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CloudA cloud is a visible mass of condensed water droplets or ice crystals suspended in the atmosphere above Earth's surface. The condensing water vapor forms small droplets of water (typically 0.01 mm) or ice crystals that, when surrounded with billions of other droplets or crystals, are visible as clouds. Clouds reflect all visible wavelengths of light equally and are thus white, but they can appear grey or even black if they are so thick or dense that sunlight cannot pass through.
Cloud formation and propertiesClouds form in areas where moist air cools, generally by rising. This can happen
The actual form of cloud created depends on the strength of the uplift and on air stability. In unstable conditions convection dominates, creating vertically developed clouds. Stable air produces horizontally homogeneous clouds. Frontal uplift creates various cloud forms depending on the composition of the front (ana-type or kata-type warm or cold front). Orographic uplift also creates variable cloud forms depending on air stability, although cap cloud and wave clouds are specific to orographic clouds. Cloud properties (mostly, their albedo and rain-out rate) are strongly dependent on the size of the cloud droplets and the manner in which these particles coalesce. This is in turn affected by the number of cloud condensation nuclei present in the air. Because of this dependence, and lack of global climatological observations, clouds are difficult to parametrise in climate models and a bone of contention within the global warming debate. Cloud ClassificationClouds 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 cumiloform, 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. High cloudsThese form above 16,500 feet (5,000 m), in the cold region of the troposphere. They are typically denoted by the prefix cirro- or cirrus. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent.High Clouds include:
A contrail is a long thin cloud which develops as the result of the passage of a jet airplane at high altitudes. Middle cloudsThese develop between 6,500 and 16,500 feet (between 2,000 and 5,000 m) and are denoted by the prefix alto-. They are made of water droplets, and are frequently supercooled.Middle clouds include:
Low cloudsThese are found up to 6,500 feet (2,000 m) and include the stratus (dense and grey). When stratus clouds contact the ground they are called fog.Low clouds include:
Vertical cloudsThese clouds can have strong upcurrents, rise far above their bases and can form at many heights. Vertical clouds include:
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cloud" and from http://www.white-on.com
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