FORECASTING WEATHER

Taranaki Daily News, 27 December 1929, Page 18

FORECASTING WEATHER

METEOROLOGICAL WAYS TARANAKI'S RAINFALL RECORD. THE CLIMATE AND FARMING. “Lovely weather to-day, Mr. Brown!" No topic of human conversation is more discussed than the weather. It serves to introduce total strangers; an offhand remark on the weather breaks down the barrier between man and man. No subject is of so much importance as the weather.- Its influence is paramount over -.the affairs of men; favourable weather-and man prospers, but harmful weather and man’s efforts are reduced to nought. The causes of weather changes are obscure and to the layman hard to unravel, but to. the scientist . and meteorologist many of these stand revealed and the daily weather forecasts are noted for their remarkable accuracy.

Weather forecasts are concerned principally with the prediction of rainfall, wind and temperature changes, the most important theme being. rainfall. Wind changee are next in importance, especially to mariners, and temperature has special consequences to farmers. Wind is a vital factor in comfort, and often causes serious. effects, as in the case of gales, but as a breeze on a hot day it makes conditions pleasant and tolerable.

In order to understand the methods of precipitation forecasting it should be i realised that the only important means by which rain can be produced is by the cooling of the air. Air always contains more or less water vapour, but at any particular temperature it can only contain a certain amount, the amount increasing as the temperature increases. If, thereiore, the air be cooled it will finally reach a temperature at which it cannot "retain/ all the moisture present in the form.of vapour. Some of the latter condenses into small drops, which are visible as cloud or fog. If the cool-

ing process is continued' the drops increase in size until they fall as rain. The. next fact to realise is that the only effective means of cooling the air sufficiently to produce rain is by raising it into the atmosphere— ; into the colder regions. . As the atmospheric height' increases. the air pressure decreases owing to a decrease in the amount of superincumbent air. It is just this fact which makes Mount Egxnont such a vital factor in the production of rain iii Taranaki. If air be raised in the' atmosphere it has to adjust itself to continually decreasing pressure and expands in the process. As a result its temperature, falls, this being at the rate of about 18 degrees F., per 3000 feet' if the air is dry and 11 degrees ifthe air is .saturated, with moisture. Air will sometimes cool'itsclf by radiation, but. this process is slow and unlikely to be a direct cause of precipitation. Rain is of. four types: Cyclonic, instability showers, drizzle and orographical (or mountain) rain. Cyclonic rain is of two kinds, according to meterorologists—warm front rain and cold front rain. The former is produced through the uplift of warm air along an inclined plane formed by the cold air, The inclination is very slight—perhaps only 1 in 109 —the line _on the surface Where the cold and warm air come in contact being described as the “warm front.” Warm air usually appears in the upper levels shortly after the highest pressure has been experienced and is often marked by the appearance of cirrus cloud. As the depression advances the clouds become lower and denser, followed by rain. .With cold front rain just as the line of lowest pressure in a depression is reached there comes a change of wind from a southerly direction. This air is cold and the boundary between the warm air and the* cold that is. overtaking it is the “cold front.” The air behind the .cold front'advances into the warm air, wedging it up. In this case the warm air is forced up rather abruptly in front of and along the cold front, producing a wall of cloud. The band of rain along the cold front is narrower than that preceding the warm front and seldom fails to produce rain. The cold front is followed shortly by\clear intervals interspersed, with showers, which gradually cease.

MOUNTAINS AND RAINFALL. . A further form of rain is that called instability showers. This is produced by heating over a warm sea surface. If a mass of air at the surface is heated to a temperature above that of its surroundings it will be forced upwards owing to its decreased density. Its temperature will fall as its height increases, but it will remain warmer than its surroundings and, therefore, continue to rise ' until it is gradually mixed with the surrounding air or meets a stable layer. In this way heated air over a warm sea may rise sufficiently for the consequent cooling to cause rain to fall as scattered showers, A similar effect may be produced by the unequal heating of cold unstable air over a warm land surface. When air is stagnant over land in summer unstable conditions may be produced after several days by the accumulated effect of the heating of the ground surface by the sun. local thunderstorms develop, particularly in mountainous regions. . Mountain rain is principally produced when a mountain range lies across the path of the wind and the air is forced up over the mountain tops and cooled. Air does not naturally tend to defy gravity and climb over a mountain; it generally tends to travel around mountains, hence the strong winds which are sometimes encountered in places like Cook Strait, where there is a concentration of air How.

But when an air current meets a long obstacle like the Southern Alps some of the air is sure to be forced over and the greater is the rainfall likely to result. The nearer the-air approaches to unstable conditions the easier it is to force over mountains. When rain is a combination of instability and mountain rain the falls are likely to be heavy. A rainfall map of New Zealand shows how important orographical rain is in the Dominion. When the air reaches the other side of the mountain after dropping much of its moisture it will be difficult for it to descend unless it-zis potentially as dense as the air below on the lee side. Therefore, a<s frequently happens in Canterbury, the current from windward continues on the lee side as an upper current only, over-riding the lower level air. Thus there may be strong northwest winds high up with light northeasters beneath them. There are numerous effects produced by mountains. For instance, a wedge-

j shaped mass of air. near a warm front may be held back, the warm air continuing to climb over the cold Wedge. In this way warm front rain may be prolonged on the weather side of a range. To be able accurately to forecast weather the meteorologist must have information as to the conditions . prevailing over a wide area, and also at varying levels in the atmosphere. Every day the New Zealand Government Meteorologist, Dr. E. Kidson, receives 75 reports from New Zealand stations, 50 in the morning and the remainder in the afternoon. An increasing number of wireless reports from ships at sea also supplies valuable data. Reports from 14 Australian stations are received at intervals, and twice daily weather messages come from Chatham and Norfolk Island respectively. Both of these stations are exceedingly useful in the weather information they supply.

The various reports received are taken in hand and plotted on a suitable map to form a weather chart. First an arrow is drawn with its point at the station whose report is being considered, this arrow pointing the direction from which the wind is blowing. The number of feathers on the arrow denotes the force of the wind on the Beaufort scale. Thus there are eight feathers for a gale, four for a moderate breeze, and so on. Then the air pressure is written down close to the point of the arrow. -

The barometer readings, taken from mercury barometers, are corrected for temperature and reduced to sea level and standard gravity so as to make the readings intercomparable. Temperature is recorded, pn the chart and the weather ■is indicated' by Beaufort' letters, R for rain, 0 for overcast, etc. Symbols are also used for further explanation.

The next thing is to draw the isofears, or lines of equal pressure, the whole being, connected by continuous lines. These charts thus serve as a guide to the weather man in making his. predictions The principal systems are the “anti-cyclone,” or centre of high pressure, and the “depression,” or area in which the pressure, is lower than in its surroundings. A little experience sbon makes one familiar with-the significance of the various pressure systems, and each day their movement is studied and compared.

./ CAUSE OF WESTERLY WINDS. The direction of the wind bears a definite relation to the isobars, being.such that if one looks, in the direction ’in which the wind’is blowing pressure is lower on. the right than on the left. The closer the isobars, or the steeper the “pressure gradient,” the - stronger is the wind. At a height of a few hundred feet the parallelism between wind and isobars is very close, and the wind'can be calculated to a close degree of approximation from the direction of the isobars and the steepness of the pressure gradient. . The moving anti-cyclones follow tracks which are almost wholly within the area covered. by the charts, so that the characteristics of their movements can be studied in detail. It is found that though individuals may depart widely from the mean tracks, there is a great deal of regularity in the latter. For instance, there is a regular annual march of the average latitude of the anti-cyclone centres. The anti-cyclones travel in their farthest south latitudes in February. From March onwards there is a northward migration until the most northward position is reached in September or. October.

This movement has an important influence on the weather. When the anticyclones are far north Now Zealand comes more completely into the zone of westerly winds, and the strength and persistence of the westerlies as experienced in Taranaki is largely associated with this fact. In February and March the zone of high pressure is nearer and the winds are lighter and the weather more settled.

There .is also a variation in the-speed of anti-cyclone movements. This is greatest over the Australian • and New Zealand region in October, November and December, and- least in May and June. This, no doubt, is another of the factors associated with the velocity of westerly winds and’is in itself an important one in forecasting. There is also considerable uniformity in the distance between anti-cyclone centres, and it is considered that anti-cyclones and the intervening depressions are due to fairly regular pressure waves existing in the upper air. Considerable accuracy is possible in general weather forecasting, but local conditions vary considerably over short •distances. This fact is very obvious in Taranaki, where rainfall varies greatly in places only ‘a few miles away. Taking the rainfall records for 40 years past at' Ohawe, on the coast near Hawera, an effort is here made to give a comparison between the different years and periods of the year and to note any'special conditions. There is a prevalent idea that rainfall it- not so great since the bush has been cut down, but this is not at all borne out by the figures. In actual fact the last fiveyear period, 1924-29, has been much wetter than . the first period under review, from 1890 .to 1894, there having been 26J inches more rain in the more recent period. Whatever the value of bush in regard to rainfall it cannot be said that its absence has made the coastal climate drier. The average annual rainfall recorded at Ohawe over 40 years is 42.66 inches. For the past six years this amount has been exceeded, while 1924, with 56.98 inches, was the second wettest year on record. It is now ten years since a. really dry year was experienced, this being in 1919, when onlv 30.76 inches fell. In the 40-year period there have been only five years showing a precipitation considerably above the average. These and the rainfall are as follows: 1893, 52.85 in.; 1901, 58.48 in.; 1907, 53.23 in.;, 1920, 55.53 in.; 1924, oG.OSin. There have been in the same periodseven years showing considerably less rainfall than the average. These are: 1890, 35.41 in .; 1898, 30.55 in.; 1902, 33.85 in.; 1908, 35.57 in.; 1914, 30.77 in.; 1915, 33.82 iii.; 1919, 30.76 in.

SEASONAL VARIATIONS IN RAIN.’ The remaining years are conspicuous for the fairly regular rainfall —a matter of considerable importance to Taranaki as a dairying district. As long as Mount Egmont stands to act as a rain gatherer a continuation of annual regular rainfall may be expected to provide water for this fertile province. From the foregoing figures it is not possible to discover any defined cycle between wet and dry years. - These gaps, appear to be at irregular intervals. Between the wet years there are intervals of 11, 3, 13 and 4 years, while between the dry years the intervals are 8,4, 6,0, I,' and 4 respectively. .Eliminating 1915, which followed a dry season in 1914, intervals of six years .would separate the dry years from 1902 onwards. In the 11-year interval between the wet years 1893 and 1904, there were

two dry years, 1898 and. 1902. In the 13-year interval between the wet:years of 1907 and 1920 there were no fewer than four dry. years, 1908, 1914, 1915 and 1919. . One point of note is that the interval between wet years hag been much longer tlian that between dry years.

The wettest year on record was 1904, when 58.48 inches fell, -and the driest 1898, when the amount was only 30.55 in. Another very dry year was 1914, the fall being 30.77 in. In connection with the wettest year it is interesting to observe that the rainfall was well distributed throughout the year, all the seasons being exceedingly wet. In the matter of seasonal variations the rainfall figures disclose much of interest, although here again there are no clearly defined weather- cycles. In the period there have been twelve dry summers in the respective years 1890, 1898, 1900, 1902, 1905, 1913, 1914, 1915, 1921, 1925, 1928 and 1929, and eleven wet summers, namely, 1891. 1895, 1895. 1897, 1904, 1907, 1909, 1910, 1920, 1923 and 1924. A dry spring was experienced in nine different years, these beino- 1891, 1893, 1894, 1896, 1901, 1908, 1914, 1917 and 1919, while on five occasions a wet spring has resulted, these beino- in'l9o4 1905, 1913, 1924 and 1926.

'The winters have usually been consistent in rainfall,- the only marked variations from normal being the following dry seasons: 1891, 11.21 in.; 1898. 12.02 in.; 1902, 12.25 in.; 1914, 12.19 in.; 1915, 12.71 in.; 1919, 11.31 in.; and 1922, 12.72 in., with wet winters in 1893, 19.57 in.; 1904, 21.04 in.; 1907, 19.60 in.; 1916, 19.49 in.; 1917, 19.62 in.; 1920, 20.14 in.; .1925, 21.85 in.; 1927; 22.72 in.; and 1929, 20.28 in. There h. .e thus been seven dry winters and nine wet winters in the period under review, four of the latter being within the last decade, one (that of 1927) being the wettest on record. From these figures it may be assumed that nature has arranged for a' fairly regular rainfall, and that periodically the distribution is balanced to maintain a consistent average. The Ohawe figures for the annual precipitation over the past. 40 .years are as follows:— .

1890 . .. 35.4Iin. 1910 .■ 44.19in. 1891 . ... 42.12in. 1911 ... . 45.05in. 1892 . .. 44.21in. 1912 ... . 45.59in. 1893 .. .. 52.88in. 1913 ... 40.94in. 1894 .. .. 37.62ih. 1914 ... ; 30.77in; 1895 . .. 48.90in. 4915 33.82in. 1890 -.. .. 36.16in. 1916 44.83in. 1897. .. .. 46.631n. 1917 37.7 lin. 1898 .. .. - 30-55in. 1918 ... •45.10in. 1899 .. .. 41.10in. 1919 ... 30.76in. 1900 .. .. 37.42in. 1920 ...■ 55.53in. 1901 .. .. 41.57 in. 1921 .... 43.01in. 1902 .. .. 33.85in. 1922 ... 40.18in. 1903 .. .. 41.05in. 1923 ... 47.35in. 1904 .. .. 58.48in. 1924 .... 56.98in: 1905 .. . 38.60in. 1925 .... 43.72in. 1906 .. .. 39.15in. 1926 ... 48.89in. 1907 .. .. 53.23in. 1927 .... 46.82in. 1908 .. .. 35.57in. 1928 .... 42.29in. 1909 .. . 45.36in. 1929 (to date) ..... . 41.17in.