PROGNOSTICS

Otago Witness, Issue 3467, 24 August 1920, Page 50

PROGNOSTICS

By H. T. G

A consistent and regular study of weather signs will, as mentioned in a previous article, enable the observer co forecast local conditions with a certain amount of accuracy. In New Zealand, for instance, a warm north wind (mois-ture-laden as it must necessarily be), on blowing over a colder area, will usually deposit its moisture in the form of rain. But such is a generalisation, and must be subjected to the various modifying influences of local conditions. In predicting, the thermometer as well as the barometer should be studied. A rapid rise in temperature frequently accompanies a rapid decrease in pressure. A barometer fall of one-tenth of an inch in, say, a couple of hours, is an almost invariable sign of approaching rain. The approach of cyclonic conditions, especially if the degree of intensity be considerable, is preceded by a rise in temperature as well as by decreased pressure. Regarding the prediction of local weather conditions, a study must be made of the surrounding physical features, and the effects of winds of various force and coming from various quarters upon adjacent hills, peaks, or low-lying valleys, must be noted for future reference. Perhaps the most reliable source upon which prognostications may be based, however, is to be found in the various cloud forms. To become "weather-wise" it is necessary to observe the gradual formation of these moisture masses, the direction from which they come, and the time of day at which the are of comparatively settled shape. It is impossible, however, to discover a methodical system or to formulate fixed rules guiding the observer to a definite decision as to the ensuing weather; he must gather all available data and com-

pare all the existing conditions with those of similar comparative nature which have produced an observed phenomenon at a previous date, and draw his own conclusion therefrom. This applies with special force to the prediction of weather in a district where any very prominent mountain peak or range of hills exerts a noticeable influence upon climate. Some general hints on cloud prognostics may bo of value, but it must be clearly understood that their range of utility is general and not particular. Cumuli usually appear in the early morning as small rounded white clouds, increasing as the day advances and diminishing towards sunset. These forms denote settled fine weather. Should they, however, increase in size and density as evening approaches (owing to increased moisture and cold in the lower regions of the air) rain may be expected to follow within a few hours. During a long spell of fine weather the ascending currents are often without sufficient moisture to form cumuli, and a cloudless blue sky is thus observed. But at great altitudes, where the ascending vapour may become frozen, the filmy cirrus clouds will readily form, and so long as they retain their feathery texture and great altitudes we may depend upon the weather remaining fine, but if they are lowered to the warmer regions by a moisture-laden wind, and if they form continuous banks (cirro-stratus), then rain, accompanied by high wind, may be expected. Should, however, these stratified banks become scattered and broken up so that the wool-pack form of cirrocumulus appears, we may safely predict fine weather.

One form of cumulo-stratus cloud, which closely resembles cirro-stratus, but which exhibits peculiar cone-like masses arising from out its midst, is seen when electric disturbances take place at great altitudes in hot weather, and its presence usually betokens an approaching thunderstorm. The ordinary cumulo-stratus clouds, howover, consisting of piled up horizontal banks, often of a dark and threatening appearance, are but the parents of the dingy nimbus clouds which will gradually develop if the conditions favourable to the change remain constant. The gathering o~ nimbus clouds from cumulo-stratus is a most interesting sight. The lower layers increase their area, and appear to become flattened during the process, as_ if the upper strata exerted a crushing influence with their dense masses of accumulated moisture. When the darkened uniform mass gathers itself together and the wind is fairlv i*egular in force a fleecy fringe, probably consisting of pure cumulus of lesser density, will often give rise to the "silver lining" appearance which lends such a beautiful contrast to the somewhat dismal appearance of such cloud forms.

Should the conditions not remain favourable to the development of nimbus from cumulo-stratus or other clouds, the threatened rain may pass off. The writer has frequently noticed that a dry wind blowing upon cloud masses during the formation of nimbus rain clouds will cause a reconstruction and a gradual decrease in density until either cumulus or lighter cumulo-stratus clouds are built up, and the rain storm is at the least postponed. But the pressure of heavy cumulus or of cumulo-stratus clouds at sunset or during the evening may alwavs be taken as a fairly sure sign of approaching rain. As the result of long-continued and careful observation, the only other "sky guides" which the writer can speak of with any degree of certainty are as follows:

Fine weather may be looked for if the day is ushered in with a clear or even with a srrey sky, or if slowly-moving light fleecy clouds are present. If these clouds retain their shape for some length of time the sign i 3 more reliable. Even then this fine weather condition is subject to the wind, for a sudden shift in the direction of an air current is an unfavourable sign. Such a shift mav be looked for when lower layers of clouds are seen to move in contrary directions to those above. Should cloud forms be of a "hard" appearance, with abruot edges, or should a flying scud, rapidly passing denser masses of clouds, be observed, then either rain ; a change of wind, or more probably both, may be expected within a short time.