Stability in the Atmosphere

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Stability in the Atmosphere
If an air parcel is displaced from its original
height it can Return to its original height
- Stable Accelerate upward because it is
buoyant - Unstable Stay at the place to which
it was displaced - Neutral
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Lapse Rates

• The Lapse Rate is the rate at which temperature
  changes with height in the atmosphere
• The environmental lapse rate is the cooling or
  warming as measured by a weather balloon.
• The parcel lapse rate is the rate at which a
  rising air parcel would warm or cool.
• This process is assumed to be adiabatic, which
  means no heat is added to or taken from the
  bounded parcel.

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Buoyancy

• If an air parcel is less dense than the
  surrounding air, it will RISE!
• The ideal gas law states that
• PV nRT or ? P/RT
• This means that as temperature increases, density
  decreases.
• Therefore, since a parcel will rise if it is less
  dense than the environment, it follows that it
  must also be WARMER than the surrounding
  environment
• Tenv lt Tparcel gt RISING MOTION
  UNSTABLE
• Tenv gt Tparcel gt SINKING MOTION STABLE
• Tenv Tparcel gt NEUTRAL

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Moist VS Dry Adiabatic Process

• Adiabatic means NO HEAT EXCHANGE!
• Gd is the dry adiabatic lapse rate (10C km-1)
• Gm is the moist adiabatic lapse rate (6C km-1)
• The moist adiabatic lapse rate is less than the
  dry adiabatic lapse rate because as vapor
  condenses into water (or water freezes into ice)
  for a saturated parcel, latent heat is released
  into the parcel, mitigating the adiabatic cooling
  --gt

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Absolute Stability

• Gd is the dry adiabatic lapse rate (10C km-1)
• Gm is the moist adiabatic lapse rate (6C km-1)
• Ge is the environmental lapse rate (0C km-1) as
  measured by a weather balloon.
• Hence, an unsaturated or saturated parcel will
  always be cooler than the environment and will
  sink back down to the ground
• This is an example of absolute stability
• The condition for absolute stability is Gd gt
  Gm gt Ge

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Example Inversion Layer

• An inversion layer is a layer in which
  temperature increases with height.
• This is an example of absolute stability.
• The environmental lapse rate will always be less
  than both the dry and moist adiabatic lapse
  rates, thus a rising air parcel will always be
  colder than the environment, thus negatively
  buoyant in an inversion layer!
• Inversion layers are key in suppressing
  thunderstorm development in the mornings.

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Absolute Instability

• Gd is the dry adiabatic lapse rate (10C km-1)
• Gm is the moist adiabatic lapse rate (6C km-1)
• Ge is the environmental lapse rate (30C km-1) as
  measured by a weather balloon.
• Hence, both a saturated and an unsaturated parcel
  will be WARMER, and thus less DENSE than the
  surrounding air. This means the parcel is
  ABSOLUTELY unstable and will be positively
  buoyant.

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Conditional Instability

• Gd is the dry adiabatic lapse rate (10C km-1)
• Gm is the moist adiabatic lapse rate (6C km-1)
• And for this example, a weather balloon has
  measured
• Ge is the environmental lapse rate (7.8C km-1)
• If a parcel is saturated, it will follow the pink
  line and be WARMER and therefore less dense than
  the environment. It will be positively buoyant,
  meaning it will RISE.
• If a parcel is unsaturated, it will follow the
  green line and be COLDER and more dense than the
  environment. It will be negatively buoyant.
• When Gd gt Ge gt Gm The environment is said to
  be conditionally unstable, depending on whether
  the parcel is saturated or not!

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Radiosondes
Twice a day at designated reporting stations
around the world upper air balloons are released
carrying an instrument called a radiosonde. The
radiosonde is a very small and light and contains
sensors, which measure temperature, moisture, and
pressure as the balloon rises. At the surface
meteorologists track the speed and heading of the
radiosonde using a dish antenna that locks onto a
radio signal that is emitted from the radiosonde.
Since the balloon drifts with the wind the wind
speed and direction of winds aloft are determined
by tracking the radiosonde. This information is
then transmitted to a ground-based receiver.
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Environmental Temperature Profile
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Skew-T
Environmental Dew Point Temp Profile
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Isotherms-Blue diagonal lines
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Dry Adiabats- Curving Green Lines
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Moist Adiabats- Curving Blue Lines
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Parcel Path- Solid Black Line
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PARCEL PATH
LCL
T sfc 24 o C 1. Draw a line starting at the
surface temperature, following the DRY ADIABAT Td
sfc 20 o C 2. Draw a line starting at the
surface dewppoint, following the MOIST
ADIABAT 3. Where these two lines cross is
called the LIFTING CONDENSATION LEVEL If air
can be lifted from the surface to this level, a
cloud will form. This will be the level of the
CLOUD BASE. 4. From here the parcel is
saturated, thereforeit will follow the MOIST
ADIABAT
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PARCEL PATH
LFC
inversion
In this case, there is an inversion just above
the surface, in which the environment is warming.
Recalling our discussion of instability, when
the environment warms more quickly than the dry
and moist adiabatic lapse rates, the atmosphere
is said to be ABSOLUTELY STABLE. This means that
an air parcel in this layer will be negatively
buoyant. This is inversion layer is sometimes
called a CAP. It caps the convective ability of
the environment. If an outside force, such as a
frontal passage, daytime heating, or topography,
can lift the air parcel through this level to a
point at which it becomes WARMER than the
environment, it is said to have reached the LEVEL
OF FREE CONVECTION.
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Forcing Air to Rise gt Cloud Development
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Polar Sounding
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Deep Convection
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Moist Capped Boundary Layeraka Loaded Gun
Sounding
CAPE Convective Available Potential
Energy. Defines the vertically integrated
positive buoyancy of a rising parcel. Basically,
it is a measure of the area between the parcel
path and the environmental lapse rate. The
greater the area, the greater the potential for
severe weather.
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Desert Sounding Typical Summer Day in Denver