CLOUDS -  the techie bit:
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, (from the Greek word nephos for 'cloud'). Nephology, is the study of clouds and cloud formation.

On Earth the condensing substance is typically water vapor, which forms small droplets or 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 grey 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 colored 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.

Clouds are divided into two general categories: layered and convective. These are named stratus clouds (or stratiform, the Latin stratus means "layer") and cumulus clouds (or cumuliform; cumulus means "piled up"). These two cloud types are divided into four more groups that distinguish the cloud's altitude. Clouds are classified by the cloud base height, not the cloud top. This cloud classification system was proposed by the British meteorologist researcher Luke Howard in 1802, in a presentation to the Askesian Society.

High clouds (Family A)
These generally form above 20,000 feet (6,000 m), in the cold region of the troposphere. In Polar regions, they may form as low as 16,500 ft (5,030 m); they are denoted by the prefix cirro- or cirrus. At this altitude, water frequently freezes so clouds are composed of ice crystals. The clouds tend to be wispy and are often transparent.
Clouds in Family A include:
• Cirrocumulus (Cc)
• Cirrostratus (Cs)
• Cirrus (Ci)
• Cirrus Kelvin-Helmholtz Colombia
• Cirrus uncinus
• Contrail, a long thin cloud which develops as the result of the passage of an aircraft at high altitudes.
• Pileus
Middle clouds (Family B)
These develop between 2,000 and 6,000 m (between 6,500 and 20,000 feet) and are denoted by the prefix alto-. They are made of water droplets and are frequently supercooled.
Clouds in Family B include:
• Altocumulus (Ac)
• Altocumulus castellanus
• Altocumulus lenticularis
• Altocumulus mackerel sky
• Altocumulus undulatus
• Altostratus (As)
• Altostratus undulatus
Low clouds (Family C)
These are found up to 2,000 m (6,500 feet) and include the stratus (dense and grey). When stratus clouds contact the ground, they are called fog.
Clouds in Family C include:
• Cumulus humilis (Cu)
• Cumulus mediocris (Cu)
• Nimbostratus (Ns)
• Stratocumulus (Sc)
• Stratus (St)

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