“Red sky at night, shepherds’ delight. Red sky in the morning, sailors’ warning.”
So goes one version of the old saying based on generations of observations of farmers and seafarers. A saying that was first documented in the Bible although probably in use long before that. In Matthew (ch. 16 v. 2),
When it is evening, ye say, It will be fair weather; for the sky is red.
And in the morning, It will be stormy today, for the sky is red and threatening.
There is some scientific truth behind the saying. It is actually the clouds that are red and appear because the morning sun’s rays shine through a clear sky to the east which has dust and small particles are trapped in it by high pressure. The dust scatters blue light and leaving only red to shine on the clouds. Assuming this high-pressure system has come to us from the west (as is most likely will have since that is the prevailing wind direction) it has already passed us and wet and windy weather will surely follow.
This photo was taken about 0715 on Cliff Rigg, by 0900 it was raining.
Similarly, a red sky in the evening means that high pressure has yet to arrive with us from the west so therefore the next day will, most likely, be dry and fair.