponding area which, to some extent at least, is robbed of its fair share, and we have mountains casting "rain shadows.' Since westerlies are the prevailing winds over New Zealand, especially in Hie upper air, tho orographic effect is most prominent in them, and since the westerlies increase with increasing latitude, tho effect is most marked in tho more southern districts. Sometimes, however, especially in connection with cyclones, there are strong easterly or south-easterly winds, and whoro these impinge on high country on tho east coast very heavy rain results. rlhe 'mountainous region between HaWkcs Bay and tho Bay of Plenty and to the riorth-east towards) East Capo receives heavy falls in botli north-west aiid south-easterly rains, to which it has an open exposure. Wo have 110 complete data on the variation of rainfall with altitudo in New Zealand, but there is no doubt tbab tho increase is very marked to well above 3000 ft. There is some slight suggestion that tho zone of heaviest fall may be'in the neighbourhood of 4000 ft —at least, in some localities. It soe'ms almost certain that beyond 5000 ft a declino commences. ' , - EFFECTIVENESS OF BAIN. Tho effectiveness of rainfall depends not only on tho amount, but also oil a number of other factors. Amongst .the most important of tlieso is evaporation. Tho kl^ta regarding evaporation in different parts of New Zealand are Ut present, except in Central Otago, very Scanty) and Will not be di&cussed here. Thore'. are, iiowover, several factors influencing evaporation to which rei'erenco might.be made. /The. evaporation, will depend to a large extent on tcihperature effect, evaporation would tend wind velocity. On account of the teniperaturo c:cct, evaporation would tend to be higher in tho north than in the south. Generally speaking, tho humidity will be lower at inland stations, and especially east of the ranges, than near the coast. . The greater the cloudiness, the loss will be the evaporation; and tills, tot), will 'Dad td increased evaporation ,in such places as portions of the interior1 of Canterbury and Otago, where there is a large proportion' of clear skfes. Owing to tho effocts of evaporation, therefore, a heavier rainfall will bo needed to produce tho same effect on vegetation in northern thau in southern districts. ' Again, a rainfall that consists oi' light or moderate falls coming on a large number of days will bo more effective, on the whole,, than heavy falls on a few days. This-factor, also, will tend to make tt'smaller rain suffice in the south than would in the north. Conversely heavy total rainfalls are not such it Berioiis drawback to a climate it they fall on a comparatively, small kitihlber of,<daVs and are" interspersed with much filie weather. On account of the daiises mentioned, some of the low rainfalls re'eorddd on the Canterbury PlainS and the eastern portions of Otag"o, for instance) produce more abundant' vegetation than would ■corresponding falls in the interior or, still morcj in thd North Islandi Another inlpftrtant factdr in determining tho effectiveness tifVthe rainfall is its annual distribution. tfo 9nOW this wouia necessitate the preparation of monthly rainfall maps, Which Would involve a great" amount of works