Re: How do we see colours in clouds?

There are several processes by which clouds can appear colored. The most familiar occurs when sunlight is reflected inside small spherical droplets of water (rain or mist). Refraction occurs as the light enters and leaving the raindrop, splitting the light into a spectrum which you see as a rainbow.

Sunlight can also be refracted through hexagonal needle-shaped ice crystals in high-altitude or wintertime clouds. This makes a (sometimes colored) halo around the sun, usually 22 degrees away from the sun. Often, air currents in the clouds cause the needles to point straight up and down rather than randomly. When this happens, only the parts of the halo directly to the left and right of the sun are visible: these are called "sun dogs".

Finally, sunlight can diffract through cloud droplets. Imagine ripples moving over calm water toward a rock, pier piling, or other obstruction. The obstruction will scatter ripples in all directions. Water droplets and haze particles will scatter light waves in the same way. When the particles are much smaller than the wavelength, short waves are scattered most strongly. This is what makes the sky blue. When the waves are roughly the same size as the wavelength, the behavior is described by Mie scattering theory. The behavior is very complex, with scattering being a complicated function of direction and wavelength. Waves of a size closely matching the particles are strongly scattered, while waves slightly larger or smaller pass through almost unchanged.

I suspect what you observed was some sort of Mie scattering. Usually, water droplets in clouds are rather large (20-30 microns) compared to a wavelength of light (.3 -.5 microns), so the Mie scattering spectrum says that all wavelengths are reflected equally. This makes clouds appear white. To form cloud droplets, water vapor must condense around tiny dust particles called 'condensation nuclei'. Usually, there's lots of water vapor but not much dust, so the drops are large.

Forest fire smoke puts lots and lots of these dust particles in the air. If dust increases but amount of vapor stays the same, the average particle size will be smaller. Smaller particles are closer to the size of a wavelength of light, so the Mie scattering will be intense and wavelength dependent. Some wavelengths (colors) will be strongly scattered, others will pass right through to your eyes. Parts of the cloud with different droplet sizes will filter different colors. Mie scattering polarizes light strongly, so the effect will be more noticeable with polarized sunglasses.

Any fairly advanced book on optics or atmospheric radiation will have a section on Mie scattering. Be warned: the math involved is fairly high-level (calculus and differential equations)

For other readers: James Anderson, who originally asked the question, sent me a note about a website with a picture similar to what he observed: the site is at the University of Illinois, and the picture is below.