Moisture and Atmospheric Stability

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Moisture and Atmospheric Stability

• AOS 101 Discussion

Discussion Leader Val
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Contouring Help

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Review

• Turn in hw 3
• Badger forecasts
• Why do you dry off faster in a desert climate?

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The biggest power plant on Earths surface-
http//www.srh.weather.gov/jetstream/atmos/hydro.h
tm
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Water can exist in all three phases in our
atmosphere

• What term do we seem to use to quantify the
  amount of water in any given volume of air at one
  time?
• Answer Moisture

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Ways to measure the moisture content of the
atmosphere (discussed in lec.)

• Absolute Humidity
• Specific Humidity
• Saturation Mixing Ratio
• Vapor Pressure
• Saturation Vapor Pressure
• Relative Humidity
• Dew Point Temperature

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The variables we will refer to most

• Mixing Ratio- mass of water vapor/mass of dry air
  (does not change).
• Relative Humidity- Vapor Pressure/ Saturation
  vapor pressure.
• Dew Point Temperature- The temperature at which
  air with the current amount of vapor in it will
  become saturated.

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Two ways to saturate the air
(or raise the
relative humidity)
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Two ways to saturate the air
(or raise the
relative humidity)

• 1. Add more water vapor to it
• 2. Decrease the temperature
• This is because warm air is capable of holding
  more water vapor molecules than cold air.
• (Remember the water vapor molecules are moving
  faster in warm air and less likely to stick and
  condense)

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Moisture

• An air parcel with a large moisture content has
  the potential for that parcel to produce a great
  amount of precipitation.
• - Air with a mixing ratio of 13 g/kg will likely
  rain a greater amount of water than air with a
  mixing ratio of 6 g/kg.

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Moisture
Two parcels of air PARCEL 1 Temperature 31
oF, Dewpoint 28 oF PARCEL 2 Temperature
89 oF, Dewpoint 43 oF
Parcel 2 contains more water vapor than Parcel 1,
because its dewpoint is higher. Parcel 1 has a
higher relative humidity, because it wouldnt
take much cooling for the temperature to equal
the dewpoint! Thus, Parcel 1 is more likely to
become saturated. But if it happened that both
parcels became saturated then Parcel 2 would have
the potential for more precipitation. RH is not
simply equal to the dewpoint divided by the temp.
but is a good representation.
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The other key component to the hydrologic cycle-
Stability

• What is stability?
• Stability refers to a condition of equilibrium
• If we apply some perturbation to a system, how
  will that system be affected?
• Stable System returns to original state
• Unstable System continues to move away from
  original state
• Neutral System remains steady after perturbed

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Stability Example
Stable Marble returns to its original position
Unstable Marble rapidly moves away from initial
position
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Stability
How does a bowl and marble relate to the
atmosphere??

• When the atmosphere is stable, a parcel of air
  that is lifted will want to return back to its
  original position

http//www.chitambo.com/clouds/cloudshtml/humilis.
html
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Stability Cont.

• When the atmosphere is unstable (with respect to
  a lifted parcel of air), a parcel will want to
  continue to rise if lifted

http//www.physicalgeography.net/fundamentals/imag
es/cumulonimbus.jpg
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What do we mean by an air parcel?

• Imaginary small body of air a few meters wide
• Can expand and contract freely
• Does not break apart
• Only considered with adiabatic processes -
  External air and heat cannot mix with the air
  inside the parcel
• Parcel warms or cools purely due to pressure
  changes (?U Q W)

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Buoyancy and Stability

• At same pressure if at same altitude!
• So if ?parcel lt ?env. Parcel is positively
  buoyant
• In terms of temperature that would mean
• T of parcel gt T of environment buoyant!
  (unstable)
• T of parcel lt T of environment sink! (stable)
• T of parcel T of environment stays put
  (neutral)

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Atmospheric Stability (Review)
This is all well and good but what about day to
day applications almost there
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Vertical Profile of Atmospheric Temperature
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Lapse Rates
Lapse Rate The rate at which temperature
decreases with height (Remember the inherent
negative wording to it) Environmental Lapse
Rate Lapse rates associated with an observed
atmospheric sounding (negative for an inversion
layer) Parcel Lapse Rate Lapse rate of a
parcel of air as it rises or falls (either
saturated or not) MALR - Moist Adiabatic Lapse
Rate Saturated air parcel DALR - Dry Adiabatic
Lapse Rate Dry air parcel
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DALR

• Air in parcel must be unsaturated
• (RH lt 100)
• Rate of adiabatic heating or cooling 9.8C for
  every 1000 meter (1 kilometer) change in
  elevation
• Parcel temperature decreases by about 10 if
  parcel is raised by 1km, and increases about 10
  if it is lowered by 1km

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MALR (or SALR)

• As rising air cools, its RH increases because the
  temperature approaches the dew point temperature,
  Td
• If T Td at some elevation, the air in the
  parcel will be saturated (RH 100)
• If parcel is raised further, condensation will
  occur and the temperature of the parcel will cool
  at the rate of about 6C per 1km in the
  mid-latitudes

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DALR vs. MALR

• The MALR is less than the DALR because of latent
  heating
• As water vapor condenses into liquid water for a
  saturated parcel, LH is released, lessening the
  adiabatic cooling

Remember no heat exchanged with environment
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DALR vs. MALR
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Absolute Stability

• The atmosphere is absolutely stable when the
  environmental lapse rate (ELR) is less than the
  MLR
• ELR lt MALR lt DALR
• A saturated OR unsaturated parcel will be cooler
  than the surrounding environment and will sink,
  if raised

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Absolute Stability

• Inversion layers are always absolutely stable
• Temperature increases with height
• Warm air above cold air very stable

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Absolute Instability

• The atmosphere is absolutely unstable when the
  ELR is greater than the DALR
• ELR gt DALR gt MALR
• An unsaturated OR saturated parcel will always be
  warmer than the surrounding environment and will
  continue to ascend, if raised

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Conditional Instability

• The atmosphere is conditionally unstable when the
  ELR is greater than the MALR but less than the
  DALR
• MALR lt ELR lt DALR
• An unsaturated parcel will be cooler and will
  sink, if raised
• A saturated parcel will be warmer and will
  continue to ascend, if raised

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Conditional Instability

• Example parcel at surface
• T(p) 30C, Td(p) 14C (unsaturated)
• ELR 8C/km for first 8km
• Parcel is forced upward, following DALR
• Parcel saturated at 2km, begins to rise at MALR
• At 4km, T(p) T(e)this is the level of free
  convection (LFC)

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Conditional Instability

• Example continued
• Now, parcel will rise on its own because T(p) gt
  T(e) after 4km
• The parcel will freely rise until T(p) T(e),
  again
• This is the equilibrium level (EL)
• In this case, this point is reached at 9km
• Thus, parcel is stable from 0 4km and unstable
  from 4 9km

LCL
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Lifting due to Topography
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How does the parcel get a lift?

• Convection
• Convergence
• Topography

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Lifting due to Topography

• This type of lifting occurs when air is
  confronted by a sudden increase in the vertical
  topography of the Earth
• When air comes across a mountain, it is lifted up
  and over, cooling as it is rising
• The type of cloud formed is dependent upon the
  moisture content and stability of the air

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Lifting by Convection

• As the earth is heated by the sun, thermals
  (bubbles of hot air) rise upward from the surface
• The thermal cools as it rises, losing some of its
  buoyancy (its ability to rise)
• The vertical extent of the cloud is largely
  determined by the stability of the environment

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Lifting by Convection

• A deep stable layer restricts continued vertical
  growth
• A deep unstable layer will likely lead to
  development of rain-producing clouds
• These clouds are more vertically developed than
  clouds developed by convergence lifting

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Lifting by Convergence

• Convergence exists when there is a horizontal net
  inflow into a region
• When air converges along the surface, it is
  forced to rise

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Lifting by Convergence

• Large scale convergence can lift air hundreds of
  kilometers across
• Vertical motions associated with convergence are
  generally much weaker than ones due to convection
• Generally, clouds developed by convergence are
  less vertically developed