A cloud is a visible mass of tiny liquid water droplets or frozen ice crystals suspended in the atmosphere. The temperature inside a cloud is highly variable, depending on its altitude, the surrounding air, and the physical state of the water it contains. The coldness of a cloud is determined by the atmospheric layer in which it forms, which dictates the environmental temperature. Clouds can range from above the freezing point to temperatures colder than a home freezer.
How Altitude Determines Cloud Temperature
The primary mechanism that makes clouds cold is the decrease in air temperature with increasing height, known as the atmospheric lapse rate. As a parcel of air rises, it enters a region of lower atmospheric pressure. This drop causes the air to expand, which requires energy.
Because the rising air parcel does not exchange heat with its surroundings quickly, this expansion process causes the air’s internal energy to decrease, resulting in a temperature drop. This process, called adiabatic cooling, is why the higher a cloud forms, the colder its temperature will be. For unsaturated air, the temperature drops by approximately 5.4°F (9.8°C) for every 3,280 feet (1,000 meters) of ascent.
Once the air cools enough for water vapor to condense and form a cloud, the process changes slightly. Condensation releases latent heat, which slows the rate of cooling as the air continues to rise. The rate of cooling in a saturated, cloud-filled air parcel is known as the moist adiabatic lapse rate, which is typically between 2°F and 5°F (3.6°C to 9.2°C) per 1,000 feet. A cloud’s temperature is nearly identical to the temperature of the surrounding air at that specific altitude.
Temperature Ranges for Different Cloud Types
Cloud classification is often based on altitude, which serves as a good predictor of the cloud’s temperature range and composition. Low-level clouds, which form up to about 6,500 feet (2,000 meters), are considered “warm clouds.” These clouds, such as Stratus and Cumulus, usually exist above 32°F (0°C) and are composed primarily of liquid water droplets.
Mid-level clouds, with bases ranging from 6,500 feet to 20,000 feet (2,000 to 7,000 meters), are often “mixed-phase.” Clouds like Altostratus and Altocumulus typically exist between 32°F (0°C) and -40°F (-40°C). This range means they can contain a combination of liquid water droplets and ice crystals.
High-level clouds, such as Cirrus, Cirrostratus, and Cirrocumulus, form above 20,000 feet (6,000 meters) where temperatures are consistently frigid. These clouds are composed entirely of ice crystals, as the ambient temperature is often well below -40°F (-40°C). The tops of tall cumulonimbus clouds can reach temperatures as low as -60°F or colder.
Supercooled Water and Ice Formation
Supercooled water is a unique phenomenon in cloud physics, referring to liquid water that remains unfrozen below 32°F (0°C). Pure water droplets can persist in a liquid state down to nearly -40°F (-40°C) because they lack a solid surface on which to freeze. This supercooled water is common in mid-level clouds and the upper parts of tall low-level clouds.
The freezing of supercooled water requires a process called ice nucleation. If the temperature drops to about -40°F, the water freezes spontaneously through homogeneous nucleation. At warmer temperatures, a solid particle, known as an ice nucleus, is necessary to trigger freezing, a process called heterogeneous nucleation.
Ice nuclei are microscopic particles, such as mineral dust, pollen, or certain bacteria, that provide a template for water molecules to arrange into an ice crystal structure. Once freezing begins, the ice crystals grow rapidly at the expense of the surrounding supercooled water droplets. This growth occurs via the Bergeron process, where water vapor deposits onto the ice, causing liquid droplets to evaporate and transferring mass to the ice crystals.