Red sky at night

This is what is said on the internet...

The 'red sky at night' rhyme is more than an old wives' tale though and has some meteorological foundation - in England at least.

To explain why we'll need to know why clouds sometimes appear red and how that may be used to predict the weather. Firstly, why do clouds often appear red in the morning and evening?

Sunlight is broken into the familiar rainbow spectrum of varying-wavelength colours as it passes through the atmosphere.

The blue/violet end of the spectrum is diverted more than the red/orange.

When the sun is low in the sky, at dawn and dusk, sunlight travels through more atmosphere than at other times of 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 is therefore less visible. So, we see the clouds as red as the light that is reaching them is primarily red.

...and how does that help predict the weather?

The weather in the UK comes from the west, i.e. the wind is primarily westerly.

The sun rises in the east and sets in the west.

If there is broken cloud in the morning we may look to the west and see red light reflecting back from the cloud, i.e. 'red sky in the morning'. As the clouds are coming towards us there must be a chance of rain, at least an increased chance compared with the cloudless period we had just enjoyed.

Likewise for 'red sky at night'. If we see red clouds in the evening they will be in the east and have already passed us by, giving a good chance of clear skies and fine weather ahead.

So there you go!