High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon. The most common form of high-level clouds are thin and often wispy cirrus clouds. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation. Though cirrostratus can cover the entire sky and be up to several thousand feet thick, they are relatively transparent, as the sun or the moon can easily be seen through them. Sometimes the only indication of their presence is given by an observed halo around the sun or moon.
Halos result from the refraction of light by the cloud's ice crystals.
Cirrostratus clouds, however, tend to thicken as a warm front approaches, signifying an increased production of ice crystals. Because of their lower altitudes, they are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are cold enough. Typically a portion of an altocumulus cloud is shaded, a characteristic which makes them distinguishable from the high-level cirrocumulus. Altocumulus clouds usually form by convection in an unstable layer aloft, which may result from the gradual lifting of air in advance of a cold front. The presence of altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms later in the day.