#2219 "November Sunset Stratocumulus"

The sunset is very yellow but the clouds have a strong red and pink colour to them. Hmm. Why? These cloud droplets may have been young and smaller than average but that cannot be the entire story. Both Rayleigh and Mie scattering painted my sunset view. This sunset view was from 4:40 pm on Wednesday November 21st, 2018. The dissipating stratocumulus were in the northwesterly flow behind a cold front. Art gives me a chance to practise science as well.

Rayleigh scattering is strongly dependent upon the size of the particle and the wavelengths. The intensity of the Rayleigh scattered radiation increases rapidly as the ratio of particle size to wavelength increases - the scatterers get bigger. The intensity of Rayleigh scattered radiation is also identical in the forward and reverse directions.

At sunset short wavelength blue light is scattered out of the direct beam by the long path of the light through the atmospheric molecules which are really the Rayleigh scatters. By the time the direct beam from the sun got over Singleton Lake after the long trek through the atmosphere, only long red wavelengths remained. However I needed something to scatter the remaining direct beam red light to my eye.

The Mie scattering model takes over from Rayleigh when the particle size becomes larger than around 10% of the wavelength of the radiation. Mie scattering is roughly independent of wavelength and it is larger in the forward direction than in the reverse direction. The greater the particle size, the more of the light is scattered in the forward direction - in this case toward my eye.

To finish the construction of my scene I needed the turbulent stratocumulus clouds. Some of the cloud particles might have been small enough by Rayleigh but really (humour intended) all I needed was Mie scattering to forward the some of the remaining red light in the direct beam from the sun to my waiting eyes. Note that the edges of the backlit cloud are much more red than the centers. Only a few scattering interaction are possible on the thin edges of the cloud where there are relatively few droplets to do the scattering. More Mie scatters in the bulk of the cloud jumble all of the remaining direct beam together so that the artist sees a mixture of the remaining wavelengths and simply a darker colour of cloud.

For this and much more art, click on Pixels. Thank you.