In the extreme front of the depression there is a blue sky, then, as the barometer begins to fall, and sometimes even before that takes place, a bank of cirro-stratus, preceded by a halo-bearing sky, makes its appearance, which gradually becomes lower and denser, and forms an overcast, dirty sky. In the whole front of the depression the temperature rises, and the atmosphere feels muggy and close. In the right-hand front the clouds assume the cumulo-stratus type, with driving rain later on. In the left-hand front the air is cooler, but still oppressive, with an easterly wind and overcast sky, succeeded by drizzling rain or ill-defined showers. When the trough of the depression has passed the barometer begins to rise, the wind changes and becomes squally, with showers of rain; the air grows cooler, and the clouds break and ultimately clear away.
Now, with regard to the prognostics with reference to Fig. 2, where the characteristic weather in the different portions of a depression are given in a diagrammatic form, it will be seen that the first indication of a coming change is the appearance of a halo round the sun or moon in the cirro-stratus clouds. Hence,
When round the moon there is a brugh,
The weather will be cold and rough.
The moon with a circle brings water in her beak.
Halos predict a storm (rain and wind or snow and wind) at no great distance, and the open side of the halo tells the quarter from which it may be expected.
Mock suns predict a more or less certain change of weather.
Fig. 2.—Cyclone Prognostics.
With regard to the open side of the halo indicating the quarter from which the storm may be expected, it does not appear that this can be much used as a prognostic. It, however, most probably originated in the fact that halos are often seen in the southwest or west, when the sun or moon is rather low, the lower portion of the halo being cut off by clouds banking up in that direction, and that our storms generally come from those quarters. As a specimen of the value of prognostics we give some details of halos. When rain does not fall within thirty-six hours, any subsequent rain probably belongs to a new depression.