A cloud is an atmospheric phenomenon often described simply as a visible mass of condensed water, but scientifically, it does not conform to any single state of matter. Rather, a cloud is a dynamic, complex mixture that simultaneously contains liquid, solid, and gaseous components. This combination makes a cloud an example of an atmospheric colloid or aerosol, where tiny particles are suspended within a gas. Understanding a cloud requires analyzing the different physical states of the water and air within its structure.
The Liquid and Solid Components
The visible part of any cloud is composed of countless microscopic particles of liquid water or solid ice. These particles represent the condensed states of matter and are responsible for scattering sunlight, which allows us to see the cloud against the blue sky. The most common components are liquid water droplets, which are incredibly small, typically ranging in diameter from about 5 to 50 micrometers. To form a single average-sized raindrop, it would take roughly one million of these cloud droplets.
The physical state of the condensed water depends largely on the temperature of the air within the cloud layer. In warmer, lower-altitude clouds, the water remains liquid, even if the temperature is slightly below freezing, a state known as supercooled water. However, in higher-altitude clouds, such as cirrus clouds, or in the upper portions of tall thunderheads, the water crystallizes into solid ice particles. These solid ice crystals can grow larger through deposition, where water vapor freezes directly onto the crystal surface.
The Essential Role of Gas
Despite the visual prominence of the liquid and solid components, the vast majority of a cloud’s volume consists of atmospheric gas, commonly known as air. This gas acts as the medium that holds the visible water and ice particles in suspension. The surrounding air is a mixture of gases, predominantly nitrogen and oxygen, and a small, but crucial, amount of invisible water vapor.
Water vapor is the gaseous state of water and serves as the source material for all cloud formation. When air rises and cools, the invisible water vapor condenses onto microscopic airborne particles, called cloud condensation nuclei, to form the visible liquid droplets. Although water vapor is present throughout the cloud, it is not what we see. If a cloud were made of only water vapor, it would be transparent. A cloud is correctly characterized as an aerosol system—a suspension of tiny liquid or solid particles dispersed within a gaseous medium.
Why Clouds Remain Suspended
The primary reason a cloud’s condensed particles do not immediately fall to Earth is their minuscule size, which dramatically increases the effect of air resistance. A cloud droplet, with a radius of just a few micrometers, has a very low mass-to-surface-area ratio. This ratio means that the drag force exerted by the air effectively counteracts the force of gravity.
As a result, these tiny particles possess an extremely low terminal velocity, which is the maximum speed an object reaches while falling through a fluid like air. For a typical cloud droplet, this descent rate is only about 60 to 120 feet per hour. This slow downward drift is easily overcome by even gentle atmospheric air currents, such as updrafts, which are rising columns of warm air. The continuous upward movement of air within the cloud system keeps the particles aloft, creating the illusion of a stationary or floating mass.