Among the many different cloud types, three are responsible for most precipitation that falls to Earth: stratus, cumulus and nimbus. These clouds are capable of producing both rain and snow, often by combining with one another in hybrid formations. While some are almost exclusively associated with specific weather events such as thunderstorms, the type of precipitation that falls from a cloud is ultimately dependent upon temperature, humidity and air pressure.
All clouds are made of moisture, and regardless of the type of cloud, thousands of tiny water droplets must condense around microscopic particles of dust or smoke in order to gain enough density and fall as precipitation. If atmospheric temperatures near the Earth's surface are at or below freezing, this precipitation falls as snow. Alternatively, a phenomenon known as the Bergeron-Findeisen process causes ice crystals to actually form within the cloud itself, which then melt and fall as rain the closer they get to the Earth's surface.
Cloud types receive names based on their position in the atmosphere, their overall shape and the weather with which they are associated. Nimbus, for example, means "rain-bearing" in Latin, and is added to cloud names as a prefix or suffix when they produce precipitation of any kind. Nimbostratus clouds, for example, are typically thick, low clouds that form a dense bank and yield steady snow or rain.
Stratus: Rain and Snow
Stratus clouds are low to mid-level clouds that develop into horizontal, flat formations. Stratus is from the Latin meaning "layer," and stratus clouds can appear dark and dense or white and puffy. Storm fronts are often preceded or followed by stratus cloud formations carrying precipitation as rain or snow. Because temperatures are warmer closer to Earth and cooler higher up in the atmosphere, low-hanging stratus clouds generally bring rain while higher stratus clouds are associated with snow.
Cumulus clouds are dense and puffy vertical cloud formations that extend as high as 15,000 meters (50,000 feet) into the atmosphere. Although cumulus clouds are common on sunny, fair-weather days, they earn the moniker of thunderheads because of their tendency to produce thunderstorms. A cumulus cloud becomes a cumulonimbus cloud capable of severe thunderstorms when sufficient heat, updraft and moisture combine in the cloud to produce lightning, thunder and heavy rains.