Most people view clouds from below, seeing their bright, contoured shapes against the sky, but the internal appearance is a different experience. A cloud is a visible mass of condensed moisture suspended in the atmosphere. The experience inside a cloud is similar to walking through a dense ground-level fog. This phenomenon is composed of countless microscopic particles that scatter light, creating an environment far from the clear, open space often imagined.
The Microscopic Ingredients
The physical composition of a cloud consists of a vast population of extremely small liquid water droplets, tiny ice crystals, or a combination of both. These particles form when invisible water vapor condenses in the air. For condensation to occur, the air must first cool to its saturation point, known as the dew point, which is often achieved as air rises and expands.
Water molecules require a surface to bond together and form droplets. This surface is provided by minute airborne particles called cloud condensation nuclei (CCN), which are a subset of aerosols. These nuclei, including specks of dust, pollen, sea salt, or soot, are typically about one micrometer in size, and every cloud droplet has one at its core. The resulting water droplets are microscopic, measuring under 20 micrometers (0.02 millimeters) in radius. The cloud’s visible form results from a million or more of these tiny droplets suspended in every cubic meter of air.
Navigating the Interior: Visibility and Light
The visual experience inside a cloud is characterized by a uniform field of whiteness or grayness and extremely limited visibility. When light enters the cloud, it interacts with the densely packed water droplets or ice crystals. Because the cloud particles are much larger than air molecules, they scatter all visible wavelengths of light relatively equally.
This non-selective scattering of all colors is why clouds appear white from the outside, and why the interior appears bright but featureless. The light is diffused so thoroughly that it eliminates shadows and definition, creating an environment akin to a “whiteout.” Visibility is often reduced to a few feet or less, making it impossible to see boundaries or the surrounding sky. Dense storm clouds, like cumulonimbus, create a more opaque and dark interior due to their high liquid water content and light absorption, while thin clouds allow for slightly better visibility.
Internal Conditions and Movement
Beyond the visual obscurity, the interior of a cloud is a dynamic environment marked by significant movement and temperature fluctuations. Within vertically developing clouds, strong air currents known as updrafts and downdrafts create turbulence. Air can be rising or sinking at phenomenal rates, which constantly mixes the cloud’s contents and makes the internal environment unstable.
Temperature changes rapidly with altitude, and conditions often involve supercooled water droplets. These are liquid water droplets that remain fluid even at temperatures well below freezing, reaching temperatures as low as -40°C. When these droplets strike a solid surface, such as an aircraft wing, they freeze instantly in a process known as structural icing. In large clouds, the constant collision of ice crystals and water droplets generates static electricity, leading to massive electrical discharges seen as intra-cloud lightning.