The Moisture Composite Chart

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The Moisture Composite Chart

• Geography 620

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Moisture Composite Chart

• The composite chart is a four-panel chart
  containing analyses of the lifted index, K-index,
  lowest freezing level, precipitable water and
  relative humidity.
• The chart is prepared daily from 00Z and 12Z
  upper air rawinsonde observation data.
• Chart applies to data between surface and 500 mb
  level.

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Stability Indices

• The upper-left panel of the composite chart
  contains stability indices the Lifted and K-
  index.
• Lifted index isopleths are drawn for intervals of
  4 units for index values of 4 and lower. The
  zero isopleth is drawn as a heavier solid line.
• Generally negative L.I. values indicate
  instability

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Obtaining the lifted index

• An imaginary air parcel is lifted from the
  surface (the surface air temperature) both dry
  adiabatically and wet adiabatically to the 500 mb
  level.
• At 500mb the parcels temperature is compared to
  that of the sounding.
• L.I. T500 - Tparcel

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K-index

• K values are below those of the lifted index.
• Isopleths of the K-index are not drawn.
• K index is more an indicator of heavy rain than
  instability. High k-index values indicate good
  chance of heavy rain as much as it means
  instability.

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Precipitable Water

• Precipitable water is shown in the upper
  right-hand corner. It is analyzed for intervals
  of 0.50" with 0.25" (dashed). Isopleths are used
  to define the pattern when necessary.
• Bottom number is of normal, which may have more
  practical information, accounting for unusual
  events in dry climates, etc.

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Calculating P.W.

• Precipitable water (surface to 500 mb)
• computed from the temperature and dewpoint
  values at each point on the rawinsonde sounding.
  
• The precipitable water observation shown on the
  chart is the sum of the individual pressure
  weighted precipitable water values for each
  sublayer defined by the temperature-dewpoint
  observations on the sounding.

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Precipitable water

• Thus
•  
• where
• and
• where W is the total precipitable water from
  surface to 500 mb (inches),
• p is the pressure difference between the top and
  bottom of each sublayer (mb),
• r is the average mixing ratio of the sub-layer
  of pressure (g/kg),
• e is the average vapor pressure of the pressure
  sub-layer in mb
• p is the average pressure of the sub-layer ?p in
  mb.

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Freezing Level Panel

• Freezing levels may occur at more than one spot
  in a rawinsonde sounding.
• The lowest freezing level is analyzed on the
  Freezing Level Panel below the station circle.
• The surface intersection (32oF) is drawn as a
  dashed line.
• The freezing level elevation above ground is also
  presented as contours drawn as solid lines at
  4000 foot intervals to 12000 ft.
• These contours are discontinuous where they
  intersect smoothed terrain contours in the
  western United States.

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• Pressure of the freezing level
• calculated by logarithmic interpolation from the
  pressure levels reporting temperatures on either
  side of zero Celsius.
• Height of the freezing level pressure is next
  calculated from heights at the standard pressure
  levels (e.g., 850 mb, 700 mb, etc.) by
  logarithmic interpolation or extrapolation,
  whichever is necessary.
• Useful for avoiding icing conditions.

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Relative Humidity Panel

• The relative humidity panel is in the lower right
  of the composite chart.
• The relative humidity is analyzed for intervals
  of 10 for humidities of 50 and higher.
• The 50 and 90 isopleths are drawn as heavier
  solid lines.

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• The average relative humidity (surface to 500 mb)
  is computed first for each sub-layer of the
  atmosphere, weighted by the pressure depth of the
  sub-layer, summed and pressure averaged over the
  layer between the surface and 500 mb. Thus

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• where
• and where H is the average relative humidity in
  the layer surface to 500 mb (in ) h-bar is the
  average relative humidity in each sub-layer ()
  e is the average vapor pressure (mb) and es is
  the average saturation vapor pressure (mb) for
  the sub-layer.

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Questions for today

• 1. Review the stability index panel. List some
  locations having the greatest potential for
  instability and thunderstorms according to the
  lifted index, including the location of the
  lowest index value. 
• 2. Where is the 1000-500 mb level precipitable
  water content highest and where is it highest in
  terms of seasonal departure from normal?
•  
• 3. What is the freezing level (full amount with
  units) at Wilmington.
• 4. Look at the soundings for northern AZ and
  Riverton WY. What characteristics of the
  sounding is occurring that produces these frz
  level observations? Do situations occurring in
  the Riverton WY sounding to produce the double
  freeze level also occur at Salt Lake UT or Grand
  Junction CO soundings?
• 5. Where (in general) is the relative humidity
  highest east of the Rockies?

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Miscellaneous charts

• http//weather.noaa.gov/fax/miscella.shtmlmhum

http//weather.noaa.gov/fax/nwsfax.html