Red sky at night
Shepherd's delight
Red sky in the morning
Shepherd's warning

Traditional English Saying - in the US it's often sailor's delight/warning.

There is actually more than a grain of truth in this centuries-old piece of meteorological poetry. The fact that the colour of the sky in the evening and morning can be a predictor of weather to come was known in Biblical times. In fact an evening reddish hue is caused by the sunlight's path through high-level atmospheric dust particles. During wet or windy weather these particles get absorbed into rainclouds and so a red sky at night is more likely prior to a fine day. A red sky at dawn would be caused by the same effect, but few people are around at sunrise except in midwinter. If the reddish colour persists for any length of time after the sun has risen then the effect is most probably caused by refraction through ice particles and water droplets in the atmosphere: a sure sign that rain is on its way.