“Red sky at night, sailors’ delight. Red sky in the morning, sailors’ warning.” Long before weather apps, people forecast the weather by looking up, and this rhyme is one of the most famous pieces of weather lore there is. So is it true? Within limits, yes. It has real science behind it, but it only works in certain parts of the world, and that includes some of Australia but not all of it.
A 2,000-year-old forecast
The saying is very old. A version of it appears in the Bible, in Matthew 16:2-3, where Jesus says that people predict fair weather when the evening sky is red, and stormy weather when the morning sky is red and threatening, as the Library of Congress notes. Shakespeare used the idea too, describing a red morning as a sign of wreck to the seaman and tempest to the field in his poem Venus and Adonis. Over time, the rhyme took different forms; in some places it is shepherds, not sailors, who are delighted or warned.
What does a red sky at night, or in the morning, mean?
Two things make the saying useful. The first is the direction the weather comes from. In the mid-latitudes, roughly 30 to 60 degrees from the equator, the prevailing winds blow from west to east, so weather systems usually arrive from the west and leave towards the east, as NOAA meteorologist Joe Sienkiewicz explains in Scientific American. The second is where the Sun is. It sets in the west and rises in the east.
So at sunset, you are looking west, towards the weather that is coming. A red sunset needs a reasonably clear path for the Sun’s light to reach you and light up the clouds, so a vivid red evening sky suggests that the air to the west is fairly clear and dry, often under high pressure, and fair weather is on its way. At sunrise, you are looking east, towards the weather that has already passed. A red morning sky often means the Sun is lighting up the high clouds of an approaching weather system from underneath, a sign that cloud, and possibly rain, is moving in behind it.
Why the sky turns red at all
The colour itself comes from the same physics that makes the sky blue. When the Sun is low, its light travels through a much longer stretch of atmosphere to reach you, and most of the blue light is scattered out along the way, leaving mainly the oranges and reds, as I explain in why is the sky blue? Dust, smoke and moisture in the air can make those colours even richer. Clouds matter too: the most spectacular red skies usually happen when high clouds are lit from below by a low Sun, with clear sky in the direction of the Sun so that its light can reach them.
Does it work in Australia?
The rhyme was born in places where weather reliably moves from west to east, and not all of Australia is like that. That pattern holds in the mid-latitudes, roughly 30 to 60 degrees from the equator in both hemispheres, as MeatEater’s fact check points out. In Australia, that takes in Perth, Adelaide, Melbourne, Hobart and Sydney, where cold fronts and weather systems regularly sweep in from the west, so the old rhyme can work reasonably well there.
The further north you go, the less reliable it becomes. The tropical north, including Darwin and Cairns, sits in the zone of the trade winds, which blow mostly from the east, so weather does not reliably come from the direction of the sunset at all. Subtropical areas such as south-east Queensland sit near the edge, where the pattern is more hit and miss, and summer storms often build up locally in the afternoon rather than marching in from the west. Even in the mid-latitudes, the saying is only a rule of thumb; Sienkiewicz notes that if weather is moving from south to north, as sometimes happens, it does not hold. So it is a charming piece of folklore, and a decent rough guide in southern Australia, but it is no substitute for the Bureau of Meteorology’s forecast.
Other weather sayings with some truth in them
Red skies are not the only weather lore with a basis in science. A ring, or halo, around the Sun or Moon is caused by light shining through high, thin ice-crystal cloud, which often arrives ahead of an approaching weather front, so “ring around the moon, rain soon” can have something to it. Similarly, high wispy clouds known as mares’ tails, and the rippled pattern called a mackerel sky, often signal a change in the weather in the next day or so, as the UK’s Met Office explains. Like the red sky rhyme, they work best as hints, not guarantees.
A rhyme with a reason
So red sky at night, sailors’ delight, is more than an old wives’ tale. Where weather comes from the west, a red sunset hints at clearer skies on the way, and a red sunrise hints that the weather is about to turn. Just remember that it was made for the mid-latitudes, so it works better in Hobart than in Darwin. If you want to know why sunsets turn red in the first place, see why is the sky blue?, and for what grey, rainy days do to your energy levels, see why does rain make you sleepy?
Sources
- Library of Congress, Everyday Mysteries: Is red sky at night, sailor’s delight true?
- Scientific American: Is there scientific validity to the saying red sky at night?
- MeatEater: Fact checker, is red sky at night, sailors’ delight accurate?
- Met Office: Red sky at night and other weather lore
- Wikipedia: Red sky at morning