chapter 6 - cloud development and forms - texas tech university
TRANSCRIPT
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Chapter 6 - Cloud Development and Forms
Understanding Weather and ClimateAguado and Burt
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Interesting Cloud
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Mechanisms that Lift Air
• Orographic lifting• Frontal Lifting• Convergence• Localized convective lifting
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Mechanisms that Lift Air
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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.
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Orographic Lifting
DALR
DALR
DALR
Ignore
#’s
DALR
MALR
MALR
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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.
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Cold Front and Warm Front
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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.
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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.
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Local Convection
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Local Convection
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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.
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Types of Air (Static Stability)
• Absolutely Unstable• Absolutely Stable• Conditionally Unstable
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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.
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Absolutely Unstable Air
ERL = 1.5° C/100m
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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.
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Absolutely Stable Air
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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).
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Conditionally Unstable Air
ELR = 0.7 ° C/100m
LCL
SALR = MALR
LFC
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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
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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.
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Advection of Cold and Warm Air
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Advection of Cold and Warm Air
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Advection of Cold and Warm Air
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Advection of an Air Mass with Different ELR
• Air masses maintain their temperature and humidity profiles as they move from one location to another.
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Limitations on Lifting
• What causes air to quit rising?– Stable air
• Inversions
– Entrainment (mixing)
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Layer of Stable Air
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Inversions
• Inversion – A layer of extremely stable air where temperature increases with height.
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Inversions
• Radiation Inversion – Cooling of the surface
• Frontal Inversion – Interface of two air masses
• Subsidence Inversion – Sinking air aloft
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Subsidence Inversion
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Entrainment
• When air rises considerable turbulence is generated. This entrainment draws in environmental air into the parcel and suppresses further growth.
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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
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Cloud Types
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High Clouds (cirro)
• Located above 6,000 m (19,000 ft).• Composed of ice crystals• Low water content because of low
temperatures (-35°C)
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Cirrus
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Cirrostratus
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Cirrocumulus
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Fall Streaks
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Middle Clouds (alto)
• Located between 2000 - 6,000 m (6,000 -19,000 ft).
• Composed mainly of liquid droplets
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Altostratus
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Altocumulus
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Low Clouds
• Located below 2000 m (6,000 ft).• Composed mainly of liquid droplets
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Stratus
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Clouds with Vertical Development
• Cumuliform Clouds– Can have violent updrafts– Can have heavy precipitation– Can have vast temperature differences
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Cumulus Humilis
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Cumulus Congestus
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Cumulonimbus
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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.