Colours in the Sky

We’ve all been stopped in our tracks by spectacular sun rises and sets. Gradually changing from yellow, to bright orange, and finally culminating in a fiery red ball, the surrounding mountains seem to glow. Although it borders on heresy to attempt to break down a sunset into simple physics, there’s a symphony going on behind the scenes.

Interesting to note is that the sun looks yellow at all – pictures from space show a white and not a yellow sun. When light travels through the atmosphere, it comes into contact with a materials that scatter and filter out certain parts of the spectrum. These include atoms of oxygen and nitrogen. As light moves through the atmosphere, the blue end of the spectrum is gradually filtered out leaving a predominance of yellow light. This causes the sun to appear yellow from the earth's surface. As the sun travels greater distances through the atmosphere, more and more of the blue light will be removed. If it travels far enough, only the red end of the spectrum will remain visible. From overhead (i.e. at noon), the light only travels far enough to provide a yellow sun.

As day slips into evening, a number of things change. The most important change can be found in the angle of the sun. As it drops down lower in the sky, the light beams must now skim along the surface of the planet rather than shine straight down on us. As a result, the light travels much further through the atmosphere and more of the blue spectrum is scattered – slowly changing the colour to orange and then red.

Of course there's much more to a good sunset than just a red ball of light. The sunset varies greatly in intensity depending upon a number of factors. If there is a lot of water moisture or dust in the air, the entire sky begins to take on the same colour as the globe of the sun itself. This is often enhanced by some cloud cover (as long as it doesn't obscure the show). These factors lead to the endless variety of sunsets experienced over the years.

This scattering of blue light also results in another atmospheric phenomenon – a blue sky. When we look upwards, we are actually seeing the scattered blue light – the light that didn't reach the earth's surface. It may seem hard to believe that the same process that causes the sunset to be red can also cause the sky to appear blue, but that is precisely what happens.

All Material © Ward Cameron 2005