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Chapter 6 - Cloud Development and Forms Understanding Weather and Climate Aguado and Burt ATMO 1300 Interesting Cloud ATMO 1300 1 Mechanisms that Lift Air • • • • Orographic lifting Frontal Lifting Convergence Localized convective lifting ATMO 1300 Mechanisms that Lift Air ATMO 1300 2 Orographic Lifting • Air flowing up a hill/mountain forces adiabatic cooling. This promotes precipitation. • The opposite occurs downwind of a mountain (leeward side) as air descends and warms by compression. This inhibits precipitation. ATMO 1300 Orographic Lifting r no I g ’s # DALR MALR e DALR MALR DALR DALR ATMO 1300 3 Frontal Lifting • Front – Transition zone between two different air masses – Cold front– cold air advances towards warmer air and displaces the lighter, warmer air upward. – Warm front – warm air advances towards cold air, the warm air is forced upward over the cold air. ATMO 1300 Cold Front and Warm Front ATMO 1300 4 Convergence • Horizontal movement (advection) towards a common location implies an accumulation of mass called convergence. • Does not lead to an increase in air density, rather an increase in vertical motions carry the mass away. ATMO 1300 Local Convection • Differential heating of the Earth’s surface can produce free convection over limited areas. • Buoyancy can initiate uplift by itself, but it can also speed or slow the uplift provided by orographic, frontal or convergence lifting. ATMO 1300 5 Local Convection ATMO 1300 Local Convection ATMO 1300 6 Static Stability • Static stability – The air’s susceptibility to lift. – Unstable – Air will continue to rise if given an initial upwards push – Stable – Air resists the upward displacement and sinks back to original level. – Neutral – Air will neither rise on its own or sink back to its original level. ATMO 1300 Types of Air (Static Stability) • Absolutely Unstable • Absolutely Stable • Conditionally Unstable ATMO 1300 7 Absolutely Unstable Air • Once a parcel is lifted it continues to move upward regardless of saturation. • Whenever the ELR exceeds the DALR (1°C/100 m) the air is absolutely unstable. ATMO 1300 Absolutely Unstable Air ERL = 1.5° C/100m ATMO 1300 8 Absolutely Stable Air • Air parcel returns to its original location after being displaced. • When ever the ELR is less than the SALR (0.5°C/100 m), the air is absolutely stable. ATMO 1300 Absolutely Stable Air ATMO 1300 9 Conditionally Unstable Air • When ELR is between the DALR and SALR, the environment is conditionally unstable. • An air parcel will become buoyant if lifted to a critical altitude called the level of free convection (LFC). ATMO 1300 Conditionally Unstable Air LFC SALR = MALR LCL ELR = 0.7 ° C/100m ATMO 1300 10 Changes to the Environmental Lapse Rates • Changes can occur in 3 ways: – Heating or cooling of the lower atmosphere – Advection of cold or warm air at different levels – Advection of air mass with a different ELR ATMO 1300 Heating or Cooling of the Lower Atmosphere • Heating of the Earth’s surface occurs rapidly and leads to a steep ELR near the surface. • The opposite occurs at night as cooling promotes the development of an inversion in the lowest portion of the atmosphere. ATMO 1300 11 Advection of Cold and Warm Air ATMO 1300 Advection of Cold and Warm Air ATMO 1300 12 Advection of Cold and Warm Air ATMO 1300 Advection of an Air Mass with Different ELR • Air masses maintain their temperature and humidity profiles as they move from one location to another. ATMO 1300 13 Limitations on Lifting • What causes air to quit rising? – Stable air • Inversions – Entrainment (mixing) ATMO 1300 Layer of Stable Air ATMO 1300 14 Inversions • Inversion – A layer of extremely stable air where temperature increases with height. ATMO 1300 Inversions • Radiation Inversion – Cooling of the surface • Frontal Inversion – Interface of two air masses • Subsidence Inversion – Sinking air aloft ATMO 1300 15 Subsidence Inversion ATMO 1300 Entrainment • When air rises considerable turbulence is generated. This entrainment draws in environmental air into the parcel and suppresses further growth. ATMO 1300 16 Cloud Types • High Clouds – Cirrus, cirrostratus, and cirrocumulus • Middle Clouds – Altostratus and altocumulus • Low Clouds – Stratus, stratocumulus, nimbostratus • Clouds with Vertical Development – Cumulus and cumulonimbus ATMO 1300 Cloud Types ATMO 1300 17 High Clouds (cirro) • Located above 6,000 m (19,000 ft). • Composed of ice crystals • Low water content because of low temperatures (-35°C) ATMO 1300 Cirrus ATMO 1300 18 Cirrostratus ATMO 1300 Cirrocumulus ATMO 1300 19 Fall Streaks ATMO 1300 Middle Clouds (alto) • Located between 2000 - 6,000 m (6,000 19,000 ft). • Composed mainly of liquid droplets ATMO 1300 20 Altostratus ATMO 1300 Altocumulus ATMO 1300 21 Low Clouds • Located below 2000 m (6,000 ft). • Composed mainly of liquid droplets ATMO 1300 Stratus ATMO 1300 22 Clouds with Vertical Development • Cumuliform Clouds – Can have violent updrafts – Can have heavy precipitation – Can have vast temperature differences ATMO 1300 Cumulus Humilis ATMO 1300 23 Cumulus Congestus ATMO 1300 Cumulonimbus ATMO 1300 24 Cloud Coverage • Overcast – Above 90% of the sky is covered with clouds. • Broken - Between 60 and 90% of the sky is covered. • Scattered – Between 10 and 60% of the sky is covered. • Clear – less than 10% of the sky is covered. 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