Why can clouds appear extremely dark when they are approaching yet lighten up when they are above us?
The colour that clouds appear to be has more to do with their type than their location. Clouds laden with abundant moisture, i.e. formed with many droplets, will tend to be relatively dark since water droplets in the cloud will absorb light passing through it. Clouds such as cirrus that we see as thin streaks high in the sky, or cirrocumulus, that we see as small, round white puffs, contain relatively little moisture. In fact, cirrus clouds are almost exclusively composed of ice crystals. Clouds such as altocumulus, also known as middle clouds, are composed mostly of water droplets that appear as grey, puffy masses. Another fact determining the appearance of clouds is their position relative to the viewer and the sun. When the sun is visible, an approaching thunderstorm may look especially dark because of the contrast between the bright foreground lit by the sun and the dark background where the storm is located. When the storm begins to obscure the sun, the storm cloud may not appear
