Red sky at night...

Red sky at night, shepherds delight
Red sky in the mroning, shepherds warning

This old saying has been around for a very long time. The first written version of it can be found in the Wyclif Bible published in 1395, and Shakepeare made reference to it in Venus & Adonis in 1593. Is there any truth in it? Apparently, yes! ... in the UK at least...
It's all to do with how we see sunlight. Imagine a rainbow. Sunlight is broken into that spectrum of colours as it passes through the atmosphere. The blue/violet end of the spectrum is diverted more than the red/orange (which is why we see the sky as blue).
When the sun is low in the sky, at dawn and at dusk, sunlight travels through more atmosphere than at other times of the day. The red wavelength is better able to go on a direct course and be reflected back off clouds, whereas the blue light is more scattered before reaching the cloud and so is less visible. We see the clouds as red because the light that is reaching them is primarily red.
The wind in the UK comes primarily from the west. The sun rises in the east and sets in the west. If there is broken cloud in the morning then, looking to the west, we may see red light reflecting back from the cloud, so "red sky in the morning". As the clouds are coming towards us, there must be a chance of rain.
If we see red clouds in the evening they will be in the east and have already passed us by, so a good chance of clear skies and fine weather ahead.
I found all this out when I googled "red sky ay night" to see where the poem came from - amazing what you learn through Blip, isn't it? :)