Cloud

The color of a cloud tells much about what is going on inside the cloud. Clouds form when comparatively warm air containing water vapor is lighter than its surrounding air and this causes it to rise. As it rises it cools and the vapor condenses out of the air as micro-droplets. These tiny particles of water are relatively densely packed, and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color. If the cloud is sufficiently large, and the droplets within are spaced far enough apart, it may be that a proportion of the light which enters the cloud is not reflected back out before it is absorbed This process of reflection/absorption is what leads to the range of cloud color from white through grey through black. For the same reason, the undersides of large clouds and heavy overcasts appear various degrees of grey; little light is being reflected or transmitted back to the observer.Other colors occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red and yellow are at the long end. The short rays are more simply scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-sized droplets in the cloud.A more ominous color is the one seen commonly by severe weather observers. A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud which shows green is a pretty sure sign of imminent heavy rain, hail, strong winds and possible tornados. Yellowish clouds are rare, but may occur in the late spring throughout early fall months during forest fire season. The yellow color is due to the presence of smoke. Red, orange and pink clouds occur almost wholly at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere itself. The clouds themselves are not that color; they are merely reflecting the long rays of sunlight which are predominant at those hours. The effect is much the same as if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads, this can produce blood-red clouds.