Are clouds cold? The answer is generally yes, but the temperature within a cloud can vary dramatically from well below freezing to above the melting point of ice. A cloud is a visible mass of tiny liquid water droplets, ice crystals, or a mixture of both, suspended in the atmosphere. The temperature of this suspended matter determines a cloud’s structure, composition, and ability to produce precipitation.
The Mechanism of Cloud Cooling
Clouds are cold because the air that forms them cools as it rises through the atmosphere. This cooling happens through adiabatic cooling, a process where temperature changes without heat being added or removed. As air ascends, atmospheric pressure decreases, causing the air parcel to expand. This expansion draws energy from the air’s internal thermal energy, causing its temperature to drop. The cooling continues until the air reaches its dew point, causing water vapor to condense onto microscopic particles called cloud condensation nuclei, forming the cloud’s water droplets or ice crystals.
Classifying Clouds by Internal Temperature
Meteorologists classify clouds based on their internal temperature relative to the freezing point of water, \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). Warm Clouds exist entirely at temperatures above freezing throughout their vertical extent. They are composed exclusively of liquid water droplets and are most common in tropical and subtropical regions.
In contrast, Cold Clouds have at least some portion, typically the upper part, below the freezing point. These clouds often contain a mixture of ice crystals and liquid water droplets, particularly between \(0^\circ\text{C}\) and \(-20^\circ\text{C}\). Liquid water existing below freezing is known as supercooled water, which remains liquid because it lacks the necessary ice nuclei to initiate freezing. At temperatures below about \(-40^\circ\text{C}\), clouds become fully glaciated, composed entirely of ice crystals.
How Cloud Temperature Determines Precipitation Type
The temperature and composition of a cloud directly control the physical process by which precipitation forms. In warm clouds, which are solely liquid, the primary mechanism is the collision-coalescence process. Larger water droplets fall faster than smaller ones and collide, merging to form progressively bigger droplets that eventually fall as rain.
In cold clouds, the Bergeron process (or ice-crystal process) is the dominant mechanism. This process relies on the coexistence of supercooled water droplets and ice crystals. Water vapor rapidly deposits onto the ice crystals, causing them to grow quickly at the expense of the surrounding supercooled droplets. These growing ice crystals become heavy enough to fall, and the final type of precipitation is determined by the temperature profile of the air column below the cloud.