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Water in the Atmosphere
Understanding Weather and Climate
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Cloud Development and Forms
Understanding Weather and Climate
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Learning Objectives
1. The various atmospheric lifting mechanisms and how
these influence cloud formation.
2. Static stability and how it influences cloud formation.
3. The major types of clouds that are produced by these
processes.
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Poem
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Clouds
Crucial Role in the Hydrologic
Cycle
Most Prominent Feature of
Daily Weather
Global Importance
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Producing Clouds
1. Adding Water Vapor into the Air
2. Mixing Warm Moist Air with Cold Air
3. Lowering the Air Temperature
Most Important for Clouds
Lifting the Air Will Cool It Adiabatically
Forming Clouds Requires Mechanisms to Lift Air
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• Cloud Development
– When an unsaturated parcel of air rises until it
becomes saturated, a cloud forms.
– The mechanisms responsible for the development of
the majority of clouds are:
•
•
•
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Convection
Topography
Ascent due to convergence of surface air
Uplift along weather fronts
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Mechanisms That Lift Air
1. Orographic Lifting:
Forcing of Air Above a
Mountain (Land) Barrier.
2. Frontal Lifting:
Displacement of One Air
Mass (Warmer) Over
Another Air Mass (Cooler).
3. Convergence: Horizontal
Movement of Air Into a area
at Low Levels.
4. Localized Convective
Lifting: Buoyancy (Heating).
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• Cloud Development (Convection and Clouds)
– When the surface is unevenly heated, a thermal
breaks away from the warm surface and rises,
expanding and cooling as it ascends.
• As it rises it mixes with the cooler, drier air around it and
gradually loses its identity.
• At this point its upward movement slows, however other
rising air parcels penetrate it and help the air to rise a
little higher.
• Only if the parcel rises to its saturation point, the
moisture will condense, and the thermal becomes visible
as a cumulus cloud.
– The level where lifted surface air becomes saturated
is called the condensation level.
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• Cloud Development (Convection and Clouds)
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Orographic Lifting
Forcing of Air Above a Mountain (Land) Barrier.
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Height of Clouds
Varies From
Day to Day As
Characteristics
of Air Change
Can Be Much
Higher Than
Barrier
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Rainshadow Effect
Windward Side (Upwind): Precipitation Greatly Enhanced
Leeward(Downwind): Low Precipitation
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Death Valley
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Frontal Lifting
Displacement of One Air Mass (Warmer) Over
Another Air Mass (Cooler)
Warmer Air Approaches Colder Air
Warmer Air Wedges Over the Colder Air
Warm Front
Smooth Slope
Colder Air Approaches Warmer Air
Colder Air Wedges Under the Warmer Air
Cold Front
Blunt
Slope
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Horizontal Convergence
Horizontal Movement of Air Into an Area at Low Levels
Mass of Air Not Evenly Distributed
Causes Areas of Higher and Lower Pressure
Pressure Difference Cause Wind
Horizontal MovementClimate
of Air
2 Into a Low Pressure Zone
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Causes Convergence Lifting
Localized Convection
Localized as Opposed to Global
Free Convection
Heating of Earth’s Surface in Localized Areas
Buoyancy: Lighter, Warmer Air Rises
Can Speed Up or Slow Down Other Lifting Mechanisms
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Static Stability
Static Stability
Air’s Susceptibility to Uplift.
Statically Unstable Air
Continues to Rise If Given an Initial Upward Push
Statically Stable Air
Resists Upward Displacement and Sinks Back to
Original Level
Statically Neutral Air
Will not Rise or Sink After Its Initial Upward Push
Comes to Rest Where It was Displaced
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Buoyancy
Static Stability is Related to Buoyancy
Parcel of Air
Less dense Than Surrounding Air: Positive Buoyancy
Tends to Rise (Warmer)
More dense Than Surrounding Air: Negative Buoyancy
Tends to Sink if Not Lifted(Colder)
A Rising Parcel of Air
Stops Rising When It Cools to Surrounding Air
Sinks When It Becomes Colder Than Surrounding Air
This Suppresses Uplift
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Lapse Rates
Lifted Parcel of Air
Cools at One of the Adiabatic Lapse Rates
Air Around it Maintains Its Original Temperature Profile
Relative Density
1. Depends on Saturated or Unsaturated
DALR or SALR
2. Environmental Lapse Rate (ELR)
Three Types of Static Stability
1. Absolutely Unstable Air
2. Absolutely Stable Air
3. Conditionally Stable
Air2
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Absolutely Unstable Air
-1.0°C
-1.5°C
ELR = -1.5°C / 100 m
Unsaturated DALR = -1.0°C / 100 m
Will Keep Rising - Cooling Slower than Surrounding Air
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Absolutely Unstable Air
-0.5°C
-1.5°C
ELR = -1.5°C / 100 m
Saturated
SALR = -0.5°C / 100 m
Will Keep Rising - Cooling More
Slowly than Surrounding Air
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Absolutely Stable Air
-1.0°C
-0.2°C
ELR = -0.2°C / 100 m
Unsaturated DALR = -1.0°C / 100 m
Will Not Rise - Cooling Faster than Surrounding Air
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Absolutely Stable Air
-0.5°C
-0.2°C
ELR = -0.2°C / 100 m
Saturated
SALR = -0.5°C / 100 m
Will Not Rise - Cooling Faster than Surrounding Air
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Stability Rule #2
1. Absolutely Unstable Air
Whenever the ELR Exceeds the DALR or SALR
(Positive Buoyancy)
2. Absolutely Stable Air
Whenever the ELR Is Less Than the DALR or SALR
(Negative Buoyancy)
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Making Conditionally Unstable Air
ELR = -0.7°C / 100 m
Unsaturated DALR = -1.0°C / 100 m
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Will Keep Rising - Cooling
Faster
than Surrounding Air26
Conditionally Unstable Air
ELR = -0.7°C / 100 m
Unsaturated DALR = -1.0°C / 100 m
Saturated
SALR = -0.5°C / 100 m
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Level of Free Convection
A Conditionally Unstable Air Mass
Rises Above the Level of Free Convection
Must be Lifted
Then Can Rise on Its Own
LFC
Clouds Increase in Depth
Yield Precipitation
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• Cloud Development (Convection and Clouds)
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Factors Influencing the
Environmental Lapse Rate
The Average Environmental Lapse Rate (ELR)
-0.65 °C / 100 meters
Highly Variable in Space and Time
Surface Air Temperature
Vertical Temperature Profile
Influences
1. Heating or Cooling of the Lower Atmosphere.
2. Advection of Cold or Warm Air at Different Levels.
3. Advection of a Different Air Mass with a Different ELR.
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Advection of Cold or Warm Air at
Different Levels
Advection or Wind Can Be Different at the Surface From That Aloft
ELR Can Be Different If Winds Are Different
Example
Same Direction: -0.5°c/100m
Perpendicular : -1.0°c/100m
Idealized Example As Winds Gradually Change With Height
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Advection of a Different Air Mass
with a Different ELR
Air Mass
Large Area Distinguished From Its Neighbors by Differences in
1. Temperature
2. Water Content (Humidity)
Maintain Their Identity (1 & 2)
Air Masses Can Migrate Into Other Large Areas Changing ELR
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Cloud Types By Form
1. Cirrus
Thin, Wispy Clouds of Ice
2. Stratus
Layered Clouds
3. Cumulus
Clouds Having Vertical Development
4. Nimbus
Rain-producing Clouds
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Cloud Types By Height
1. High Clouds
Cirrus
Thin, Wispy Clouds of Ice
Cirrostratus
Layered, Thin, Wispy Clouds of Ice
Cirrocumulus Thin, Wispy Clouds of Ice with Vertical
Development
2. Middle Clouds
Altostratus
Altocumulus
3. Low Clouds
Stratus
Stratocumulus
Nimbostratus
Higher, Layered Clouds
Higher Clouds with Vertical Development
Layered Clouds
Layered Clouds with Vertical Development
Rain-producing, Layered Clouds
4. Extensive Vertical Development
Cumulus
Clouds Having Vertical Development
Cumulonimbus Rain-producing Clouds with Vertical
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High Clouds
Above 6000 meters
Height a Little Dependent on Temperature
Composed of Ice (Ave. Temp. -35°C)
1. Cirrus (Ci)
2. Cirrostratus (Cs)
3. Cirrocumulus (Cc)
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High Clouds - Cirrus
Above 6000 meters
Cirrus (Ci)
Thin, Wispy Clouds of Ice
Simplest
1.5 km Thick
Little Water Vapor - 0.025 g/m3
Individual Ice Crystals - 8mm (0.3in)
Fall at speeds of 0.5 m/s (1 mi /hr) Making Streaks
Falling Ice Sublimates
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Cirrus
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Cirrus
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High Clouds - Cirrostratus
Above 6000 meters
Cirrostratus (Cs)
Layered, Thin Clouds of Ice
More extensive horizontally
Lower Concentration of Ice
Surface Objects cast shadows
Halo (22°) around Sun and moon
Whitish, milky disk with sharp outline
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Cirrostratus
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High Clouds - Cirrocumulus
Above 6000 meters
Cirrocumulus (Cc)
Individual, Puffy Rows of Clouds of Ice
Form When a Wind Shear Exists
Wind Speed or Direction Changes with Height
Precursor of Precipitation - Warm Front
Resemble Fish Scales - “Mackerel Sky”
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Cirrocumulus
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Middle Clouds
2000 to 6000 meters
Composed of Liquid Droplets
Alto means Middle
1. Altostratus (As)
2. Altocumulus (Ac)
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Middle Clouds - Altostratus
2000 to 6000 meters
Altostratus (As)
Middle-level Counterparts to Cirrostratus
Liquid Water Droplets
Scatter a Lot of Insolation Back to Space
Diffused Light
Absence of Shadows
Sun and Moon (If Seen) Are Bright Spots With No Outline
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Altostratus
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Middle Clouds - Altocumulus
2000 to 6000 meters
Altocumulus (Ac)
Liquid Water Droplets
Layered Clouds Forming Long Bands, or
Contains a Series of Puffy Clouds in Rows
Gray in Color With Possibly One Part Darker
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Altocumulus
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Low Clouds
Bases below 2000 meters
Composed of Liquid Droplets
1. Stratus (St)
2. Stratocumulus (Sc)
3. Nimbostratus (Ns)
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Low Clouds - Stratus
Bases below 2000 meters
Composed of Liquid Droplets
Stratus (St)
Layered Clouds Formed From Large Areas of Stable Air
Slow Uplift (Few10s Cm/s), or
Turbulence From Strong Winds
Forced Convection
Low Water Content (0.1 G/m3)
0.5 to 1.0 Km Thick
100s of Km Horizontally
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Stratus
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Low Clouds - Nimbostratus
Bases below 2000 meters
Composed of Liquid Droplets
Nimbostratus (Ns)
Much Like Stratus, Except for Presence of Precipitation
Low Moisture Content Produces Light Precipitation
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Nimbostratus
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Low Clouds - Stratocumulus
Bases below 2000 meters
Composed of Liquid Droplets
Stratocumulus (Sc)
Layered Clouds With Vertical Development
Darkness Varies Because of Vertical Thickness
Darker Is Thicker
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Stratocumulus
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Clouds with Vertical Development
Bases Below 2000 Meters but Extend Into Middle-level
Composed of Liquid Droplets
Cumuliform Clouds
Those With Substantial Vertical Development
Vertical Velocities Exceed 50 M/s (100 Mi/hr)
Updrafts Have Speeds Greater Than Weak Hurricanes
Water Content ~1 G/m3 (Much Larger Than Stratiform
Clouds)
1. Cumulus (Cu)
a. Cumulus humilis
b. Cumulus congestus
2. Cumulonimbus (Cb)
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Clouds with Vertical Development
Cumulus
Cumulus (Cu)
a. Cumulus humilis
Fair Weather Cumulus
Does Not Yield Precipitation
Single Raising Plume - Zone of Raising Air
Clear Sky - Zone of Sinking Air
b. Cumulus congestus
Multi-Towers with Several Cells of Uplift
Strong Vertical Development from Unstable Air
Individual Towers Last Only Tens of Minutes
Constantly Replaced by New Ones
Large Temperatures from Bottom to Top
Supercooled Droplets then Ice
Ice Visible WhereClimate
No Distinct
Edges of Clouds 56
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Washed Out Appearance
Clouds with Vertical Development
Cumulus humilis
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Clouds with Vertical Development
Cumulus congestus
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Clouds with Vertical Development
Cumulonimbus
Cumulonimbus (Cn)
Most Violent
Warm, Humid, and Unstable Air
Produce Thunderstorms
Tops Can Extend Into Stratosphere
Anvil Top (Blacksmith)
Composed of Ice in High Winds of Stratosphere)
Anvil Pushed out from Column
Hailstones Fall from End
Strong Updrafts are Not Uniform
Highest Speeds in Top Third
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Cumulonimbus
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Unusual Clouds
1. Lenticular: Lens-like
From Downwind of Mountain Barriers
From Disruption of Air Flow
Series of Waves
Adiabatically
Droplets evaporate on Downwind Side, Form on Upwind
Usually Only Two or Three Form, but Six Have Been
Observed
2. Banner
Similar, but Are Located Above Isolated Peaks
3. Mammatus
Cumulus Clouds That Seem to Have Sack-like Hangings
Places that are Heavy with Water
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Lenticular
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Banner
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Mammatus
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Unusual Clouds Above the Troposphere
1. Nacreous
Seen in the Winter at Twilight in the Polar Regions
Supercooled Water or Ice Crystals
Height: 30 Km (20 miles) in Stratosphere
2. Noctilucent
In Mesosphere
Illuminated After Sunset or Before Sunrise
Ice Crystals
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Nacreous
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Noctilucent
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Cloud Coverage
Other Characteristic of Clouds Is Coverage
Overcast
More Than Nine-tenths (9/10)
Broken
Six-tenths to Nine-tenths (6/10 to 9/10)
Scattered
One-tenth to Five-tenths (1/10 to 5/10)
Clear-sky
Less Than one-tenths (1/10)
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Summary
1. Lifting Mechanisms: frontal uplift, convergence, orographic
uplift, and convection.
2. Frontal uplift, convergence, orographic uplift are enhanced or
hindered by the static stability of the atmosphere, whereas
free convection necessarily occurs only when the air is
unstable.
3. Instability implies that if a parcel is given an initial boost
upward, it will become buoyant and continue to rise. On
the other hand, if the air is stable, a parcel displaced
vertically will tend to return to its original position.
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Summary
4. Static stability or instability is determined by the air column's
rate of temperature decrease with altitude. When the
temperature lapse rate is less than the saturated adiabatic
rate, the air is statically stable; when it exceeds the dry
adiabatic lapse rate, it is unstable. Conditional instability
arises when the lapse rate is between the two adiabatic
rates. When the air is conditionally unstable, a lifted
parcel will rise on its own accord only if it is lifted above a
certain critical point called the level of free convection.
5. Three processes modify the lapse rate: the inflow of warm and
cold air at different altitudes, the advection of a different
air mass, and heating or cooling of the surface.
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Summary
6. Environmental lapse rates vary not only through time, but also
with elevation. Thus, a column of atmosphere might be
unstable at one level but stable aloft.
7. No matter what the condition of the troposphere, the
stratosphere is always statically stable and thereby limits
the maximum height of updrafts.
8. Inversions are a special case in which the temperature
increases with altitude. Because of their strong static
stability, inversions suppress the vertical motions
necessary for cloud formation and for the dispersion of
air pollution. Inversions are formed by subsidence
(sinking air), the emission of longwave radiation from the
surface, and the presence of fronts.
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Summary
9. Clouds have been categorized into ten distinct types grouped
according to their height and form.
Height
1. Cirrus
2. Stratus
3. Cumulus
4. Nimbus
Form
1. High Clouds
Cirrus, Cirrostratus, Cirrocumulus
2. Middle Clouds
Altostratus, Altocumulus
3. Low Clouds
Stratus, Stratocumulus, Nimbostratus
4. Extensive Vertical Development
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Cumulus,
Cumulonimbus
Adiabatic Lapse Rate
• The First Law of Thermodynamics can be expressed
as:
dU = dq + dw
where dU is the change in internal energy, dq is the
heat supplied to the system, and dw is the is the work
done on the system.
• dH, the change in enthalpy, can be written as
dH = dU + pdV + Vdp
• When we raise a parcel of air there is no heat input,
hence dq=0 (adiabatic) and dw=pdV
• Therefore dH = -Vdp
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Adiabatic Lapse Rate
•
•
•
•
The heat capacity of a gas at constant pressure, Cp, is defined as
(dH/dT) so that
Cp dT= Vdp
From the hydrostatic equation we get
dp = -g σ dz
Hence
Cp dT = -V g σ dz
For a unit mass of gas V=1/σ and we get
 dT
g

 d
dz
Cp
•
The dry adiabatic lapse rate plays an important role in atmospheric
stability.
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Fig. 3.17
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Lapse Rates and Stability
• Lapse rate is the rate at which the real atmosphere
falls off with altitude – the environmental lapse
rate
• An average value is 6.5 ºC per kilometer
• This should be compared with the adiabatic lapse
rate of 10 ºC.
• If the environmental lapse rate is less than 10 ºC,
then the atmosphere is absolutely stable
• If greater than 10 ºC, it is absolutely unstable
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Wet adiabatic lapse rate
• The presence of condensable vapors, such as water
vapor, complicates the process.
• As the parcel of air ascends it cools at the dry
adiabatic lapse rate until the water vapor reaches
saturation – then condensation takes place.
• This releases latent heat – which can raise the
temperature of the air parcel.
• Now the lapse rate depends on the amount of water
vapor – wet adiabatic lapse rate.
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Role of atmospheric stability
Temperature inversions produce very stable atmospheric
conditions in which mixing is greatly reduced. There are
two general types of inversions: surface inversions and
inversions aloft.
Surface inversions are the result of differential radiative
properties of the Earth’s surface and the air above. The Earth
is a much better absorber and radiator of energy than air; thus,
in the late morning and afternoon hours the lower atmosphere is
unstable. The opposite is true in the evening; a stable
atmosphere with little vertical mixing prevails.
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The Nocturnal Inversion
• On clear nights, a temperature inversion develops near
the surface.
- Air temperature usually decreases with height.
An inversion is a layer of air where temperature
increases with height.
- Because the layer of air in the inversion is warmer than
the air below it, the cooler air below the inversion
cannot rise above it. Pollutants near the surface are
therefore trapped below the inversion in the overnight
hours.
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Fig. 3.18
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Role of Atmospheric Stability
Inversions aloft are associated with prolonged, severe pollution
episodes. These types of inversions are caused by the sinking air
associated with the center of high pressure systems (subsidence).
As the air sinks it is warmed adiabatically. Turbulence at the very
lowest part of the atmosphere prevents subsidence from warming
that portion of the atmosphere.
Los Angles pollution episodes as well as those over the Mid-Atlantic
region are the result of inversions aloft associated with strong
high pressure systems.
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(Unstable)
(Near neutral stability)
Γ (dashed) – DALR
Solid - ELR
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