The cirrus cloud is usually seen on the outskirts of an anticyclone, if in the front it gradually disappears, but if in the rear it is a sign that there will be a change in the weather, hence:
If cirrus clouds dissolve and appear to vanish, it is an indication of fine weather.
If cirrus clouds form in fine weather with a falling barometer, it is almost sure to rain.
When, after a clear frost, long streaks of cirrus are seen with their ends bending towards each other as they recede from the zenith, and when they point to the northeast, a thaw and a southwest wind may be expected.
Both these latter prognostics refer to a depression coming in and “breaking up the weather” and the anticyclone.
CONCLUSION.
We have endeavored in this paper to deal with such prognostics as can be readily classified, but besides these there are many more, what may be termed unclassified prognostics. Our object has been to show the relation between certain prognostics and certain forms of isobars, and by this means to assign them their proper value. It cannot be doubted that if careful attention were given to the observation of the aspect of the sky, the different forms of cloud and local signs, these would prove, even in conjunction with a single barometer reading, of great assistance in predicting the weather. Of course those persons who live in the neighborhood of large towns labor under a great disadvantage, as the sky has nearly always a dirty appearance, and is frequently obscured by smoke. For this reason London is about the worst place to reside in for studying the weather.
We have only been able to give the rudiments of the new method of considering prognostics, for the method is capable of great extension, and we hope that we may succeed in interesting some of the Fellows sufficiently to induce them to observe prognostics in conjunction with the daily weather charts.
In conclusion, we would venture to express our opinion that the observation of local signs of weather ought to form a most important element in all arrangements for telegraphic reporting for the purpose of forecasts, and that the duty should be impressed upon the observers of reporting at once important changes in the local and general signs of the weather. Great attention should also be paid to the observation of the forms and motion of clouds, and as considerable lack of knowledge prevails, even on the part of good observers, regarding the different forms and modifications of clouds, we are glad to see that the meteorological office has already commenced systematic observations of cirrus clouds. Telegraphic observers should be specially instructed in cloud observations and prognostics by some one thoroughly well versed in the subject, so that there may be strict uniformity among all the observers, but we admit that the proposal presents serious difficulties in the way of realization.
Theoretically, when the isobars are well defined we ought to be able to write down the prognostics which might be visible, but practically we cannot do so. Besides, there are sometimes cases of isobars which have no well-defined shape, but with which thunder-storms or heavy showers often occur. These, as is well known, do not affect the barometer, but are abundantly forewarned by the commonest prognostics, and as the rainfall is usually heavy in them the failure of the forecast which omits to notice them is very conspicuous.