Introduction to Thermodynamic Diagrams

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Introduction to Thermodynamic Diagrams
How thermodynamic diagrams are used to forecast
thunderstorms?
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Introduction to Thermodynamic Diagrams

• Outline
• Basic Idea of Thermodynamic Diagrams
• Possible Diagrams
• Skew-T Log-P Diagram
• Rawinsondes
• Dropsondes
• Skew-T Applications

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Basic Idea of Thermodynamic Diagrams

• Advantages
• A visualization tool
• We can always use the mathematical formulas
• Many of us learn better through visualization
• Eliminates or simplifies the equations
• Can determine many quantities in a graphical
  format
• Desired Qualities in a Thermodynamic Diagram
• 1. For cyclic processes, the area should be
  proportional to the work done
• or the heat exchanged
• 2. The lines should be straight (easy to use)
• 3. The angle between adiabats and isotherms
  should be as large as possible
• (easy to distinguish variations in
  atmospheric stability ? will air rise or sink)
• (more on stability later)

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Possible Thermodynamic Diagrams

• P-V Diagrams?
• Pros
• Satisfies Requirement 1
• Good for illustrating basic concepts
• Cons
• Angle between isotherms and
• adiabats is very small
• Isotherms and adiabats are not
• straight lines
• We dont observe volume
• We need to use a different diagram
• that satisfies all three requirements
• and uses a coordinate system for

p
Isobar
i
Isochor
Adiabat
f
Isotherm
V
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Basic Idea of Thermodynamic Diagrams

• Area-Equivalent Transformations
• P-V diagrams only satisfy Requirement 1
  Enclosed area proportional to energy
• Thus, we need to consider other variables for
  the coordinate systems
• Create a generic transformation from P, V ? A, B

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Basic Idea of Thermodynamic Diagrams
Area-Equivalent Transformations
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Possible Thermodynamic Diagrams

• Tephigram
• Area proportional to energy
• 3 sets of nearly straight lines
• Isotherms (T)
• Adiabats (?)
• Saturation Mixing Ratio (w)
• Isobars (p) are curved
• Pseudo-adiabats (?e) are curved
• 90º angle between adiabats
• and isotherms

Note We will talk about the pseudo-adiabats
(?e) and saturation mixing ratio (w)
lines later in the course
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Possible Thermodynamic Diagrams

• Emagram
• Area proportional to energy
• 4 sets of nearly straight lines
• Isobars (p)
• Isotherms (T)
• Adiabats (?)
• Saturation Mixing Ratio (w)
• Pseudo-adiabats (?e) are curved
• 45º angle between adiabats
• and isotherms

Note We will talk about the pseudo-adiabats
(?e) and saturation mixing ratio (w)
lines later in the course
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Possible Thermodynamic Diagrams

• Skew-T Log-P Diagram
• Area proportional to energy
• 3 sets of nearly straight lines
• Isobars (p)
• Isotherms (T)
• Saturation Mixing Ratio (w)
• Adiabats (?) are slightly curved
• Pseudo-adiabats (?e) are curved
• 90º angle between adiabats
• and isotherms

See Example on Next Slide
Note We will talk about the pseudo-adiabats
(?e) and saturation mixing ratio (w)
lines later in the course
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Possible Thermodynamic Diagrams
Skew-T Log-P Diagram
Isobars (p) Isotherms (T) Saturation Mixing Ratio
(w) Adiabats (?) Pseudo-adiabats (?e)
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The Skew-T Log-P Diagram

• Skew-T Log-P Diagram
• Most commonly used diagram (we will use it too)
• Come in a variety of shapes, sizes, and colors.
• All Skew-T Log-P diagrams provide the exact same
  information!

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The Skew-T Log-P Diagram
Note how the lines of constant temperature
slope (or are skewed) toward the upper
left Hence, Skew-T These lines are always
solid and straight but vary in color Our
Version Red solid lines
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The Skew-T Log-P Diagram
Note how the change in pressure along the
Y-axis in non-uniform Rather, it changes
logarithmically Hence, Log-P These lines are
always solid and straight but may vary in
color Our Version Blue solid lines
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The Skew-T Log-P Diagram
The dry adiabats, or lines of constant
potential temperature slope at almost right
angles to the isotherms These lines are
always solid and slightly curved, but may
vary in color Our Version Light Blue Solid
Lines
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The Skew-T Log-P Diagram
The lines of constant saturation mixing ratio
are also skewed toward the upper left More on
these in a future lecture These lines are
always dashed and straight, but may vary in
color Our Version Pink dashed Lines
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Skew-T Applications
The saturation adiabats are lines of constant
equivalent potential temperature and they
represent pseudo- adiabatic processes More on
these in a future lecture These lines are
always dashed and curved, but may vary in
color Our Version Dashed bluish-green
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Skew-T Log-P Diagram
Pressure (200 mb)
Pseudo-Adiabat (283K)
Dry Adiabat (283K)
Isotherm (T-10ºC)
Saturation Mixing Ratio (10 g/kg)
10ºC 283K
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Skew-T Log-P Diagram
Plot Rawinsonde or Dropsonde Observations
Temperature
Dewpoint Temperature
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The Rawinsonde

• Instrument Package attached to a Balloon
• Launched twice daily (00 and 12 UTC)
• Regular launch locations
• Rise from the surface into the stratosphere
  before the balloon bursts
• Observe pressure (p), temperature (T), dewpoint
  temperature (Td),
• altitude (z), and horizontal winds (speed,
  direction) at numerous
• regular levels through the atmosphere.

Temperature and Humidity Sensor
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The Global Rawinsonde Network
Standard 1200 UTC Rawinsonde Sites
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The Dropsonde

• Instrument Package attached to a Parachute
• Launched from aircraft or hot air balloons
• over data sparse regions (e.g. the oceans)
• Used to improve high-impact forecasts
• Hurricane forecasts
• Winter storm forecasts
• Irregular launch times and locations
• Fall the from launching platform down to
• the surface using a parachute that
• controls the rate of descent
• Observe pressure (p), temperature (T),
• dewpoint (Td), altitude (z), and horizontal
• winds (speed, direction) at numerous
• regular levels through the atmosphere.

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Skew-T Applications
Identify Temperature Inversions
Inversions are layers where temperature increases
with height
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Skew-T Applications
Identify Dry Adiabatic Layers
Dry-adiabatic layers have the temperature profile
parallel to a dry adiabat
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Skew-T Applications
Determine the Potential Temperature (?) of any
Air Parcel
Bring air parcel down a dry-adiabat to 1000
mb Add 273 K to the T-value
Begin with parcel at 400 mb
T 40ºC ? 313 K
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Introduction to Thermodynamic Diagrams

• Summary
• Basic Idea of Thermodynamic Diagrams
• Possible Diagrams
• Skew-T Log-P Diagram
• Rawinsondes
• Dropsondes
• Skew-T Applications

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References
Petty, G. W., 2008 A First Course in
Atmospheric Thermodynamics, Sundog Publishing,
336 pp. Tsonis, A. A., 2007 An Introduction to
Atmospheric Thermodynamics, Cambridge Press, 197
pp.   Wallace, J. M., and P. V. Hobbs, 1977
Atmospheric Science An Introductory Survey,
Academic Press, New York, 467 pp. Also (from
course website) NWSTC Skew-T Log-P Diagram and
Sounding Analysis, National Weather Service,
2000   The Use of the Skew-T Log-P Diagram in
Analysis and Forecasting, Air Weather Service,
1990