1. Introduction to Thermodynamic Diagrams
How thermodynamic diagrams are used to forecast thunderstorms?
Thermodynamics
M. D. Eastin

2. Introduction to Thermodynamic Diagrams
Outline:
Basic Idea of Thermodynamic Diagrams
Possible Diagrams
Skew-T Log-P Diagram
Rawinsondes
Dropsondes
Skew-T Applications
Thermodynamics
M. D. Eastin

3. 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…)
Thermodynamics
M. D. Eastin

4. 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 don’t observe volume
We need to use a different diagram
that satisfies all three requirements
and uses a coordinate system for
observable variables p
Isobar i
Isochor
Adiabat f
Isotherm V
Thermodynamics
M. D. Eastin

5. 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 P A B V W
Thermodynamics
M. D. Eastin

6. Basic Idea of Thermodynamic Diagrams
Area-Equivalent Transformations: P A B V
WpV
WAB
Thermodynamics
M. D. Eastin

7. Possible Thermodynamic Diagrams
Tephigram:
Area proportional to energy
3 sets of nearly straight lines
Isotherms (T)
“Dry” Adiabats (θ)
Saturation Mixing Ratio (w)
Isobars (p) are curved
Moist adiabats (θe) are curved
90º angle between dry adiabats
and isotherms
1000 mb
800 mb
600 mb
400 mb
Pressure
-40oC
Temperature
-20oC
0oC
q = 0oC
20oC
40oC
60oC qe w
Note: We will talk about the moist adiabats (θe)
and saturation mixing ratio (w) lines later
in the course
Thermodynamics
M. D. Eastin

8. Possible Thermodynamic Diagrams
Emagram:
Area proportional to energy
4 sets of nearly straight lines
Isobars (p)
Isotherms (T)
“Dry” Adiabats (θ)
Saturation Mixing Ratio (w)
Moist adiabats (θe) are curved
45º angle between dry adiabats
and isotherms
1000 mb
800 mb
600 mb
400 mb
-20oC
0oC
20oC
40oC
-40oC
Pressure
Temperature
q = 0oC
60oC
80oC
100oC qe w
Note: We will talk about the moist adiabats (θe)
and saturation mixing ratio (w) lines later
in the course
Thermodynamics
M. D. Eastin

9. See Example on Next Slide
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)
Dry adiabats (θ) are slightly curved
Moist adiabats (θe) are curved
~90º angle between adiabats
and isotherms
See Example on Next Slide
Note: We will talk about the moist adiabats (θe)
and saturation mixing ratio (w) lines later
in the course
Thermodynamics
M. D. Eastin

10. Possible Thermodynamic Diagrams
Skew-T Log-P Diagram:
Pressure (mb)
1000
900
800
700
600
500
300
200
400
Temperature (oC) 30 40 20 10
-10
Isobars (p)
Isotherms (T)
Saturation Mixing Ratio (w)
Dry Adiabats (θ)
Moist adiabats (θe)
Thermodynamics
M. D. Eastin

11. The 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!
Thermodynamics
M. D. Eastin

12. 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
Thermodynamics
M. D. Eastin

13. 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
Thermodynamics
M. D. Eastin

14. 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
Thermodynamics
M. D. Eastin

15. 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
Thermodynamics
M. D. Eastin

16. Skew-T Applications The moist 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
Thermodynamics
M. D. Eastin

17. Saturation Mixing Ratio (10 g/kg)
Skew-T Log-P Diagram
Pressure (200 mb)
Moist Adiabat (283K)
Dry Adiabat (283K)
Isotherm (T= -10ºC)
Saturation Mixing Ratio (10 g/kg)
10ºC = 283K
Thermodynamics
M. D. Eastin

18. Skew-T Log-P Diagram Plot Rawinsonde or Dropsonde Observations:
Temperature
Dewpoint
Temperature
Thermodynamics
M. D. Eastin

19. 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
Thermodynamics
M. D. Eastin

20. Standard 1200 UTC Rawinsonde Sites
The Global Rawinsonde Network
Standard 1200 UTC Rawinsonde Sites
Thermodynamics
M. D. Eastin

21. 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.
Thermodynamics
M. D. Eastin

22. Skew-T Applications Identify Temperature Inversions Inversions are
layers where
temperature
increases
with height
Thermodynamics
M. D. Eastin

23. Skew-T Applications Identify Dry Adiabatic Layers Dry adiabatic
layers have the
temperature
profile parallel
to a dry adiabat
Thermodynamics
M. D. Eastin

24. Skew-T Applications
Determine the Potential Temperature (θ) 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
θ = 313 K
Thermodynamics
M. D. Eastin

25. Introduction to Thermodynamic Diagrams
Summary:
Basic Idea of Thermodynamic Diagrams
Possible Diagrams
Skew-T Log-P Diagram
Rawinsondes
Dropsondes
Skew-T Applications
Thermodynamics
M. D. Eastin

26. References Thermodynamics M. D. Eastin
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
Thermodynamics
M. D. Eastin