In meteorology a cloud (more commonly called cloud in scientific language) is a hydrometeora consisting of minute particles of condensed water vapor and / or ice crystals, suspended in the atmosphere thanks to updrafts or in a buoyant state and usually not in contact with the ground.Precipitation originates from clouds, one of the phases of the water cycle in the hydrosphere together with the condensation of water vapor.
The shape of some types of cloud is similar to a fractal with a typical self-similarity, while the color of the clouds is essentially due to optical phenomena of radiation-matter interaction, i.e. light-water vapor (mainly reflection, refraction and diffusion) , in turn a function of the density and thickness of the cloud. The branch of meteorology that studies clouds and related phenomena is called nephology.
Clouds are commonly referred to as masses visible above the earth's surface, but clouds also form on other planetary bodies or satellites. Cloud-like structures can also form in interstellar space, but in this case they are not clusters of water vapor droplets. In this case we speak of interstellar clouds. Clouds are representative of, and in turn cause, complex meteorological phenomena, such as rain, snow and hail. By analogy of form the term has also been extended to smoke fumes in the atmosphere or accumulations that can be of dust, sand or insects. From the point of view of thermodynamics, clouds represent the visual aspect of phenomena occurring inside a gaseous fluid known as the atmosphere; the quantities involved are: temperature, humidity, pressure.
The appearance of a cloud is determined by the nature, size, number, distribution in space of the particles and material that constitute it; it also depends on the intensity and color of the light received by the cloud and on the relative position of the observer and the light source.
Luminance is a technical term adopted for use in photometry and indicates the quotient of the intensity of light, in a given direction, and of the projection of the area of the emission surface of a plane perpendicular to that direction. In the field of clouds it is determined by the light reflected, diffused and transmitted by its constituent particles. The luminance can be modified by a haze that intervenes between the cloud itself and the observer, as well as by optical phenomena such as halos, crowns, glories, etc.
Usually the luminance of the clouds during the day allows them to be observed clearly, while during the night it is possible to observe them only when the Moon is illuminated for more than a quarter.
The clouds that form on Earth are generated by water vapor which, when condensing, forms small droplets or ice crystals, usually 0.01 mm in diameter. When clusters of billions of these droplets are formed, the cloud appears visible, of a typical white color due to the high capacity of light reflection (between 60% and 85%) on the surface of these droplets.
Due to the high dispersion of light in the droplets that make up the cloud, it can also appear gray or sometimes dark blue, almost black. The greater the density of the cloud and the greater its thickness, the darker it will appear. This is why a thunderstorm cloud, usually a cumulonimbus, appears very dark at the base.
At sunrise and sunset, the clouds can take on a color similar to that of the sky, especially orange and pink. Around the infrared wavelength, clouds would appear darker because the water that constitutes them would strongly absorb sunlight at this wavelength.
Cross classification table