Like most of us, meteorologists recognize that hovering, slate gray clouds are a sure sign that rainy weather is approaching and that white, puffy clouds signal a clear and pleasant day ahead. What the local weatherman knows that most of us may not is why some clouds are light and others dark, and what the cloud colors indicate.
Clouds are composed of tiny droplets of water that have condensed on particles present in our atmosphere. Though water droplets are basically transparent, they scatter sunlight which produces the pearly white cloud color we see most often in the sky.
As clouds accumulate more moisture, the space between droplets becomes larger and larger, permitting more light to penetrate deeper into the cloud. This causes light to be absorbed rather than reflected, making the cloud appear darker. The amount of light absorbed versus light reflected is what produces clouds that range from bright white to almost black.
Clouds often take on a blue-gray hue when light is scattered by rain droplets. They can turn an ominous green when light is scattered through ice, and greenish clouds often signal heavy rain and hail, as well as a potential tornado. Large quantities of smoke in the air produce yellowish clouds and are often the result of a forest fire.
Although clouds may appear pink, red, or orange at sunrise or sunset, they don’t actually change color. They simply reflect the long, reddish rays of sunlight that are predominant at those times of day and thus produce a beautiful backdrop for the beginning or ending of our day.