ChicagoLand Glider Council

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ChicagoLand Glider Council
Soaring Weather and Data Analysis Greg
Chisholm December 19, 2000
Al McDonald Photo
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Outline

• The internet - so much data so little time
• Brief introduction
• Introduce SkewT/LogP
• Examples
• Discussion on how to improve our page

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Whats out there! Meteo-Speak

• Sounding diagrams provide a important means for
  determining the stability of the atmosphere above
  a specific location. By using the concept of an
  air parcel, lifting it or lowering it and
  comparing the resulting parcel conditions to the
  conditions of the surrounding environment as
  defined by the balloon sounding.
• Parcels of unsaturated air tend to follow the dry
  adiabat lines as they ascend or descend. The
  saturation adiabats lines show how parcels
  saturated with water vapor will ascend or
  descend. Descending parcels will tend to
  unsaturate immediately. The mixing ratio lines
  relate to the amount of water vapor in a parcel
  in grams of water vapor per kilograms of dry air.
  Parcels of air attempt to maintain a constant
  mixing ratio as they ascend or descend.
  Generally, a parcel will rise, following the dry
  adiabat until it saturates. This occurs when dry
  adiabat crosses the initial mixing ratio line.
  This is considered the LCL. If lifting continues,
  the parcel cools following the saturation
  adiabat. If the parcel descends, it will always
  follow the dry adiabat as it will immediately
  unsaturate if saturated. By then comparing the
  parcel temperature to the environment, you can
  determine whether it is stable (parcel cooler) or
  unstable (parcel warmer). An unstable parcel will
  accelerate upwards and is the primary means for
  thunderstorm development. A stable parcel will
  decelerate and eventually descend. This is the
  typical atmospheric condition and it the primary
  condition in high pressure areas. The descending
  air desaturates the atmosphere and leads to
  clearing skies and calm conditions

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What we want

• Top and strength of the lift
• Thermal Indices
• Cloud base estimates
• Perturbations, i.e.,
• Clamp
• Sea Breeze Front
• Conditions over our airfield and changes over
  time
• Thermals are a product of instability -
  their height depends on the depth of the unstable
  layer and their strength depends on the degree of
  instability. To arrive at an estimate of the
  thermal height and strength, a thermal index (TI)
  is computed ... (Extract from SSA's Soaring
  Flight Manual)

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Kevin Fords Offering - Abbreviated
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Partial output for an ILX request
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Introduction SkewT/LogP

• Temperature as a function of height (denoted by
  pressure)
• Pressure is plotted horizontally on an inverse
  log scale LogP
• Temperature is plotted with a skew to the right
  SkewT
• Dry and saturation adiabats are drawn to aid
  analysis
• Winds as a function of height are also plotted
• Additional esoteric meteo-stuff is provided

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GOES SkewT
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Thermal Soaring and the SkewT Valley Soaring
Association P.J. Kelley

• Want to predict height and strength of thermals
• Most SkewT diagrams are too complicated for our
  use
• especially since were only interested in the
  bottom 25
• For example -- 0-50K pressure/alt
  temperature dry adiabat sat. adiabat mixing
  ratio dewpoint hodogram wind)
• Note the following is provided by soaring pilots
  for soaring pilots meteorologists be warned

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Blank Soaring SkewT
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Building Your Own SkewT

• Plot the forecast high temperature for the day,
  e.g., 86F., at airfield elevation
• Plot the forecast temperatures aloft, e.g.,
• 3,000, 260/8, no temp
• 6,000, 270/7, 12C ? 53F
• 9,000, 300/15, 7C ? 45F
• 12,000, 290/15, -2C ? 28F
• Connect the dots
• The top of the thermals is the intersection of
  the dry adiabat for todays high and the
  temperature aloft plot (sounding)

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Example
12K ?,28F

• Forecast Hi
• Temps aloft
• Connect dots
• Intersection Top

9K ?,45F
This Days Thermal Top
6K?,53F
Forecast Hi
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Thermal Strength

• Thermal Index (RAOB based ala Kevin Ford)
• At any altitude below the top of the lift, the
  difference between
• the dry adiabatic temperature (i.e., the
  intersection of the adiabat eminating from the
  forecast high temperature/field altitude point on
  the graph and the altitude of interest), for the
  days forecast high temp. and
• the sounding temp.
• Thermal Index (time dependant)
• ..., the difference between
• the dry adiabat temperature (i.e., the
  intersection of the adiabat eminating from the
  ground temperature/field altitude point on the
  graph and the altitude of interest), and
• the sounding temperature

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Kevin Ford Example
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SkewT/LogP Example
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SkewT/LogP Example (Earlier Time)
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Clouds

• Will there be clouds?
• Follow the morning dew point from the ground,
  along the line of constant mixing ratio.
• If this line never crosses the sounding, a parcel
  will not saturate and clouds will not form.
• If the relative humidity is 100 percent (i.e.,
  dew point temperature and actual air temperature
  are the same), this does NOT necessarily mean
  that precipitation will occur. It simply means
  the air is holding as much moisture as it can at
  the particular temperature the air is at.
  Saturation may result in fog (at the surface)
  and clouds aloft (which consist of tiny water
  droplets suspended in the air).

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Will there be clouds?

• The LCL may have little realistic significance if
  the air parcels do not lift.
• Therefore, it is useful to anticipate convective
  lifting that may be realized when the air is
  heated from the earth's surface, most likely
  during sunshine.
• This leads to the concept of convective
  condensation level (CCL). This is the level where
  the saturation mixing ratio isopleth through the
  surface wet-bulb temperature intersects the
  ambient temperature. The dry adiabat through the
  CCL point on the ambient temperature goes through
  the temperature (convective temperature) at the
  level po.
• This is the temperature that the air at the
  surface must attain to form the first cumulus. In
  other words, if the air at the surface is heated
  to the convective temperature, then the
  convection will be deep enough to form clouds at
  the CCL

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Where is cloudbase?

• Unsaturated air rises along the dry adiabat. As
  it rises, its dew point follows a line of
  constant mixing ratio.
• At some point, the temperature and dew point
  curves meet. At this point, the rising air is
  saturated and clouds form.
• The level at which this occurs is known as the
  lifting condensation level (LCL), and it defines
  the cloud base, if the air is lifted from the
  surface
• (Surface Max. Temp. Surface DewPoint)/4.5
  Cloudbase Altitude (temps. n degrees F.)
• (60F-40F)/4.54444'

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Where are the tops of the clouds?

• If the air is lifted still further, both the dew
  point and temperature follow a pseudoadiabat
  (TTd for this saturated air).
• If this pseudoadiabat becomes warmer than the
  environmental air, the rising volume of air will
  be buoyant at that level and will begin to rise
  freely.
• In such circumstances, cumulus clouds, or even
  cumulonimbus (thunderstorms) are likely.
• Continuing along the pseudoadiabat, the ascending
  air eventually returns to meet the environmental
  temperature curve, and is no longer buoyant. This
  point is a good estimate for cloud top height

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http//www.arl.noaa.gov/ready-bin/metgram1.pl
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Fig
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CGC Wx, part 2
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Short List
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But what about?

• Clamp
• Wind
• Inversion Layers
• ...

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Summary

• The web is a wonderful source of information
• However, distillation of information leaves more
  time for flying
• Please offer suggestions on what youd like to
  see presented on the Wx page