Skywatch Friday: The Colors of The Sky

Bandung's sky has been cloudy most of the time this week. It's been raining almost everyday. So, the photo for my participation in this week's Skywatch Friday meme is not exactly a photo I took this week. It is another photo that I took at the same time as the one I posted on 2 January 2009. Again, this is a straight shot or an as-is image as recorded by the camera.

Many of us already know why the sky change colors at different parts of the day (yellow, orange, amber, or red at sunrise and sunset and blue at a clear day time). For those who forget, here is a reminder:

The change in sky's colors as we perceive it (blue in a clear day time and amber or red at sunrise and sunset) takes place because of a phenomenon that physicists call "scattering", a process by which the light coming from the sun is scattered by the earth's electromagnetically-charged atmospheric particles.

Here is how it happens. The light that comes from the sun actually consists of a spectrum of lights of different wavelengths. On a clear day, blue and violet rays that have shorter wavelengths are dispersed more than the other colors in the light spectrum. Hence, we see blue sky. (The violet is barely visible because the retina of our eyes cannot perceive it as well as they do blue.) At sunrise and sunset, because the sun is low on the horizon, the sunlight has to travel a longer distance to reach our eyes and pass through more "layers" of atmospheric particles. Because of this, the blue and violet rays are exhaustively scattered to the point where they become nearly invisible to our eyes. That is why we only see longer wavelength spectrum light elements of red, orange, amber, and yellow, which are less dispersed.

The explanation for the change of sky colors at different parts of the day was first found by John Tyndall in 1859. That is why this explanation is called the Tyndall Effect.