A cloud shadow occurring at sunset is formed when light is cast from a
low level cloud onto a
higher level cloud. During a sunset, the suns rays will sometimes be blocked by a cumulus cloud below the horizon. Like any shadow, a darkened area will form. These types of cloud shadows are especially dramatic and can appear as darkened rays of light, sometimes referred to as anti-crepuscular rays. An example of a cloud shadow is shown in the photo above.
Like any shadow, the size of the shadow changes with the size of the cloud and the time of day. In fact, seeing a dark and ominous cumulonimbus storm cloud is another way of seeing a cloud shadow. The darkness of the cloud itself at the base is an indication that sunlight is not penetrating through the thickness of the cloud.
