The darkest layer

Article 9 Module 3

Fluffy cumulus clouds

Fluffy cumulus clouds

The basic make up of clouds

To understand why we have white as opposed to gray clouds, let us go back to the basic makeup of clouds:

Clouds are a mixture of cooled down (and connected) water vapor molecules, microscopic water droplets, and air.

It is the different ratios of those elements that create the varying colors and shapes of clouds.

Keep in mind that there are secondary elements that also play an important role in cloud formation/structure. 

These include dust, soot, airplane exhaust, and even bacteria.

Clouds, because of the typical sequential development described in the last lesson, will generally have layers.  

Also, there is a gradient of temperatures and pressures from the bottom of the cloud to the top.

Clouds above sand dunes

Clouds above sand dunes

This separates clouds into roughly three layers:

  1. A bottom most layer, with relatively higher temperatures and pressures (the darkest layer).
  2. A middle layer that is colder, with lots of liquid droplets, interspersed with air pockets (the white/grey layer) 
  3. A topmost layer with little water droplets and lots of cooled connected water vapor molecules that entrap air pockets (the whitest layer)

Why don't clouds sink lower?

Note that even heavy nimbostratus clouds are still less dense than the dry air that holds them up (water vapor decreases the overall density of the cloud).

That is why clouds look as if there is a large invisible hand stopping them from going lower down in the sky.

Stratus clouds - rain

Nimbostratus clouds - rain

The Darkest Layer

The bottom-most layer in the cloud is at the forefront as it receives the rising warm and humid air. This crams more and more water vapor molecules into the base of the cloud.

Many of these condense into microscopic liquid droplets using microscopic specks (cloud condensation nuclei).

This creates a relatively airless cloud base that is stacked full of liquid water droplets suspended in a netting of connected water vapor molecules.

Such cloud bases look moist and heavy - and dark.  

Clouds with dark bottoms acquire the prefix of "nimbo", such that one can have nimbocumulus and nimbostratus (rain) clouds.

Nimbus comes from Latin meaning cloud or halo, from classical Mythology, surrounding a deity when on earth. 

 The reason the clouds are dark at their base is because the mass of water water droplets absorbs green and red lights, just as any large body of water.

However, the chaos within the base of the cloud also prevents blue light from escaping. The net result is that the dense base of the cloud looks dark and sometimes even black.  

Lakes and oceans can look black:

This is the same reason lakes and oceans look black in rough weather; the turbulence of the water creates too much internal chaos for the light to reflect back to our eyes. Notice what happens when the water becomes calm: it reflects a blue color once again.


There are more liquid water droplets

than air pockets in dark clouds. 

Light is absorbed by the water in the cloud

and also gets bounced around between the 

different interfaces within the cloud.

As little light comes out, the cloud

looks dark, even black.  

Picture credits: 

  1. Nicholas A. Tonelli. Imminent Gathering storm clouds, Walpack Valley, Sussex County, within the Delaware Water Gap National Recreation Area. Flickr photo-sharing. Taken on May 20, 2006.
  2. Nicholas A. Tonelli. Fluffy Midday cumulus clouds near Carroll, Clinton County. Flickr photo-sharing. Taken on July 31, 2010.
  3. David. The Great Sand Dunes National Park Colorado. Flickr photo-sharing. Taken June 21, 2008.
  4. Frank Boston. Before the Rain. Flickr - photo sharing. June 28, 2010.