We had a spectacular sunset on May 7 and plenty of Northern Rivers residents were ready to document it.
We had a spectacular sunset on May 7 and plenty of Northern Rivers residents were ready to document it.

Why sunsets are pretty and the sky is really purple

WE had an amazing sunset last night and Northern Rivers residents from Casino to the coast pulled out their cameras to get these wonderful photos.

It got me wondering - why are some sunsets pretty? For that matter, why are they pretty at all?

A US academic, Stephen F Corfidi, who wrote a paper on this very subject, supplied the answer.

Some of it would be obvious, even to those of us without a scientific bent. At sunset (and sunrise for that matter), light is entering the atmosphere at an angle. That means it has longer to travel before it can reach our eyes and during that journey some interesting things can happen to it.

Beams of light entering the atmosphere collide with oxygen and nitrogen particles in the air and these particles "scatter" the light.

Because the particles are very small (we're talking molecules here), the shortest waves of light are the ones that are most affected by those collisions and most likely to scatter. Those are blue and purple wavelengths. For most of the day, the result of this is that the sky appears blue. It would actually appear purple except, Corfidi says, our eyes are more sensitive to blues than purples.)

When the light hits the sky at an angle at sunset and sunrise the same process happens, but the distance the light travels means the blues and purples are largely gone by the time the light reaches our eyes, so what we see are longer wavelength hues of red and orange.

The intensity of the sunset or sunrise comes down to the amount of pollution (be that artificial, such as smog, or natural such as dust in a desert) in the sky. A clear sky will generally produce more vivid sunrises and sunsets. (You can see this for yourself travelling in a passenger jet, where at cruising altitude you are in clearer air and, as a result, get more spectacular sunsets than you see on the ground.)

This means when we see a spectacular sunset, what we are actually seeing is a pocket of clear air in the western sky - or in the eastern sky at sunrise. Thin cloud layers can help boost the effect further by effectively providing an aerial projection screen to highlight the colours.

If you'd like more detail, you can read Corfidi's paper here, or an interview he did with National Geographic here.


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