Red sky in the morning....

...is the shepherd's warning (or the sailor's warning, according to your inclination).  In other words, it is a harbinger of bad weather.  In full, the old weather saying goes:

Red sky at night, shepherd's delight;
Red sky in the morning, shepherd's warning. 


The following explanation was found on the website of the Met (Meteorological) Office:

Why does red sky appear at sunrise and sunset?

The saying is most reliable when weather systems predominantly come from the west as they do in the UK. "Red sky at night, shepherds delight" can often be proven true, since red sky at night means fair weather is generally headed towards you.

A red sky appears when dust and small particles are trapped in the atmosphere by high pressure. This scatters blue light and leaving only red light to give the sky its notable appearance.

A red sky at sunset means high pressure is moving in from the west so therefore the next day will usually be dry and pleasant. "Red sky in the morning, shepherds warning" means a red sky appears due to the high pressure weather system having already moved east meaning the good weather has passed, most likely making way for a wet and windy low pressure system.