Clouds...

A "mackerel sky" adds to a striking sunset scene in Grimsby, Lincolnshire, England. Ripples of altocumulus or cirrocumulus clouds—resembling the markings of a king mackerel—give the phenomenon its name. Fluffy altocumulus undulatus clouds stain the sky red above a row of trees. These mid-level clouds, which form above 6,500 feet (2,000 meters), often herald an approaching storm. San Francisco's Golden Gate Bridge appears to float on a sea of surrounding clouds. Such low-level clouds, called stratus clouds, occur below 6,500 feet (2,000 meters) and often bring rain or snow. Clouds that touch the ground are known as fog. The summit of Washington's Mount Rainier lies hidden beneath a stack of horizontally layered lenticular clouds. These clouds are formed by high winds blowing over rough terrain and are sometimes described as a "stack of pancakes."  The beauty of a Tassajara, California, sunset is enhanced by towering cumulonimbus clouds—but they will likely spoil the serene scene. These tallest of all clouds often produce violent storms of rain, thunder, lightning, hail, and high winds. Cumulonimbus clouds are the highest of all clouds and may soar above 50,000 feet (15,000 meters). Their distinctive anvil-top shapes occur because of the high lateral winds encountered at such altitudes. The bright lights of Tucson, Arizona, are more than matched by a flash of lightning far above the city skyline. The sunset scene shows a classic cumulonimbus cloud formation. Cumulonimbus clouds, which are associated with severe storms, build in the skies above Mitchellton, Saskatchewan, Canada. Clouds form when warm, humid air cools enough that water vapor condenses around tiny particles to form water droplets or ice crystals. Colorful clouds and calm water create a striking sunset scene for canoeists on Winisk River in Ontario, Canada. A single cloud may hold billions of pounds of water—but not all clouds bring rain. A horizontal bank of cloud cover enhances an aerial view of Crater Lake, Oregon. Clouds appear white because they reflect sunlight.