The Science of a Sunset

August 1, 2016 | By More


Red sky at night
Sailor’s delight
Red sky in the morning
Sailors take warning

This ancient rhyme is so old that it predates the Bible, but did you know it has a scientific explanation? Why do red skies at night indicate calm seas, while red skies in the morning forecast squalls and stormy weather?

To understand why the adage holds true, first we need to understand what makes a sunset red. As the sun’s light travels through the atmosphere, oxygen and nitrogen act as a prism, splitting the white light of the sun into separate rays of different colors. This is called Rayleigh scattering. The molecules in nitrogen and oxygen are best at scattering blue light, but they have little effect on the longer wavelengths of red light. When the sun is at the horizon, Rayleigh scattering is most pronounced, and when the sky is free of dust and other pollutants, the sky turns a beautiful golden color. In certain conditions, atmospheric dust scatters the yellow hues and the sun shines a deep red color. In other words, light traveling through a dusty atmosphere often appears red by the time it reaches our eyes.

What do red sunsets have to do with predicting weather? They are caused by high and low pressure weather systems, and weather travels in patterns of low and high pressure. High pressure systems are associated with fair skies and low chance for precipitation while low pressure brings squalls and other stormy conditions. Calm, high pressure systems tend to trap atmospheric dust near the surface while low pressure systems indicate cleaner skies. High pressure sunsets are often a brilliant red while low pressure sunsets are a golden-yellow.

Weather in our part of the world travels from west to east. When the setting sun gives off a spectacular red light, it is a telltale sign that a high pressure system is coming in and bringing along fair weather. Likewise, a red sky in the morning shows a high pressure system to the east. Since low pressure systems always follow a high pressure system, you can expect stormy weather to follow.

With this knowledge in hand, you can use weather forecasts to predict when the most vivid sunsets (and sunrises) will take place. This is useful knowledge for a variety of situations, from photography to dating.

Of course, other elements contribute to the perfect sunset. Humidity and smog can dull a sunset. The most vivid colors occur in dry places far from polluted cities. Desert sunsets can bathe the sky in light. Winter skies with lower humidity also produce glorious sunsets.

The lower the sun is on the horizon, the further light has to travel through the atmosphere to meet our eyes, and the more pronounced the ray scattering effect is. This means that valleys are not ideal for seeing those spectacular colors, since the apparent horizon is so elevated. On the other hand, sunsets from atop a mountain are much more vivid.

For the most spectacular sunsets, the sun needs a canvas to paint with light. Clouds are that canvas. Nothing can compare to golden rays of light, illuminating the thin, wispy strands of a cirrus cloud. Thin clouds, at mid to high altitudes tend to be the best for reflecting light while heftier clouds can sometimes appear dark.

In summary, the best sunsets have the following elements: an impending high pressure system, low smog, low humidity, and lots of wispy clouds. These sunsets are also best seen from an elevated vantage point.

If you like, there are even websites that predict colorful sunsets for you. One site,, uses NOAA weather models to forecast brilliant sunsets.

Happy hunting!l


Category: Outdoors

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