This article was first published in The Skeptical Inquirer.
As it turns out, this apothegm dates at least back to biblical times. In Matthew 16:1–3, Jesus replies to the Pharisees and Sadducees, “When evening comes, you say, ‘It will be fair weather, for the sky is red,’ and in the morning, ‘Today it will be stormy, for the sky is red and overcast.’ You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times.” While indeed not the point of his speech, what this statement tells me is that this familiar meteorological maxim was already in the common consciousness. While our instinct might be to reject most knowledge that hasn’t been updated in millennia, it turns out that the people of Judea were largely correct.
The basic tenet of this saying is that a red sunset means good weather the next day, whereas a red sunrise means lousy weather for that day. Why the sky has any color at all is due to a phenomenon called Raleigh scattering. The light from the sun hits the particles of our atmosphere, causing much of it to scatter across the sky. Only about one-third of light is scattered, but the smallest wavelengths tend to do so more easily. We get a blue sky because these short wavelengths correspond to blue hues. At sunset and sunrise, the angle at which sunlight enters the atmosphere is significantly changed, and light must travel through many more atmospheric particles to reach us. As a result, most of the shorter blue and green wavelengths are scattered before reaching the lower atmosphere, meaning we see more of the orange and red colors in the sky.
A particularly red sunset or sunrise is typically the result of an atmosphere charged with more particles. According to the UK Meteorological Office, this tends to happen during systems of high pressure, which are associated with clear skies. Since the weather in the United Kingdom, United States, and Canada tends to travel from west to east, a red sunset in the west means that a high-pressure system, a.k.a. clear weather, is incoming. On the other hand, a red sky in the east means that a high-pressure system has just passed, meaning that a low-pressure system, a.k.a. cloudy or stormy weather, is on its way.
This adage wouldn’t hold in places where the weather isn’t mainly from the west and can’t account for the amount of time that good or bad weather will take to arrive. And it definitely shouldn’t be used to override any professional forecast info. But if you’re without your weather app, then looking to the skies at dawn and dusk will give you a decent indication of what’s to come weather-wise.
