The adage 'Red sky at night, shepherd's delight; red sky in the morning, shepherd's warning'has high-powered support. Jesus mentions it in the Bible (Matthew 16, verses 2 and 3), and it appears in Shakespeare's epic poem Venus And Adonis. But how does it work?
Remember that the sky is blue. This is because the air scatters bluer light from the Sun more than it scatters light of longer, redder wavelengths. So if you see a red sky at sunset, it means that sunlight has travelled a long way in the atmosphere.This shows that the air is very clear to the west, where the Sun sets. Because most weather patterns move from west to east, that means that calm weather is probably on its way to you.
But if you have a red sky at dawn, the clear air must be to the east. This means that the calm weather has passed. It is a good bet that conditions for shepherds will soon worsen as bad weather arrives from the west to replace it.