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University of South Australia Clouds Scott Adams Bed PE and Outdoor Education specialisation 1999 Clouds • • • • Made of tiny drops of water or ice Change due to synoptic conditions Appear white, grey or near black in different lights Classified based on height, appearance, make-up and formation process • May recur in the same areas • Convective clouds tend to dissipate at night • Stratiform clouds tend to develop at night Principles Of Cloud Formation • Air is ‘saturated’ with water vapour • Saturated cool air holds less water vapour than saturated warm air • When saturated warm air cools, some of the water vapour must condense • Condensation of water vapour results in cloud formation Mechanisms Which Result To Form Clouds • Air being heated from below • Air flowing up slopes • Air forced to rise due to cold/ warm fronts • Air converging into an area of low pressure Cloud Forms Stratiform Cloud • Layers of stable cloud formed by widespread ascent e.g., Stratus, Nimbostratus, Altostratus, Cirrostratus Cumuliform Cloud • Piles of cloud formed as “bubbles” of warm air rise by convection e.g., Cumulus, Stratocumulus, Cumulonimbus, Altocumulus, Cirrocumulus Cirriform Cloud • Fibrous or wispy clouds composed of ice crystals e.g., Cirrus, Cirrostratus, Cirrocumulus The Latin Language Of Clouds Altum • Latin for height Cumulus • Latin for “heap” - describes ‘fluffy’ or ‘lumpy’ clouds Cirrus • Latin for “curl of hair” - describes ‘wispy’ cloud Stratus • Latin for “layer/ spread out” - describes sheet-like cloud Nimbus • Latin for “violent rain” - describes rain clouds Cloud Heights In Australia High Cloud 1) Cirrus 2) Cirrocumulus 3) Cirrostratus >20,000 ft. (6 km.) Middle Cloud 8,500 - 20,000 ft. (2.5 - 6 km.) 1) Altocumulus 2) Altostratus 3) Nimbostratus (may be reported as middle or low cloud) Low Cloud 1) Stratocumulus 2) Stratus 3) Cumulus 4) Cumulonimbus 0 - 8,500 ft. (0 - 2.5 km.) Cirrus Characteristics • Detached, thin white patches or bands • Fibrous, hair-like or silky appearance Precipitation • Nil Formation • High level ascent • Shape may be ‘smeared’ by the wind Cirrus Cirrocumulus Characteristics • Thin white patch, sheet or layer • Very small grains and ripples apparent Precipitation • Nil Formation • High level ascent • Either merged or separate • Predominantly regular in shape Cirrocumulus Cirrostratus Characteristics • Transparent ‘whitish’ veil • Fibrous or smooth appearance • Commonly produces lunar halo Precipitation • Nil Formation • Widespread, upper level ascent • May cover whole sky Cirrostratus Altocumulus Characteristics • Variable in form - continuous or patchy • Usually waved or in lumps or laminae • May appear lens shaped near mountains Precipitation • Virga or light showers occasionally ‘Virga’ - precipitation streaks which evaporate before reaching the ground Formation • May be formed by convection (internal instability) or wave flow near mountains Altocumulus Altostratus Characteristics • Grey, featureless layer of cloud • May be fibrous or uniform sheet • Covers whole or part of sky • Sun shines weakly through Precipitation • Rain or snow may form Formation • Widespread ascent Altostratus Nimbostratus Characteristics • Dark grey layer of cloud - generally covering sky • Thick enough to hide the sun or moon • Classed as either middle or lower level cloud • Base indistinct due to continuous rain or snow Precipitation • Almost continuous rain or snow Formation • Widespread ascent Nimbostratus Stratocumulus Characteristics • Grey or white layer with darker areas • Usually regular, in rolls or rounded masses • Lower level cloud Precipitation • Occasional light rain, drizzle or snow Formation • Spreading out of Cumulus cloud (usually evening) • Turbulent mixing under stable layer of cloud Stratocumulus Stratus Characteristics • Grey uniform layer of cloud • May be continuous or patchy • May appear as fragments below Nimbostratus or Altostratus Precipitation • Drizzle (may be rain or snow falling from above) Formation • Low level ascent or cooling • Precipitation may saturate air Stratus Cumulus Characteristics • Tops domed or “cauliflower” in appearance • Bright, sharp outlines • Base nearly horizontal and relatively dark • Sized small to very large (Cumulonimbus) Precipitation • Showers, rain or snow from larger Cumulus Formation • Convection - surface heating or instability Cumulus Cumulonimbus Characteristics • Huge Cumulus cloud • Fluffy, fibrous top • Base dark and stormy looking • Thunder and lightning common Precipitation • Showers, rain, snow and hail - may be heavy Formation • Convection - surface heating or instability Cumulonimbus
