Depression and Frontal System Pictures
Appearance of Cirrus
The first sign of an approaching frontal system is the appearance of Cirrus, increasing and thickening as it moves across the sky.
Veil of Cirrostratus
Two or three hours later the sky is covered with a veil of Cirrostratus and quite often a halo or ring of light can be seen round the sun or moon (hence the sailors' omen of stormy weather foretold by a halo.
Altocumulus and Altostratus
The cloud layer gradually thickens and lowers to form layers of Altocumulus and Altostratus.
When this stage is reached the rain is not far away. Soon the sun is lost to view, covered by thick Altostratus from which rain begins to fall.
Whilst the cloud remains like this, the rain continues; but when Fractostratus appears below it, clearance can be expected. In the late spring, summer and early autumn, the warm sector usually brings breaks in the cloud and sunny periods.
At the colder times of the year the sky usually remains covered by Stratocumulus.
In less vigorous depressions, the cold front passes with little rain to give a clearing sky.
When the cold air current behind the depression is very much colder than the warm sector, the cold front is more vigorous and passes with a line of heavy showers and sometimes thunderstorms to give a clearance.