What causes clouds to appear blue?

Question

In a thunderstorm cloud about sunset time, I saw these clouds, including some (in the upper right) that were a unique shade of blue. I don't think I've seen clouds quite that color before. I tried to capture the color as best I could with a digital camera, though the blue is slightly washed out in the image. I took these images facing East at sunset time, during summer. There were rivers (that you could probably throw a rock over if you had a good arm) in the general vicinity, but no large bodies of water such as lakes or oceans.

Optically zooming in on the blue cloud:

This answer suggests that the sunlight should be missing those wavelengths ("at sunset, when only the red/yellow end of the spectrum hits") which is consistent with the brilliant warm colors to the left. But...the cloud was a strong and unique shade of blue, different from the blue behind it. How does this happen? That is, what natural processes underly the occurrence?

(Although that question is on Physics.SE, there seem to be more questions about cloud appearances on this site.)

Answer 1

The true reflected colour of a cloud doesn't change from white unless there is serious pollution, such as Saharan dust, wildfire soot or industrial emissions. None of these produce a blue colour, so the question really becomes 'what reflected light looks blue', and that is a function of the direction of view (towards or away from the sun), the subject's perception of colour (which can be surprisingly deceptive), and how much atmospheric blue back-scatter the cloud is reflecting. The latter accounts for most of the 'blue cloud' that we see.

Answer 2

The cloud is inherently white/gray (it's clear water droplets). It's shadowed from the red light to the west, but exposed to the blue light from the rest of the (apparently mostly clear) sky, so it is mostly illuminated with blue light, and that's what it will reflect.

As another answer observed, there are also perceptual effects. Your eyes aren't colorimeters; put a gray object next to an orange expanse, and it will look bluish. In a photo, the effect may be less pronounced; that's why the photo looks "washed out" to you in comparison to your memory.

Answer 3

The color of a cloud depends primarily upon the color of the light it receives. The Earth's natural source of light is the sun which provides 'white' light. White light combines all of the colors in the 'visible spectrum', which is the range of colors we can see.

Each color in the visible spectrum represents electromagnetic waves of differing lengths. The colors change as the wavelength increases from violet to indigo to blue, green, yellow, orange, red and deep red.

Visible light is only a small portion of the full electromagnetic spectrum. As a light wave's length increases, its energy decreases. This means the light waves that make up violets, indigo and blue have higher energy levels than the yellow, orange and red.

One way to see the colors of sunlight is by the use of a prism. The velocity of light decreases slightly as it moves into the prism, causing it to bend slightly. This is called refraction. The degree of refraction varies with the energy level each wave.

Answer 4

I have seen blue clouds, but not in connection with storms. They come about in some circumstances when the Sun is about to rise* but the local weather is overcast.

What happens is this: when the Sun is approaching the horizon, scattered light from the sky filters through the clouds and this filtered light appears dark blue, most notably over water away from city lights. But this indirect filtered light is faint; it will get obscured when the Sun rises and the filtered white light coming directly through the clouds takes over. The clouds gray out.

*I live in a city where a largw body of water is to the East, so the effect is easier for me to see at dawn than at dusk.