Stratus clouds are low, gray rainclouds that cover almost the entire sky. Fog is a dangerous stratus cloud that reduces visibility on the ground; thick fog is blinding. Stratus clouds produce rain and snow. Stratocumulus clouds are other low-level clouds that generally appear white and only produce a light drizzle. Nimbostratus clouds are dark gray in appearance, have ragged edges and produce heavy rain and snow but not electrical storms.
Cumulonimbus clouds are made of cumulus clouds that develop vertically, gradually swelling with water vapor until they burst with precipitation. Called "thunderstorm clouds," cumulonimbus clouds lie low in the sky -- below 6,500 feet, but their cauliflower-shaped towers can grow as high as 16 km -- and produce electrical storms. They are very large and anvil-shaped by high-altitude winds, and the direction in which the anvil is pointing is indicative of the direction in which the cloud is moving.
Air moving upwardly because it is hotter than the surrounding air is called unstable air. When this combines with water vapor, cumulonimbus clouds form and thunderstorms are the result. Thunderstorms are famous for hail and lightning as well as heavy rains.
Air that is forced upward by a mountain or hillside creates thunderstorms that appear to wrap around mountaintops. These thunderstorms are called orographic thunderstorms. Air mass thunderstorms are formed when the sun heats a large amount of air, typically over flat ground. The heat and moisture combine to form those well-known "late afternoon" thunderstorms.
Frontal thunderstorms occur along front lines -- large moving quantities of hot or cold air. These lines of intense thunderstorms are called squall lines.
The most dangerous cloud is the funnel-shaped tornado. These violent funnel clouds form at the ends of cumulonimbus cloud-laden "supercell" thunderstorms. Tornadoes form from heated air that rises very rapidly and collides with cooler wind. The wind at 1,000 feet moves much more slowly than the wind at 5,000 feet. This creates a horizontal rotating column of air in between the two air masses. When the heated updraft runs into the column, it rotates vertically, creating a rapidly spinning mass of air that can reach speeds of 318 mph and destroy anything it touches on the ground.