This spectacular red sunrise observed yesterday morning over London has a scientific explanation. The same explanation also lends some support to the well-known piece of folklore saying that a red sky at night is a shepherd's delight, whereas a red sky in the morning is a sailor's warning.
Rays of light from the sun come in all colours of the spectrum but some colours or wavelengths are easily scattered by tiny particles in the atmosphere. On a cloudless day at midday, for instance, a large amount of the incoming blue and violet wavelengths are scattered by the abundant oxygen and nitrogen of the air, making the sky look blue.
At sunset and sunrise, the solar radiation has to pass through a thicker chunk of the atmosphere to reach the observer on the ground. This results in an even greater selective scattering of blue and violet wavelengths of light, resulting in a proportionately greater amount of the redder wavelengths reaching the ground – hence the red skies at dawn and dusk.
The more particles there are in the air, the redder the sunsets and sunrises. Pollutants and aerosols from a volcanic eruption are particularly good at making redder skies at the start and end of the day.
In Britain, weather systems typically move from west to east. So a red sky in the morning suggests that there are clearer skies to the east shining on the moisture-laden clouds coming in from the west – hence the sailor's warning.
At sunset, conversely, a red sky suggests clearer conditions to the west, where the weather tends to come from, which is likely to mean fine weather the following day – good news for shepherds tending their flocks.
Like all predictions about the British weather, however, this one from folklore is subject to the vagaries of natural variability. The saying may prove to be better than a 50:50 guess at predicting the coming day's weather, but probably not that much better.