Moisture and Atmospheric Stability

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Moisture and Atmospheric Stability

• AOS 101 Discussion 305
• March 28th, 2008

Discussion Leader Dan Hartung
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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
• Definition - Refers to the water vapor content in
  the atmosphere, or the total water, liquid, solid
  or vapor, in a given volume of air. (NWS glossary)

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

• Absolute Humidity- The ratio of the mass of water
  vapor to the volume occupied by a mixture of
  water vapor and dry air.
• Specific Humidity- The mass of water vapor per
  unit mass of air, including the water vapor.
• Mixing Ratio- mass of water vapor/mass of dry
  air.
• Saturation Mixing Ratio- mass of water vapor when
  a parcel is saturated/mass of dry air in the
  parcel.
• Vapor Pressure- Pressure of water vapor
  constituent of the atmosphere.
• Saturation Vapor Pressure- The pressure of water
  vapor constituent when the atmosphere is
  saturated.
• Relative Humidity- Vapor Pressure/ Saturation
  vapor pressure or mixing ratio/ saturation mixing
  ratio
• Dew Point Temperature- The temperature at which
  air with the current amount of vapor in it will
  become saturated.

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The variables we will refer to most

• Absolute Humidity- The ratio of the mass of water
  vapor to the volume occupied by a mixture of
  water vapor and dry air.
• Specific Humidity- The mass of water vapor per
  unit mass of air, including the water vapor.
• Mixing Ratio- mass of water vapor/mass of dry air
  (does not change).
• Saturation Mixing Ratio- mass of water vapor when
  a parcel is saturated/mass of dry air in the
  parcel.
• Vapor Pressure- Pressure of water vapor
  constituent of the atmosphere.
• Saturation Vapor Pressure- The pressure of water
  vapor constituent when the atmosphere is
  saturated.
• 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)

• 1. Add more water vapor to it
• 2. Decrease the temperature
• This is because warm is capable of holding more
  water vapor molecules then 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 (Review)

• An air parcel with a large moisture content
  usually signifies 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 (Review)
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. However, 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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Moisture and the Diurnal Temperature Cycle

• Review Water has a high heat capacity (it
  takes lots of energy to change its temperature)
• As a result, a city with a dry climate (like
  Sacramento, CA) will have a very large diurnal
  (daily) temperature cycle
• A city with high water vapor concentration (like
  Key West, FL) will have a small diurnal cycle

Late July averages Sacramento 94/60 Key
West 90/79
Avg. Daily temps. In two locations of similar
lat. RENO, NV 72/40 DAYTON, OH - 74/53
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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.maltaweather.info/cumulus.jpg
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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//blogs.trb.com/news/weather/weblog/wgnweathe
r/archives/051906_cumulus_clouds.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 (first law of thermodynamic
  implies then that the parcel warms or cools
  purely due to pressure changes) (?U Q W)
• Space occupied by air molecules inside parcel
  defines the air density
• Average speed of molecules directly related to
  air temperature
• Molecules colliding against parcel walls define
  the air pressure inside

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Bouyancy and Archimedes Principle

• Archimedes Principle states that a body immersed
  in a fluid experiences a buoyant force equal to
  the weight of the displaced fluid.
• Log example from class. Remember the volume was
  the same when the log was submerged. The only
  difference was the masses. Since the log had a
  mass less the water it floated. I.E. the density
  of the log was less the density of the water.
  This is not so easy to measure in the atmosphere,
  so we use the ideal gas law P ?RT to compare
  temps at the same pressure level.

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Bouyancy and Stability cont.

• Since Pressure is the same, the only other
  variable changing is temperature. But remember
  they are on the same side of the equation so they
  are inversely proportional
• So if ?parcel lt ?env. Then the parcel floats or
  in other words is positively buoyant
• In terms of temperature that would mean
• If
• T of parcel gt T of environment the parcel is
  positively buoyant (less dense and will rise)
  (unstable) (or less stable)
• T of parcel lt T of environment the parcel is
  negatively buoyant (more dense and will sink)
  (stable)
• T of parcel T of environment the parcel is
  neutrally buoyant (will not rise or sink)
  (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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Review Atmospheric Soundings

• Vertical profiles of the atmosphere are taken
  at 0000 UTC (7 AM CDT) and 1200 UTC (7 PM CDT) at
  80 stations across the country, and many more
  around the world. Sometimes also launched at
  other times when there is weather of interest in
  the area.
• Weather balloons rise to over 50,000 feet and
  take measurements of several meteorological
  variables using a radiosonde.

• Temperature
• Dew point temp.
• Wind
• Direction and Speed
• Pressure

http//www2.ljworld.com/photos/2006/may/24/98598/
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Vertical Profile of Atmospheric Temperature,
Allow us to assess Atmospheric Stability
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Remember we touched on lapse rates during our
talk about convection?
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 (Relative
  Humidity 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

• 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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Rising Air

• Consider an air parcel rising through the
  atmosphere
• The parcel expands as it rises
• The expansion, or work done on the parcel causes
  the temperature to decrease
• As the parcel rises, humidity increases and
  reaches 100, leading to the formation of cloud
  droplets by condensation

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Rising Air

• If the cloud is sufficiently deep or long lived,
  precipitation develops.
• The upward motions generating clouds and
  precipitation can be produced by
• Convection in unstable air
• Convergence of air near a cloud base
• Lifting of air by fronts
• Lifting over elevated topography

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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

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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 Along Frontal Boundaries

• Front The transition zone between two air
  masses of different densities
• Lifting occurs along two different types of
  fronts
• Cold Front
• Warm Front

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Lifting Along Cold Fronts

• A colder,denser air mass lifts the warm, moist
  air ahead of it
• As the air rises, it cools and condenses,
  producing clouds and precipitation
• The steep slope of the cold front leads to more
  vigorous rising motion
• Hence, cold fronts are often associated with
  thunderstorms

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Lifting Along Cold Fronts
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Lifting Along Warm Fronts

• A warmer, less dense air mass rises up and over
  the cold air ahead of the warm front
• Air rises, cools and condenses
• Warm fronts have gentler slopes and move slower
  than cold fronts
• Generally, precipitation is more steady and
  widespread

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Lifting Along Warm Fronts
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• More to come next week on identifying different
  vertical characteristics of the atmosphere based
  on soundings including cloud formation and severe
  weather