WIND AT BULLS

Evening Post, Volume CVIII, Issue 121, 18 November 1929, Page 13

WIND AT BULLS

CRITICAL ANALYSIS

IMPORTANCE TO AVIATION

A WEATHEE STUDY

An analysis has been made of wind observations taken »t Waitatapia, Bulls, for the last ten years.- Mr. A. Thomson, director of the Apia Observatory, who for the past year has been working ou winds and upper-air observations in Wellington, has made tnis analysis, which is an important one from a point of view of aviation. Tho development of flying both from a view point of national defence and for transport of mails and passengers demands a knowledge of the winds aviators will probably encounter. This includes primarily statistics of tho frequency wind blows from various directions, the average wind velocity from these directions and tho occurrence of gales. Unfortunately, wind measurements require more elaborate equipment than either rain, pressure, or temperature. Even with accurate anemometers, it is difficult to secure an open exposure where the winds arc not seriously deflected from certain directions by forests, hills, or local obstructions. The area over which wind statistics for any staton is true is probably much less than that for either rainfall or pressure. The location of the observing station must be fully taken into account in interpreting its wind statistics. .

Bulls lies in gently rolling country, midway between the Wauganui and Wairarapa plains, and the winds observed there are probably general over both these populous areas. Members of the Dalryrnple family have been responsible for continuous wind obser-.-Nations,there from July, 1919, to June, 1929. Each day at 7.30 a.m. the wind direction and force has been recorded, notes being mado of any change during the day., Waitatapia is about 30 feet abovo sea level, and is .about seven miles inland from the sea coast of the South .Taranaki Eight, somewhat south of the point.where the coast line turns from 'an easterly to a southerly. direction. Waitatapia lies midway between the towns of Wanganui and Palmerston North on a low, rolling plain. . The wind velocity was estimated according to the Beaufort scale, the observed velocities ranging from calm to .force 8. The Beaufort scale, transformed into miles per hour, is as follows:— i • • Beaufort ■ Scale Q 1 2 3 4 5. 6 7 8. Miles per hour 02 5 10.15 21 27 35 42. It is ■ a remarkable fact that north winds were observed only once and. north-east winds only twenty-seven times in ten years. Winds from the north-west occur more frequently than from any other direction, being observed slightly more often than the two next" most frequent winds, easterly and westerly combined. The twelve months of the year may be divided conveniently into three seasons during which the winds have fairly definite characteristics. From May to August light : winds prevail with . frequent spells of calm. During September to December strong northwest and west winds blow, frequently reaching gale force. From January to April noTth-west winds decrease ■in "both strength and frequency, while the number ■of calms and east winds increases. Conditions during January to April are midway between those occurring during the" other two seasons. - ; .. WIND, VELOCITIES. The wind velocities as measured at 7.30 a.m. are probably less than, those prevailing later in the day. Southerly winds have, the highest mean velocity, but blow comparatively rarely. In order of decreasing velocity, winds are from S.W., W., N.W., S.E., E., and N.E. On account of the north-west winds occurring much more frequently than other winds, the increased force oj: this ''"■wind from September to December makes this the windiest season of the year. Soutu and south-west winds ' ive' their greatest velocity in the winter from May.to August, while south-east winds; reach their mean maximum velocity in the winter from May to August, while south-east winds reach their mean maximum velocity during January to April. '■■•' ■ '■; ' ■ The air movement is an index of the- atmospheric circulation, and is equivalent to a weighted mean wind velocity. The average air movement has a maximum value of 8.80 in November and a minimum of 3.40 in July. The number of gales was found from the observer's notes on the weather during the day, and does not deal with conditions existing at 7.30 a.m. In ton years 290 gales were recorded, of which 187 occurred in the last four months of the.year.. April, July, and August are .months .practically free, from gales. May had, on the other hand, an unenviable record of thirty-four, eonipax-od with eight in. April and thirteen in June.. . .Further investigation showed that May, although iv some years free from sales, was often a very boister- • ous month, with gales chiefly from tho west or north-west. The number of May gales varies widely; four years had not a single gale in May, while May, 1920, had no loss thru thirteen. The high figure jfor 192G was confirmed: by New Plymouth records, where gales were reported on eleven days in May, the highest monthly aggregate for that year. PASSAGE OF DEPEESSIONS. May marks the transition from summer to winter conditions as October and November are the transition months from winter to summer. Tho same pressure conditions recur at those periods. The cause of the groat majority of gales is the passage of a "V" depression. A vast area of high pressure continues for days from Now Zealand northward to the Tropics, while a succession of "V" depressions move across the Dominion at a rate of 300 to 400 miles per day, bringL- g gales to the Taranaki' Bight. \Vhil" the apex of the "V" depression approaches New Zealand across the Eastern Tasman Sea the north-west winds increase in force. While the trough is over-New Zealand -the winds at Waitatapia remain northwest. After passing Nj.v Zealand the winds veer to west. The winds reach gale force while the trough is over New Zealand, but frequently, do not diminisn in intensity until the trough has reached the latitude of Chatham Islands. The passage of intense circular depressious with closed isobars frequently ■ causes winds to rcacW gale force with--in a radius of three hundred miles othe centre of the depression. In 1920 gales were reported on 37 days, 33 of which were duo to "V" depressions, and tour to circular depressions. Those which affect Bulls and the Tarana-.i Province have their origin most frequently in the north-western portion of the Tasman £■'