Low clouds Stratus clouds are uniform grayish clouds that often cover the sky. Usually no precipitation falls from stratus clouds, but they may drizzle. When a thick fog “lifts,” the resulting clouds are low stratus. Nimbostratus clouds form a dark gray, “wet” looking cloudy layer associated with continuously falling rain or snow. They often produce light to moderate precipitation.
Middle clouds Clouds with the prefix “alto” are middle-level clouds that have bases at 6,500 to 23,000 feet up. Altocumulus clouds are made of water droplets and appear as gray, puffy masses, sometimes rolled out in parallel waves or bands. These clouds on a warm, humid summer morning often mean thunderstorms by late afternoon. Altostratus clouds, gray or blue-gray, are made up of ice crystals and water droplets. They usually cover the sky. In thinner areas of them, the sun may be dimly visible as a round disk. Altostratus clouds often form ahead of storms that produce continuous precipitation.
High clouds Cirrus clouds are thin, wispy clouds blown by high winds into long streamers. They are considered “high clouds,” forming at more than 20,000 feet. They usually move across the sky from west to east and generally mean fair to pleasant weather. Cirrostratus, thin, sheetlike clouds that often cover the sky, are so thin the sun and moon can be seen through them. Cirrocumulus clouds appear as small, rounded white puffs. Small ripples in the cirrocumulus sometimes resemble the scales of a fish, creating what is sometimes called a “mackerel sky.”
Vertical clouds Cumulus clouds are puffy and can look like floating cotton. The base of each is often flat and may be only 330 feet above ground. The top has rounded towers. When the top resembles a cauliflower head, it is called “cumulus congestus.” These grow upward and if they continue to grow vertically can develop into a giant cumulonimbus, a thunderstorm cloud, with dark bases no more than 1,000 feet above ground and extending to more than 39,000 feet. Tremendous energy is released by condensation of water vapor in a cumulonimbus. Lightning, thunder and violent tornadoes are associated with them.
