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Water on the Earth evaporates (turns into an invisible gas) and rises up into the sky. Higher up where the air is colder, the water condenses: it changes from a gas to drops of water or crystals of ice. We see these drops of water as clouds. The drops fall back down to earth as rain, and then the water evaporates again. This is called the "water cycle".
The atmosphere always has a little bit of water vapor that humans can not see. Clouds form when the atmosphere can no longer hold all the invisible air vapor.[page needed] Any more water vapor condenses into very small water drops. That is what a cloud is.
Warm air holds more water vapor than cool air. So if warm air with lots of water inside cools, it can form a cloud. These are ways air can cool enough to form clouds:
- when it is cooled by the ground at night, making a fog;
- along weather fronts where warmer air is cooled as it runs into colder air;
- when air goes up the side of a mountain and cools as it goes higher;
- when warm air goes over something colder (such as cool water in a lake).
Clouds are heavy. The water in a cloud can have a mass of several million tons. Every cubic metre (m3) of the cloud has only about 5 grams of water in it. Cloud droplets are also about 1000 times heavier than evaporated water, so they are much heavier than air. They do not fall, but stay in the air, because there is warm air all round the heavier water droplets. When water changes from gas to droplets, this makes heat. Because the droplets are very small, they "stick" to the warm air.
Sometimes, clouds appear to be brilliant colors at sunrise or sunset. This has shown to be due to pollution in the air.
Clouds are classified according to how they look and how high the base of the cloud is in the sky. This system was suggested in 1802. There are different sorts of clouds because the air where they form can be still or moving forward or up and down at different speeds. Very thick clouds with large enough water droplets can make rain or snow, and the biggest clouds can make thunder and lightning.
There are three basic types of clouds based on how they look:
- Stratus clouds are like flat sheets. They may be low-level clouds, medium-level (with alto added to the name), high-level (with cirro added to the name), or medium-thick vertical clouds that make rain or snow (with nimbo added to the name).
- Cumulus clouds are puffly, lumpy, or wavy. They may be low-level clouds (with strato added to the name), medium-level (alto), or high-level (with cirro added). Larger cumulus clouds may be medium-vertical (nothing added to the name), or towering vertical clouds (with towering or nimbus added to the name). When a towering vertical cumulus cloud grows into a thunderstorm, it is called a cumulonimbus cloud.
- Cirrus clouds are high and thin. The air is very cold at high levels, so these clouds are made of ice crystals instead of water droplets. Cirrus clouds are sometimes called mares' tails because they look like the tails of a horse.
The following is a summary of the main cloud types arranged by how high they form:
High clouds will form from 10,000 to 25,000 ft (3,000 to 8,000 m) in cold places, 16,500 to 40,000 ft (5,000 to 12,000 m) in mild regions and 20,000 to 60,000 ft (6,000 to 18,000 m) in the very hot tropics.
High-level clouds include:
- Cirrus (Ci)
- Cirrocumulus (Cc)
- Cirrostratus (Cs)
Middle clouds usually form at 6,500 ft (2,000 m) in colder areas. However, they may form as high as 25,000 ft (8,000 m) in the tropics where it's very warm all year.
Medium-level clouds include:
- Altocumulus (Ac)
- Altostratus (As)
Low-level stratus clouds are usually seen near ground level. When very low stratus cloud touches the ground, it is called fog. Cumulus clouds are usually higher and can be found as high as 6,500 ft (2,000 m) and sometimes higher. They may rise into the medium level when the air is very dry. Medium-level cumulus that forms in this way does not have alto added to its name. Layers of very flat cumulus are called stratocumulus.
Low-level clouds include:
- Stratocumulus (Sc)
- Stratus (St)
These are clouds of medium thickness that can form anywhere from near ground level to as high as 10,000 ft (3,000 m). The tops of these clouds are usually not much higher than 20,000 ft (6,000 m). Vertical clouds often create rain and snow. They are made mostly of water droplets, but when they push up through cold higher levels they may also have ice crystals.
Moderate-vertical clouds include:
- Cumulus (Cu)
- Nimbostratus (Ns)
These clouds are very tall with tops usually higher than 20,000 ft (6,000 m). They can create heavy rain and snow showers. Cumulonimbus, the biggest clouds of all, can also produce thunderstorms. These clouds are mostly made of water droplets, but the tops of very large cumulonimbus clouds are often made mostly of ice crystals.
Towering-vertical clouds include:
- Towering cumulus (Tcu)
- Cumulonimbus (Cb)
Clouds and cloud bow above the Pacific Ocean.
As a sign
- Oard, Michael (1997). The Weather Book. P.O. Box 126, Green Forest, AR 72638: Master Books. .
- "NWS JetStream MAX - The Structure of the Ionosphere". srh.weather.gov. http://www.srh.weather.gov/srh/jetstream/synoptic/clouds_max.htm. Retrieved 22 April 2010.
- "Plymouth State Meteorology Program Cloud Boutique". vortex.plymouth.edu. http://vortex.plymouth.edu/clouds.html/. Retrieved 22 April 2010.
- "BadMeteorology's explanation of why clouds form". ems.psu.edu. http://www.ems.psu.edu/~fraser/Bad/BadClouds.html. Retrieved 22 April 2010.
- "Photographs and information about clouds". chitambo.com. http://www.chitambo.com/clouds/. Retrieved 22 April 2010.
- "Cloud Fraction : Global Maps". earthobservatory.nasa.gov. http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_CLD_FR#. Retrieved 22 April 2010.