Cloud

A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity (clouds can also occur as masses of material in interstellar space, where they are called interstellar clouds and nebulae.) The branch of meteorology in which clouds are studied is nephology or cloud physics.

On Earth the condensing substance is typically water vapor, which forms small droplets of ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths: they thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance of the clouds at their base. Thin clouds may appear to have acquired the colour of their environment or background, and clouds illuminated by non-white light, such as during sunrise or sunset, may be coloured accordingly. In the near-infrared range, clouds would appear darker because the water that constitutes the cloud droplets strongly absorbs solar radiation at those wavelengths.
Condensation

As air parcels cool due to expansion of the rising air mass, water vapor begins to condense on condensation nuclei such as dust, ice, and salt. This process forms clouds. Sometimes, an elevated portion of a frontal zone forced broad areas of lift, which form clouds decks such as altostratus or cirrostratus. Stratus is a stable cloud deck which tends to form when a cool stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions. Clouds can also be formed due to lifting over mountains and other topography.[1]

Classification of clouds

Clouds are divided into two general categories: layered and convective. These are named distinguish the cloud's altitude. Clouds are classified by the cloud base height, not the cloud top. This system was proposed by Luke Howard in 1802 in a presentation to the Askesian Society.