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Cloud Formation and Cloud Types

Clouds are formed when water evaporates into vapor from oceans, lakes, and ponds or by evapotranspiration over Earth’s land surface and rises up into colder areas of the atmosphere due to convective, orographic, or frontal lifting.

Condensation or deposition of water above the Earth’s surface creates clouds. In general, clouds develop in any air mass that becomes saturated (relative humidity becomes 100%). Saturation can occur by way of atmospheric mechanisms that cause the temperature of an air mass to be cooled to its dew point or frost point. The following mechanisms or processes can achieve this outcome causing clouds to develop:

 

(1). Orographic uplift occurs when air is forced to rise because of the physical presence of elevated land. As the parcel rises it cools as a result of adiabatic expansion at a rate of approximately 10° Celsius per 1000 meters until saturation. The development of clouds and resulting heavy quantities of precipitation along the west coast of Canada are mainly due to this process.

(2). Convectional lifting is associated with surface heating of the air at the ground surface. If enough heating occurs, the mass of air becomes warmer and lighter than the air in the surrounding environment, and just like a hot air balloon it begins to rise, expand, and cool. When sufficient cooling has taken place saturation occurs forming clouds. This process is active in the interior of continents and near the equator forming cumulus clouds and or cumulonimbus clouds (thunderstorms). The rain that is associated with the development of thunderstorm clouds is delivered in large amounts over short periods of time in extremely localized areas.

(3). Convergence or frontal lifting takes place when two masses of air come together. In most cases, the two air masses have different temperature and moisture characteristics. One of the air masses is usually warm and moist, while the other is cold and dry. The leading edge of the latter air mass acts as an inclined wall or front causing the moist warm air to be lifted. Of course the lifting causes the warm moist air mass to cool due to expansion resulting in saturation. This cloud formation mechanism is common at the mid-latitudes where cyclones form along the polar front and near the equator where the trade winds meet at the inter tropical convergence zone.

(4). Radiate cooling occurs when the Sun is no longer supplying the ground and overlying air with energy derived from solar insolation (e.g., night). Instead, the surface of the Earth now begins to lose energy in the form of long wave radiation which causes the ground and air above it to cool. The clouds that result from this type of cooling take the form of surface fog.

 

Tropospheric clouds can be divided into three main categories based upon the Latin root words which refer to the process of formation and physical structure of the clouds. The first of the three is Cirrus, which has only one genus and therefore shares the same name. It is in the high altitude range and occurs mostly in the form of filaments. The other two are Stratus that are mostly sheet-like in structure, and Cumulus that appear heaped, rolled, and/or rippled. Stratus and Cumulus occur in the high, middle, and low levels of the troposphere, so each of these categories have several cloud genera.

 

Cloud formations are grouped into four families: high, middle, low, and vertical.

High cloud formations develop between 3,000 and 8,000 m in the polar regions, 5,000 and 12,000 m in temperate areas, and 6,000 and 18,000 m above the tropics. There are three members of the high cloud family: Cirrocumulus, Cirrus, and Cirrostratus.

 

High： base ca. 16,500 to ca. 40,000 ft /5 to 12 km in temperate latitudes

Genus cirrus

 

Cirrus clouds                                                          Cirrus uncinus clouds

 

 

Abbreviation: Ci

Cirrus clouds form in the highest and coldest region of the troposphere. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent. Isolated cirrus clouds often do not bring rain, however, large amounts of cirrus clouds can indicate an approaching storm system eventually followed by fair weather.

 

Cirrocumulus cloud formations signify convection. They include some water droplets that are in a super cooled state, which means they are colder than freezing but not frozen yet. They are rapidly filling up with ice crystals and will soon become cirrostratus clouds.

Cirrocumulus

Cirrus cloud formations develop when water vapor completely freezes into ice crystals above 8000 m. Because there is very little moisture that high, these clouds are thin and wispy looking. Airplanes have been known to leave condensation trails that turn into cirrus clouds.

Cirrostratus cloud formations are thin clouds completely made up of ice crystals. They often form a halo around the Sun. Many times they signal a warm front and mean that precipitation will fall in the next 12 to 24 hours.

Cirrostratus

 

Middle： base ca. 6,500 to ca. 23,000 ft /2 to 7  km in temperate latitudes

Genus altocumulus

Altocumulus cloud formations are usually in globs or rolls that are in patches. They often resemble cotton balls. They are bigger than cirrocumulus clouds and smaller that stratocumulus. They usually mean a cold front has arrived and will signify thunderstorms if they appear on a warm summer day.

 

 

 

Altocumulus lenticularis

 

Altocumulus castellanus

Abbreviation: Ac

Clouds of the genus altocumulus are not always associated with a front but can still bring precipitation, usually in the form of virga which does not reach the ground. This genus is generally an indicator of limited convective instability at the altitude of its formation, and is therefore more closely related to stratocumulus than to the more freely convectice cumulus genus.

Altostratus cloud formations are made of ice crystals are  gray in color. They can be dangerous because they are the cause of ice formation on the wings of airplanes. They are usually a sign of an incoming weather front.

 

Low： base near surface to ca. 6,500 ft /2  km in temperate latitudes

Genus stratocumulus

 

Stratocumulus                                                Stratocumulus cumulogenitus

 

 

 

Stratocumulus lenticularis                           Stratocumulus castellanus

 

 

 

Abbreviation: Sc

Clouds of the genus stratocumulus are lumpy, often forming in slightly unstable air following a cold front, and they can produce very light rain or drizzle.

Stratocumulus cloud formations cover much of the subtropics and polar oceans. In general, these clouds will not produce precipitation. When they do, it is only light rain or snow. The kicker is that these clouds are often at the front or tail end of worse weather and may indicate storms to come in the form of thunderstorms or high, gusty winds.

 

 

Genus stratus

 

At level with some stratus clouds                      Stratus fractus cloud

 

[[File:Stratus-Opacus-Nebulosis.jpg|thumb Abbreviation: St

Clouds of the genus stratus form in low horizontal layers having a ragged or uniform base. Ragged stratus often forms in precipitation while more uniform stratus forms in maritime or other moist stable air mass conditions. The latter often produces drizzle.

Moderate vertical： low to middle base from near surface to ca. 10,000 ft /3  km; tops mostly middle level

Genus nimbostratus

Clouds of the genus nimbostratus tend to bring constant precipitation and low visibility. This cloud type normally forms above 6,500 feet from altostratus cloud but can thicken into the lower levels during the occurrence of precipitation.