Blue Sky
The full background blue image on this page is a photograph taken on Feb 29, 2024 at 11:00 am EST of a very clear sky pointing the camera (a Samsung S23 Ultra in standard photo mode)
high in the sky ~ 70° above the horizon and in a direction away from the sun. A quick "color-picker" grab from the image using a basic image editing
application provides a crude estimate of the RGB (red/green/blue) color intensities in the light typical of a very clear blue sky. In this case of course the
value will change across the image (even though it looks very uniform in the image).
Since the image color captured and processed by a
digital camera can vary quite a bit due to sensor variations in different cameras and digital processing in picture synthesis,
the RGB numbers will vary. Nevertheless, it is interesting to get an idea of the relative amount of red, green and blue components present in blue-sky images.
A typical decimal RGB value (max value 255) is:
RGB = (42, 95, 187)
or in the hexadecimal number system (max value FF):
RGB = (#2A, 5F, BB) in hex numbers.
This means that the intensity of the green component (95) is about half of the blue component value 187, and the red component in turn is nearly half as strong as the green component. (The photometric RGB system involves averaging over 3 different wavelength ranges to "weight" the light distribution in wavelength to get the
3-color component RGB values).
The blue-sky effect is now well known and understand, dating back to John Tyndall's 1869 pioneering experimental demonstrations of color in scattered light. This was followed
closely by Lord Rayleigh's 1881 theoretical calculations demonstrating a strong wavelength dependence of light scattering from air molecules of 1/λ4,
explaining why blue light (shorter wavelength) is scattered more strongly than other colors of the fairly uniform continuous visible color spectrum of solar light impinging on our atmosphere. A simple estimate of the ratio of scattered light intensity for typical blue (0.44nm) and red (0.65nm) wavelengths using
1/λ4 gives 0.21, in good agreement with the RGB ratio above of 42/187 ~ 0.22.
Conversely, the color-dependent scattering law also explains why direct sunlight propagating through thick atmosphere (sunrise or sunset) is reddish, and more
so with atmospheric pollution.
Rayleigh's calculations leveraged James Clerk Maxwell's theoretical foundation (Maxwell's Equations 1865) for
the electromagnetic wave nature of light. It is amazing how closely in time, a mere 20 years, these advances were made.
For excellent overall descriptions of sunlight and blue-sky physics see: