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Chapter 6 Cloud Development and Forms • Creating clouds needs air to rise and cool • 4 Mechanisms that lift air – ________________ – lifting caused by a mountain • Air hits a a mountain and is forced upwards • Windward side – upwind side of the mountain rising air causes cooling – Abundant precipitation • Leeward side – – Lack of precipitation (________________) – Frontal lifting • Boundaries between different air masses • Warm/moist air is forced to rise and form clouds • Cold (a) and warm (b) fronts occur – • Air converges into low pressure regions • Convergence causes air to rise and cool – Localized convection • Air heated at the surface becomes warmer and less dense and rises freely into updrafts • Creates small localized summer thunderstorms • Stability of the air – Static stability – determines if air rise, falls, or remains at a certain level • Related to temperature and controlled by buoyancy – Positive and negative buoyancy – Types • Statically unstable • Statically stable • Statically neutral • Conditionally unstable • – If air is forced to rise and it continues rising freely – Positive buoyancy – Parcel of air cools slower than that of surrounding air • Parcel of rising air is always warmer than surrounding air • DAR and WAR less than ELR – – Associated with low pressure • Absolutely stable air – If air is forced to rise and it sinks back to its original position – – Parcel of air cools faster than surrounding air • Parcel of air is always warmer than surrounding air • – – Associated with high pressure • Absolutely Neutral air – If air is forced to rise and parcel remains in the location where lifting ceased – Parcel of air temperature equal to surrounding air • Conditionally unstable air – Parcel of air forced to rise is initially stable but eventually becomes unstable after saturation is reached – ELR is between the DAR and the WAR – • Factors Influencing the ELR – Heating/cooling the lower atmosphere – Advection of Cold/Warm Air at Different Levels – Advection of an Air Mass with a Different ELR • Limitations on the Lifting of Unstable Air – Stability • Eventually a rising stable parcel runs into stable air, especially at the stratosphere • Strong thunderstorm updrafts can penetrate the stratosphere – Entrainment • Rising air pulls in dryer air near its edges causing evaporation and cooling • Extremely Stable Air – • – Creates radiation fog and traps pollutants • Frontal interactions – Fronts moving through cause inversions • Subsidence – – Established stable polluted skies especially over Los Angeles • Cloud Classifications – Based on appearance and height – High clouds (______________) • Bases above 6000m (19,000 ft) • • _________________ – thin white wispy clouds • Cirrostratus (Cs) – more extensive often produces a halo around Sun or Moon • Cirrocumulus (Cc) – layered clouds with billows or rolls. Often look like fish scales. – Middle clouds (prefix = alto) • Bases between 2000 and 6000m (6-19,000 ft) • Composed of liquid drops and ice crystals • Altostratus (As) – more extensive than cirrostratus. Sun and Moon are slightly visible. • Altocumulus (Ac) – a series of puffy clouds arranged in rows. Allows less light through than cirrocumulus and the scales are larger in size. – Low clouds (________________) • Bases below 2000m (6,000 ft) • • Stratus (St) – uniform layer of low clouds often a darker gray in color. Covers a wide region. • ____________________ – low layered clouds that produce light precipitation • Stratocumulus (Sc) – low layered clouds with some vertical development – Clouds with vertical development • • Cumulus humulus (Cu) - local updrafts create individual clouds. “Fair weather” clouds • Cumulus congestus - contains numerous updrafts and form towering cumulus • __________________ – towering cumulus clouds otherwise known as “thunderheads”. – – Characteristic anvil shape made of ice – Extreme updrafts and downdrafts – Unusual clouds • Lenticular - form from mountains disturbing the wind flow. Flying saucer shape. • Mammatus – • Shelf clouds – large horizontal roll clouds leading a strong downdraft from thunderstorms • Observations of cloud coverage – Surface-Based Observations • _____________ – lasers used to measure cloud coverage at different heights – Cloud Observation by Satellite • Visible images • Infrared images • Enhanced IR images End of Chapter 6