Clouds

Abstract

Clouds consist of liquid and solid water and form as air rises to a height at which it cools below the dew point or frost point temperature, usually in the presence of abundant microscopic solid particles called condensation or freezing nuclei. Stratiform clouds are wide, thick, clouds with a layered, blanket-like appearance that tend to form a single sheet rather than separate clouds and suggest moderate instability. Cumuliform clouds are taller than wide and suggest greater instability and free convection. Clouds are also categorized by family—the height at which they form. The highest clouds are cirrus and consist largely of ice, while mid-level clouds (altus) and low clouds (stratus) consist mostly or entirely of water, respectively. The “vertical” cloud family includes those that may defy categorization into a single level because of their wide vertical extent. These range from fair-weather cumuliform clouds or “cumulus humilis” to “cumulus congestus,” to cumulonimbus—or thunderstorm clouds. Cumulonimbus clouds can have special features within them that are particularly indicative of severe weather, such as the wall cloud, mesocyclone, mammatus cloud, shelf cloud, outflow boundary, and gust front. The combination of cloud form and family yields the cloud name. Other rare but interesting cloud-like features are also described.