Red Skies in the morning threaten with bad weather. Red Skies in the evening are a sign of fine weather. This is one of the best — known sayings about the weather. And in most cases this rule fits, especially if the expression “Red Skies means the intensive lighting of clouds by the setting or rising sun. In our latitudes the weather is mainly ruled by westerly winds. A beautiful red sky in the evening can only be generated when the sky in the west is clear while there are still a lot of clouds in the east which may be the remnants of a precipitation area that is moving away. During the night the clouds often draw off to the east, followed by a sunny day. In the morning it‘s exactly the other way around:
Then the sun is in the east where the sky must still be clear, but there are already clouds coming up from the west. While the day goes on, the clouds are going to cover the whole sky.
The red colour of the clouds is caused by the scattering of the sunlight by the different constituents of the atmosphere. The air molecules scatter the blue light more effectively than the red part of the sunlight. This causes the orange-like colour of the setting or rising sun. But to generate an intensively red sky, there also have to be lots of water droplets in the air. Not only water droplets but also dust particles (maybe from a volcanic eruption) can contribute to colourful twilight effects.