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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.