Download Chapter 6 - Cloud Development and Forms Interesting Cloud

Chapter 6 - Cloud Development
and Forms
Understanding Weather and Climate
Aguado and Burt
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Interesting Cloud
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Mechanisms that Lift Air
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•
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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
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DALR
MALR
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DALR
MALR
DALR
DALR
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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
LFC
SALR = MALR
LCL
ELR = 0.7 ° C/100m
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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.
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