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CINDY DAY: Debunking a meteorological myth

Earlier in April Sheila Edwards noticed this intriguing spot of green in the late day sky over Eastern Passage, N.S.
Earlier in April Sheila Edwards noticed this intriguing spot of green in the late day sky over Eastern Passage, N.S. - Contributed

While I was on vacation and completely off the grid, an email came in from Gordon Bryce of Dartmouth, N.S. Mr. Bryce wanted to know more about the elusive “green flash.”

That’s not an easy one to explain, so I put it aside. About 100 emails later, I came across a photo taken last week by Sheila Edwards in Eastern Passage, N.S. It showed a flash of green at sunset. Could we be so lucky?

People, mostly seafaring folk, have told tales of the green flash for centuries. Many people believe it’s just a myth, but it is real. Green flashes and green rays are meteorological optical phenomena that sometimes occur just after sunset or right before sunrise.

Green flashes occur when the Earth's atmosphere causes the light from the sun to separate out into different colours; this happens because the air is not uniform. The air above the ground is made up of invisible layers at different temperatures and with different densities. Refraction of the sun’s rays causes the colours to separate.

The various colours of light bend different amounts based on their wavelengths; shorter wavelengths like blue, violet and green refract more strongly than longer wavelengths like yellow, orange and red. That means, in most cases, blue and violet light are scattered by the atmosphere while red, orange and yellow are absorbed, leaving green light the most visible during the few seconds when the sun sets below or rises above the horizon.

There is one more piece to this puzzle: the atmospheric mirage. Green flashes are enhanced by mirages, which increase refraction.

There are four categories of green flashes but nearly all of the sightings fall into one of these two: inferior mirage or mock mirage.

Inferior mirage flashes are oval and flat and occur close to sea level when the surface of the water is warmer than the air above it.

Mock mirage flashes, on the other hand, occur higher up in the sky and when conditions on the surface are colder than the air above. The flashes appear to be thin, pointy strips being sliced from the sun, and they last about one to two seconds.

In both cases, the flash occurs just after the sun has dipped below the horizon or just before it appears above it. Unfortunately, that rules out Shelia’s green light.

While there isn’t an optimal condition that will guarantee a green flash sighting, the best way to potentially observe one is to go somewhere that provides a clear view of the horizon and is free of pollution, such as over the ocean. A green flash is more likely to be seen in stable, clear air, when more of the light from the setting sun reaches the observer without being scattered.

Be sure to check out tomorrow’s Weather University column where I will attempt to explain the green glow that Sheila saw over Eastern Passage, N.S.

Cindy Day is the chief meteorologist for SaltWire Network.


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