Light reflected, scattered and transmitted by constituent particles determines the luminance of a cloud. This light comes, for the most part, directly from the luminary (Sun or Moon or stars) or from the sky; it may also come from the surface of the Earth, which is particularly strong when ice fields, snowfields or water bodies reflect sunlight or moonlight.
The luminance of a cloud may be modified by intervening haze. When haze is present between the observer and the cloud, it may, depending on its thickness and the direction of the incident light, either diminish or increase the luminance of the cloud. Haze also diminishes the contrast that reveals the shape, structure and texture of the cloud. Luminance may also be modified by optical phenomena such as haloes, rainbows, coronae, glories, etc.
During the day, the luminance of clouds is sufficiently high to make them easily observable. On a moonlit night, clouds are visible when the Moon is more than a quarter full. In its darker phases, the Moon is not bright enough to reveal clouds far from it, especially when the clouds are thin. On a moonless night, clouds are generally invisible; sometimes, however, their presence may be deduced from the obscuration of stars (noting that stars near the horizon can be obscured due to haze), of polar aurora, of zodiacal light etc.
Clouds are visible at night in areas with sufficiently strong artificial lighting. Over large cities, clouds may be revealed by direct illumination from below. An artificially illuminated layer of clouds may provide a bright background against which lower cloud fragments stand out.
When a slightly opaque cloud is illuminated from behind, its luminance is at a maximum in the direction of the luminary. It decreases away from the luminary; the thinner the cloud, the more rapid the decrease. Clouds of greater optical thickness (measure of the degree to which the cloud prevents light from passing through) show only a slight decrease in luminance with distance from the luminary. Still greater thickness and opacity make it impossible even to determine the position of the luminary. When the Sun or Moon is behind a dense isolated cloud, the latter has a brilliantly illuminated border, and luminous streaks alternating with shadowed bands may be seen around it.
As the optical thickness of a cloud layer often varies from one portion of the layer to another, the luminary may be perceptible through one part of the cloud but not through another. The varying optical thickness and the luminance of the cloud layer, especially at short angular distances from the Sun or Moon, may change considerably with time owing to the movement of the cloud.
In the case of a uniformly and sufficiently opaque cloud layer, the luminary may be perceptible when it is not too far from the zenith, but may be completely masked when close to the horizon. Sufficiently opaque cloud layers sometimes show maximum luminance at the zenith when the Sun or Moon is at a low elevation.
The light reflected from a cloud to the observer is at a maximum when the cloud is opposite the luminary. The luminance is stronger when the denseness of the cloud and its thickness in the line of sight are greater. When sufficiently dense and deep, the cloud reveals shades of grey showing a more or less clear relief; the more tangential the direction of illumination, the more extensive the range of shades.
Finally, appreciable differences in luminance exist between clouds composed of water droplets and ice crystals. Ice crystal clouds are usually more transparent than water droplet clouds owing to their thinness and to the sparseness of the ice particles. However, certain ice crystal clouds, occur in thick patches and can have a high concentration of ice particles. When these clouds are illuminated from behind, they show marked shading. They are, however, brilliantly white in reflected light.