A cloud is a visible collection of minute liquid water droplets, tiny ice crystals, or a combination of both, suspended within the Earth’s atmosphere. While the air surrounding these particles is a gas, and invisible water vapor is also a gas, the visible component of a cloud consists of water in its liquid or solid state. This distinguishes clouds from purely gaseous phenomena, as the visible mass is formed by condensed water.
Water’s Changing Forms in Clouds
Water in the atmosphere exists as invisible water vapor, which is a gas. Clouds are composed of countless extremely small liquid water droplets or microscopic ice crystals. These tiny particles scatter light, making the cloud visible.
The state of water within a cloud is determined by temperature. In warmer atmospheric conditions and at lower altitudes, clouds consist of liquid water droplets. As temperatures decrease with increasing altitude or in colder environments, clouds are more likely to contain ice crystals. Clouds can also be a mixture of both liquid droplets and ice crystals, particularly in transitional temperature zones.
These water droplets are incredibly small, making them subvisible individually. Ice crystals, found at higher, colder levels, can be slightly larger. The air, a gas, surrounds these suspended liquid and solid particles, but it is the suspended water particles that create the cloud’s visible appearance.
Cloud Formation and Suspension
Clouds begin to form when invisible water vapor cools and transforms into visible liquid water droplets or ice crystals. This process, known as condensation or deposition, requires microscopic particles for the water vapor to condense or deposit upon. These particles, such as dust, pollen, or salt, are called condensation nuclei.
Air typically rises in the atmosphere due to convection. As this air rises, it expands and cools due to lower atmospheric pressure at higher altitudes. When the rising air cools to its dew point—the temperature at which it becomes saturated with water vapor—the water vapor begins to condense onto the available condensation nuclei, forming the tiny liquid droplets or ice crystals that constitute a cloud.
These incredibly small water droplets or ice crystals remain suspended in the atmosphere. Their minute size means they encounter significant air resistance relative to their weight. This air resistance, combined with constant updrafts or rising air currents within the cloud, helps to balance the force of gravity, keeping the cloud particles aloft. While cloud particles have a slow downward velocity, the continuous upward movement of air within and around the cloud effectively counteracts this descent, allowing clouds to appear to float.