Images by John 'K'. "Flat bottomed clouds". Flickr.com, March 4, 2009.
Why are clouds bright white?
You know those fluffy white clouds that look like wooly sheep or whipped cream? Why are they so white?
When I type in "why are clouds white?" into a Google search, the top 10 links all agree that the whiteness is due to the scattering of light by water droplets within the cloud.
I would politely like to disagree. My goal in this article is to show you why this does not make sense.
There is a better explanation, a simpler one. Of course, the caveat is that I am not a meteorologist. But, read on, and see what you agree with.
Clouds are white because disorganized air bubbles produce the white color
First: A lot of what we see as white is due to trapped air bubbles
Drinks can be colored; however, shake them up or carbonate them, and they will develop a white froth. The first picture shows a beer head that is pure white, even though the beer is amber.
Water is clear (unless it is deep; then it is blue). On the other hand, whitewater, wave crests, frothy saliva, and frozen water molecules with entrapped air (ice or snow) will appear white. The second picture shows a crab frothing at the mouth as the air from its gills mixes with water.
Many animals and birds have white camouflage. The examples include: Polar bears, Ermines, arctic foxes and hares, and myna birds. They do this as their fur and feathers are hollow and entrap air bubbles. The third picture shows a polar bear with white fur. The fourth shows a white, wooly sheep.
Of note, not all white color in nature is due to entrapped air. Read more in this course.
Martin Garrido. Beer. Flickr.com, Dec 27, 2011.
Better Than Bacon. Polar bear in California. Apparently their hairs aren't white, they're actually translucent but hollow to better insulate, Flickr.com, Aug 4, 2007.
Crab foaming "at the mouth":devra . Spittling crab in Morro State Park Marina. Flickr.com, July 5, 2010.
David Masters. Happy Sheep. One of the sheep at Styal Country Park, Cheshire. Flickr.com, Feb 15, 2009.
Clouds are dark when water absorbs light
Second: Larger water droplets eat up the light --> darkness
Next, let's take the instance of dark or grey clouds, such as nimbostratus or cumulonimbus clouds. As soon as the sky darkens above us, we instinctively look for cover. The darkening signifies a heavy downpour.
Rain clouds are thicker and deeper. For example, a nimbostratus cloud often develops from the higher alto stratus cloud as it encounters a warm front. The excess warm water vapor creates increasing condensation within the cloud. This in turn encourages the formation of larger water droplets.
As the cloud gets heavier, it lowers itself down in the Troposphere. The air mass of the lower Troposphere pushes upwards against the lower surfaces of the cloud. This added pressure increases the condensation of water vapor into droplets. The droplets in the cloud grow even larger.
This is probably the point at which the sky starts darkening. You can see in the third picture how altostratus clouds converge into nimbostratus clouds.
At this point, the water droplets are large enough to absorb the light creating a dark and foreboding presence. Thus, the heavily-laden water clouds are dark; the light and airy clouds (cirrus clouds) very high in the troposphere are pure white.
Maxwell Hamilton. Unusual Clouds; Altostratus undulatus, Flickr.com, March 30, 2015.
Paulo O. Storm Clouds Over Fairfax. Flickr.com, April 27, 2011.
Nicholas A. Tonelli. Convergence, Nimbostratus streaks, Easton. Flickr.com, March 24, 2007.
Snow and ice without air appear clear or blue
Third: Add air to snow and ice ------ whiteness
This is last argument to support the fact that clouds are white because of the disorganized air bubbles. Ice cubes without air bubbles are clear as shown in the second image below (it took some effort to do this at home, but I had help from here).
An ice cube produced in a regular freezer with somewhat cold temperatures will freeze with air bubbles within it. It will look as if you have trapped a miniature cloud or fog within it.
However, if the ice is thick or deep, it appears blue. Just like ocean water looks blue. While snow looks white because of the air trapped between the snowflakes, packed snow will lose the air and appear blue.
Hijab, J. Ice cubes with trapped air bubbles, 2018.
Hijab, J. White snow on deck, 2018.
Hijab, J. Clear ice cubes (no air bubbles), 2018.
anoldent. Blue ice cave, 2000 New Zealand 108. Flickr.com, September 23, 2008.
Conclusion
In summary, it does not matter how much water/ice the cloud has. As long as there is a mass of disorganized air bubbles at the forefront of the cloud, the cloud will appear white.
Thus, the tops of clouds - seen from an airplane - appear very white. That is the edge of the cloud with an expanding air mass. The bottom of a storm cloud will appear dark or grey; that is where water droplets are getting larger and weighing down the cloud. The dense number of large water drops absorb light creating the darkness effect.
Vladimer Shioshvili. Clouds in Dusheti, Georgia, Flickr.com, May 21, 2006.