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CINDY DAY: Spectrum of sunset colours

What a sight!  Timothy O'Brien snapped this photo just before the sun dipped below the horizon along the Nova Scotia/New Brunswick border. The photo was taken from the Amherst side. "This gorgeous sunset was capping off a beautiful spring day," O'Brien says.
What a sight! Timothy O'Brien snapped this photo just before the sun dipped below the horizon along the Nova Scotia/New Brunswick border. The photo was taken from the Amherst side.

Red sky at night is quite a delight, but what if it’s yellow or pink? We are spoiled with some spectacular sunsets across Atlantic Canada. They can be as changeable as the weather, but can they be forecast? They can if you think about what causes the rotating colour palette.

Cindy Day
Cindy Day

The colours of the sunset result from a phenomenon called Rayleigh scattering. Small particles and water molecules can change the direction of light rays, causing them to scatter. The colours are determined by the wavelength of the light and the size of the particles. The short-wavelength blue and violet are scattered by molecules in the air much more than other colours of the spectrum. This is why blue and violet light reaches our eyes from all directions on a clear day and it’s why the sky is blue!

At sunrise and sunset, when the sun is low on the horizon, sunlight passes through more air than it does when it’s high in the sky; more atmosphere equals more molecules. If the path is long enough, all of the blue and violet light will scatter out and the other colours will reach your eyes, and that’s why sunsets are often yellow, orange and red.

Red has the longest wavelength of any visible light, so the sun appears red when it’s on the horizon, where its extremely long path through the atmosphere blocks all other colours.

What makes a really great sunset is the right amount of water vapour, wispy clouds, and even a few particles that direct that scattered light back toward our eyes. At sunset, the angles and amounts of light scattered from the sun to a particular spot in the sky change very rapidly. Clouds can go from white to brilliantly coloured to dull grey in just a few minutes.

Check out the beauty of the late evening sky and be sure to snap a few photos. You can share them with all of us by sending your photos to Weathermail@weatherbyday.ca

Cindy Day is the chief meteorologist for SaltWire Network.

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