FORECASTING THE WEATHER.

Taranaki Daily News, 21 February 1922, Page 8

FORECASTING THE WEATHER.

FACTORS THAT GOVERN CLIMATE. (By F. L. Wooles, in the Lyttelton Times.) The weather is a' perpetual topic of conversation. There is every reason why that should be so when one realises how profound is the influence of the weather on the welfare and prosperity of the pople. An unreasonable storm or a long dry spell may mean the loss of thousands of pounds’ especially to the farming community, while in lighter vein how often is a holiday made or marred by the state of the weather? To be forewarned is to be forearmed and it is obvious if one can foretelLwhat the weather is going io be even for a day or two ahead it max be of inestimable value under certain conditions. However, to gain an idea of weather forecasting a knowledge of climate and other factors is essential. The principal factor governing the climate of New Zealand is the surrounding ocean. The insularity of New Zealand and its comparative isolation in the great southern ocean have a decidedly modifying influence on the climate. The climate of other parts of the world as well as the Dominion depends as much upon the movement and temperature of the ocean as it does upon the position and elevation of the masses of land. The cold of the polar regions would be more rigorous and the heat of the tropics more intense if the currents of the ocean did not convey water from the polies to the Equator and vice versa, thus constantly tending to an equalisation of temperature. By the action of these currents enormous volumes of water are moved across the oceanic basins and by washing the shores of continents and islands modify tiieir climates according to the temperature of the water.

Every difference of level which is produced on the ocean’s surface in consequence of prolonged winds, heavy rains of very active evaporation results in the formation of a current, as water whether salt or fresh ever seeks its own level and unceasingly flows from the more elevated places towards the depressions. Every atmosphere variation results in a displacement of the superficial water. The greater currents, how%ever, which flow With a regular movement around the basins of the ocean between the poles and the equatorial zones are determined by genera) causes acting at the same time upon the entire planet. These causes are the sun’s heat and the rotation of the earth on its axis.

It will be realised how susceptible New Zealand, with its small land area and its lengthy coastline, must be to the varying temperature and currents that wash its shores. As an illustration of these oceanic influences, there is a marked difference in the normal temperature of the sea on the east coast of New Zealand as compared with the west coast. Along the east coast as far north as East Cape a cold Antarctic current is usually in evidence, though not always, while the west coast is washed by a warm equatorial current that is known to exist in the Tasman Sea. As a consequence, |he west coast of the South Island in the Winter and early spring is usually several degrees warmer than the east coast of the South Island.

Apart from oceanic influences there are other factors that modify climate. The physical features of a country or locality have an important effect. Weather phenomena are distinctly influenced by such factors. The configuration of the various parts of the Dominion has a great deal to do with the varied climate experienced. Owing to the mountainous character of a great portion, certain parts experience an abundant rainfall, while in other localities the average annual precipitation is small. The South Island and the southern half of the North Island come within the normal belt of westerly rains. Owing to the Southern Alps, the west coast of t)ie South Island has about four times the rainfall annually as the east coast. The same thing applies in a' lesser degree to the North Island. North of East Cape and Raglan, however, the rainfall is generally abundant on both coasts, that region sometimes receiving westerly rains from Antarctic disturbances, but more generally the easterly rains of sub-tropical storms. Latitude exercises an important influence on climate; New Zealand, stretching through 13 degrees, is susceptible in the north to sub-tropical influences and in the south to Antarctic influences. With regard to foretelling the weather, which is part of the important science of meteorology it w r ill be readily understood that to make a reliable weather forecast many factors have to be taken into consideration. The fundamental factors have already been outlined, but allied to these mustbe a knowledge of the incidence of high and low pressure areas. The intensity or otherwise of such systems an! their relation to one another in the steepening or lessening of the barometric gradient are an important part of the science 1 upon which weather forecasting is based. 1 For that purpose barometer readings and other information are obtained from over a wide area synchronously »,f a certain hour bo correlation of such infomation xvi! he other factors already 1 enumerated 1 --•ng the basis upon which I weather forecasts are arrived at. The * information from which a reliable forecast can fee based is not- available 10

every one, but for the bertefit of those •who may be interested a few hintr on foretelling Canterbury weather from twelve to twenty-four hours ahead, local observation, may be of use; but of course cannot be guaranteed to produce the best results, as important infonnation is necessarily lacking. A falling barometer with northerly or westerly winds indicates fair weather on the plains and Banks Peninsula, but rain likely in back country. Barometer falling with easterly winds and a clear or fairly clear sky means probably a change of wind to north-west. However, if heavy clouds work up from the east rain is more likely. A rising barometer with south-west winds indicates coastal showers chiefly about Banka Peninsula, the weather generally clearing quickly unless the barometer remains comparatively low afid steady,’ when south-west squalid may continue for two or three days. When the barometer is rising With south-east winds showery conditions to a heavy rainfall may be expected along the coast, this weather often extending well inland. A steady or falling barometer with southeast winds indicates, especially if the weather has set in wet, more or less continuous heavy rain for two or three days and possibly floods. The barometer falling rapidly and very low’ usually portends a north-w’est gale; when rising rapidly a south-west gale with little or no rain.

Cirrus cloud first making an appearance in the south-west or western sky during fine weather portends north-west weather. When it appears first in the northern sky and works south it indicates easterly rains. During north-west weather if the nor’-west “arch” does not lift and the wind changes to southerly with the “arch” still in evidence it is almost an infallible sign of rain. If, however, the sky is- clear or virtually clear when the wind changes from northwest to south-west little rain is to be expected. During . the . warmer months of the year and the day. after a nor’wester has been in evidence heavy masses of cumulus cloud may sometimes be seen developing in the western sky, the lower current at the time being an easterly breeze and the barometer low and steady. When such conditions obtain, especially if the clouds become more dense and ominous looking towards the middle of the ,dny, ai-<l the wind freshens from the east, thunder storms are indicated.