It occurred to me that the fluffy cumulus clouds were white but the rainclouds were looking ominously dark. Previous discussion on this blog has noted that the water particles scatter all wavelengths of light equally well. This adds up to white light and the scattering effect means that what we see is ill-defined. I was thinking that rainclouds would have bigger droplets and so more effective scattering. I've read online that raindrops might absorb light but that the main thing is that rainclouds are thicker so scattering effectively blocks the sunlight coming down from above. Not much light comes to our eyes from the bottom so they look dark.
