Cirrus clouds form at higher altitudes and consist entirely of ice crystals. They appear as long, thin, wispy streamers that people often call "mare's tails." Cirrus clouds do not produce precipitation, and are associated with fair weather.
Cirrocumulus clouds are high-altitude clouds comprising long rows of small, rounded puffs. These puffs are about the width of your little finger when held at arm's length. They tend to form in cold weather, and may be white or gray in color. When cirrocumulus clouds cover a large area, they resemble the scales of a mackerel. Therefore, this cloud formation is known as a "mackerel sky."
Cirrostratus clouds comprise a thin layer of clouds that typically covers the whole sky. They are usually thin enough that the sun or moon is visible through them. When this happens, ice crystals in the clouds may cause a halo to appear around the light source. Cirrostratus clouds may indicate rain or snow, particularly if lower clouds are in the area.
Altocumulus clouds form at medium heights. They are puffy gray and white, and can be about 3,000 feet thick. The gray and white areas result from variations in the cloud's thickness. Altocumulus clouds are about the width of your thumb held at arm's length. They tend to form in warm, humid weather and can be precursors of thunderstorms.
Altostratus clouds are medium-height clouds that appear as a layer covering most or all of the sky. They are gray or bluish gray, and thick enough to obscure or hide the sun and moon. Altostratus clouds are forerunners of rain or snow. When precipitation falls from an altostratus cloud, it is classified as a nimbostratus cloud.
Stratus and stratocumulus clouds are low-level clouds. Stratus clouds are generally widespread and gray in color, like fog that is not reaching the ground. Stratocumulus clouds are lumpy and gray; they look larger than altocumulus clouds because they are lower in the atmosphere. Stratocumulus clouds may form in lines, or have a scattered appearance. Light precipitation is associated with both of these cloud types.
Cumulus and cumulonimbus clouds demonstrate considerable vertical development. Cumulus clouds look like fluffy cotton balls, and can signify either good weather or coming rain showers. Cumulonimbus clouds have the potential to produce thunderstorms with lightning, rain, hail and strong winds (including tornadoes). Both cloud types have a flat base and distinct outlines. The tops of cumulus clouds typically look like a cauliflower head, while cumulonimbus clouds have a distinctive anvil-shaped top. Cumulus clouds can reach 3,000 feet in height; the tops of cumulonimbus clouds may be 30,000 feet or higher.
Lenticular clouds form when wind flows over the ridge of a mountain. Water vapor condenses as the air rises over the ridge, and then evaporates as it travels downward. This forms a stationary lenticular cloud on the downwind side. The shape of the cloud may resemble a flying saucer, or the lens of a magnifying glass. Kelvin-Helmholtz clouds also form when air flows over a mountain, but the mechanism is different. In this case, the condensation results from turbulence as air currents with different speeds and trajectories interact. Kelvin-Helmholtz clouds visually represent the wave forms that result from these interactions.
Mammatus clouds got their name from the fact they can resemble women's breasts. They can also look like melons or tennis balls. They usually form at the base of cumulonimbus clouds, and indicate severe weather.
Contrails are artificial clouds formed when the water vapor from jet engines condenses and freezes around the particles in the jet's exhaust flow. Contrails remain in the air as human-created cirrus clouds until the ice crystals melt and evaporate. Humid air results in larger and longer-lasting contrails.