Stephen Hall-Jones of Dunedin, asks :-
Elizabeth Andrews, of Christchurch, asked:-
Does the old saying "Red sky at night Shepherd's delight: red sky in the morning Shepherd's warning" have any basis?
Recently we had a superb red sky in the morning followed later in the day by a violent storm. Does the old saying "Red sky in the morning, sailor's warning" have a basis in fact?
Erick Brenstrum, a physicist with MetService and the author of "The New Zealand Weather Book", responded.
Clouds take on red and gold colours at sunrise and sunset because light is scattered in all directions by molecules of gas in the atmosphere. Blue light is scattered to a greater extent than the other colours because it has a shorter wavelength. This is why the sky appears blue during the day in directions away from the sun.
At sunrise and sunset light enters the atmosphere at a long slant and has to pass through a much greater depth of the atmosphere than at midday. As it does so, much of the blue light is scattered away sideways leaving mostly red and yellow colours in the direct beam from the sun.
The folk saying "Red sky at night Shepherd's delight: red sky in the morning Shepherd's warning" forecasts fine weather to follow a colourful sunset but rain to follow a colourful sunrise. In mid-latitude countries, such as England and New Zealand, most of the weather systems come from the west.
For a colourful sunset to occur the sunlight needs clear skies to the west in order to reach the clouds over New Zealand. These clear skies would then move over the country during the next day bringing fine weather. For a colourful sunrise clear skies are necessary to the east of New Zealand so that sunlight can reach us. In which case the clouds over New Zealand are likely to be the leading edge of the next rain bearing weather system approaching from the west.
This forecasting method does work sometimes in New Zealand, but in eastern areas like Otago and Canterbury rain approaching from the west often gets stopped by the Southern Alps and it remains dry despite a brilliant sunrise.