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A cloud is a visible mass of droplets, in other words, little drops of water or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology. It is composed for more than 20° of gas. On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm (0.00039 in) 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 at the cloud base. Thin clouds may appear to have acquired the color of their environment or background and clouds illuminated by non-white light, such as during sunrise or sunset, may appear colored accordingly. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.



Cloud types are divided into two general categories: layered and convective. These names distinguish a cloud’s altitude. Clouds are classified by the base height, not the cloud top, and bases may differ depending on the geographical zone. This system was proposed in 1802, when it was presented to the Askesian Society by Luke Howard.


High clouds (Family A)


High clouds will form between 10,000 and 25,000 ft (3,000 and 8,000 m) in the polar regions, 16,500 and 40,000 ft (5,000 and 12,000 m) in the temperate regions and 20,000 and 60,000 ft (6,000 and 18,000 m) in the tropical region.

  • Cirrocumulus (Cc)
  • Cirrus (Ci)
  • Cirrostratus (Cs)


Middle clouds (Family B)


Middle clouds tend to form at 6,500 ft (2,000 m) but may form at heights up to 13,000 ft (4,000 m), 23,000 ft (7,000 m) or 25,000 ft (8,000 m) depending on the region. Nimbostratus clouds are sometimes included with the middle clouds.

  • Altostratus (As)
  • Altocumulus (Ac)


Low clouds (Family C)


These are found up to 6,500 ft (2,000 m) and include the stratus (dense and grey). When stratus clouds contact the ground, they are called fog.

Clouds in Family C include:

  • Cumulus (Cu)
    • Cumulus humilis
    • Cumulus mediocris
  • Stratocumulus (Sc)
  • Nimbostratus (Ns)
  • Stratus (St)


Vertical clouds (Family D)


These clouds can have strong up-currents, rise far above their bases and form at many heights.

Clouds in Family D include:

  • Cumulonimbus (associated with heavy precipitation and thunderstorms) (Cb)
    • Cumulonimbus calvus
    • Cumulonimbus incus
    • Cumulonimbus with mammatus
  • Cumulus (Cu)
    • Cumulus congestus (TCu)
  • Pyrocumulus


Other clouds

A few clouds can be found above the troposphere; these include noctilucent and polar stratospheric clouds (or nacreous clouds), which occur in the mesosphere and stratosphere respectively.

Some clouds form as a consequence of interactions with specific geographical features. Perhaps the strangest geographically-specific cloud in the world is Morning Glory, a rolling cylindrical cloud which appears unpredictably over the Gulf of Carpentaria in Northern Australia. Associated with a powerful “ripple” in the atmosphere, the cloud may be “surfed” in glider aircraft.



6 Thoughts to “CLOUDS”

  1. Pattie.Rose

    Cool wildlife photo

  2. Rose

    Really fantastic photography

  3. Gerri

    Just like looking up at heaven! Love your work!

  4. Alan S. Hochman

    Thanks Gerri, I appreciate the comment.

  5. grant hilton-barber [savanaman]

    Hi Alan

    Clouds have to be the most underrated natural phenomena , don’t they ?!!

    We look at them daily mostly without a second thought but I reckon they remain one of natures most spectacular displays.

    Thanks for connecting on twitter and I will look forward to going through your website, looks great !


  6. Alan S. Hochman

    Hi Grant
    I totally agree with you on clouds! Definitely underrated

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