A SCIENTIFIC EXPLANATION.

Auckland Star, Volume XXV, Issue 145, 19 June 1894, Page 2

A SCIENTIFIC EXPLANATION.

WEATHER FORECASTS

HOW THEY ARE MADE.

The severe gale and heavy rain which prevailed all over the colony on Saturday night and Sunday, are " explained away " on scientific principles by Captain Edwin, the Government Meteorologist at Wellington. The Captain, according to a Press Association telegram from Wellington, seems to account for the gale and tremendous downpour a day or two ago by the meeting of two elements which were not affinities, a " cyclonic " and an " anti-cyclonic " centre somewhere in the South. The consequent disturbance, it would appear, arose through these two mysterious forces of the air disagreeing with each other. The latest telegram from Wellington states: — " Captain Edwin statos the severe southerly gale which has been prevalent all over the North Island since Saturday evening has been caused by there being a cyclone centre to the westward of the Three Kings about noon on that day, and travelling to the south-east, meeting an area of high pressure, or anti-cyclone, which was to the westward of the South Island at the game time, advancing to the east. Both systems have so acted on each other as to cause a change of route, which has taken the cyclone centra more towards the east, and the anti-cyclone more to the southward than usual. The lowesb pressure of the cyclone, which was from 80 to 100 miles north of the North Cape at noon yesterday is 29in., and the highest pressure of the anti-cyclone, which was south-east of Stewart Island, was estimated at 30.40 in." PREDICTING THE WEATHER. We are deluged with weather forecasts and predictions of gales in this age of precision in the science of meteorology, but probably few have an idea of how these prophecies of the weather are arrived at. We are familiar enough with Captain Edwin's stereotyped warning, " North to nor'-east gale, with heavy saa and rain," but how few there be who know on what data the weather prophet bases his conclusions. Mr Clement Wragge, the Queensland Government meteorologist, recently gave some very interesting information in a public lecture at Brisbane as to how he arrived at his weather forecasts. Seated in his office chair in the telegraph office in Brisbane (says a report in a contemporary) he receives from all parts of Australia, Tasmania, New Zealand and New Caledonia reports on the weather at nine o'clock each morning. The observers are'officers of the Post and Telegraph Department, who are supplied with copies of standard rules for observation. They have to send particulars of the state of the barometer, the wind, the clouds, the weather at the time and during the previous 24 hours, and it at the seaside, the state of the sea. The force of the wind is denoted by numbers covering all gradations, from 0, signifying absolute calm, to 12, hurricane, and the amount of cloud is also estimated in a somewhat similar manner. To these particulars the observer has alfeo to add the speed at which clouds are travelling. THE PROPHET AT WORK. Armed with all thoße data Mr Wragge proceeds to work out big forecasts.' On a

charb of Australasia bearing the- names 'of the observing stations; but otherwise blank, he places against each station the barometric reading received that day, and draws lines through those places where the readings are the same. Having marked his charb, the next thing Mr VVragge has to do is to deduce from the isobars — the lines of equal barometric pressure —the probable kind of weather which may be expected in various parts of Australasia. By carefully calculating the rate of motion and course of the anticyclonic and cyclonic waves, and taking into consideration the features of the country over which they have to pass, he is able to forecast the weather for all the colonies and adjacent seas to 85 per cent, of accuracy from one day to fourteen days, bub he admitted that until meteorology, which is one of the youngest of the sciences, had made still more progress, 15 per cent, of the forecasts he issued might prove either partial or total failures. Mosb people, however, will, we think, agree thab 85 per cent, is a very good proportion of successes. CYCLONES AND " ANTLCYCLONES.'' The anti-cyclone spoken of by Mr Wragge he defined as a region of high atmospheric pressure and dense atmosphere, and the cyclone as a region of low pressure and rarefied atmosphere. . He further explained them as follows :—" An anti-cyclone w&s a greab mountain of atmosphere, .nearly always more or less circular in shape, and the barometer reads highest at the centre or sucleus, the isobars decreasing in value on the surrounding sides, and thus producing barometric gradients. Anti-cyclones were fed or maintainedgby a downflow of air from the upper regions of the atmosphere. A cyclone, in the language of the .meteorologist, might be regarded as an atmospheric trough or valley; ib was, in facb, a low-pressure system, nob necessarily a tropical hurricane, although thab type of disturbance came under the same category. Cyclones were usually circular in oubline, and the barometer read lowest ab the centre, the isobars increasing in value on all sides. This type of disturbance was maintained by an uprush of atmosphere ; it was, In facb, the very opposite to the anti-cyclonic form. A constant intercirsulation was kept up between the anti-cyclones and cyclones, the one feeding or maintaining the other, and hence ib was impossible to have an anticyclone without its opposite type (the cyclone) being in comparatively close proximity." " SEASONAL " FORECASTS. Referring to whab are known as •'seasonal" forecasts—predictions of the probabla weather for month's ahead—Me Wragge explained thab ab present such long forecasbs woio impossible here. " The meteorological conditions of Australia," he said, •* were largely influenced by antarctic factors, of which we knew but little, bat it was certain thab the positions of the ice-floes and bergs in high latitudes operated to no small extenb in determining the character, of coming seasons. He could nob undertake longrange forecasting and the predicting the nature of seasons from three to six months ahead until he was sure of the necessary data. In India, on the other hand, long-range forecasting has been attempted with some measure of success, but then the geographical position of thac country favoured the solution of the problem." It eeems thab a half-way station between New Zealand and the South Pole, with cable communication, would be neeeßBary to ensure accurate seasonal forecasts of the weather here. BAROMETRIC READINGS. With regard to the barometric readings in the gale of June, 1889, and of last Sunday, a correspondent writes as follows :— 11 In June, 1889, the lowest barometer reading ever recorded in Auckland occurred during the storm which raged on that occasion. Can any observer give the lowest reading of the barometer then, and also the lowest point reached during the storm on Sunday, 17th inst. ? The fall of the glass on Sunday must have closely approached that of the former one." On enquiry from Mr Cheesemanj Observer at the Auckland Museum, we find that the lowest barometric reading ever known in Auckland occurred here on June Bth, 1889, when the glass registered as low as 28.9. On Sunday last the barometer fell to 29.2—three-tenths higher than the reading of 1889.