Skew T Log P Diagram
AOS 330 LAB 7
• Difference between ( x or dx),  x and  x
• Hypsometric Equation
• Equal Area Transformation
3 Desirable Characteristics of a
thermodynamic diagram
1. They are designed so that area on the diagrams
is proportional to energy.
2. The fundamental lines are straight and thus easy
to use.
3. On a skew-T log P diagram the isotherms(T) are
at almost 90o to the isentropes (q).
We want the isentropes and isotherms to be far
apart to help us to see small temperature
changes relative to the dry lapse rate , which
is important in determining stability. d
Atmospheric Soundings Plotted on
Skew-T Log P Diagrams
Allow us to identify stability of a layer
Allow us to identify various air masses
Tell us about the moisture in a layer
Help us to identify clouds
Allow us to speculate on processes occurring
What are all these lines?
Isobars and Isotherms
Dry Adiabats and Isotherms
1000 hPa 
q  T 
R /cp
Dry Adiabatic Lapse Rate
• The rate which T decreases with height for a dry
parcel that rises adiabatically.
2 km
d 
12.4 deg. C
 9.8K / km
1 km
22.2 deg. C
1000 hPa 
q  T 
R /cp
0 km
30 deg. C
ascent or
Pseudoadiabats and Saturated Mixing
Ratio lines
Hey, what are Pseudoadiabats?
To make certain calculations easier, we assume that
condensate falls from the parcel as soon as it forms.
This obviously isn’t entirely realistic, but the resulting
lapse rate only differs from the moist adiabatic rate by
about 1% in most instances.
Now, let’s see what adiabatic and pseudoadiabatic
processes look like when we plot them up.
An Adiabatic Process
Is this process reversible?
Adiabatic / Pseudoadiabatic process
Now, is this process reversible?
Common Inversions
• Subsidence Inversion
- typically form in regions of large scale sinking motion (under
the subtropical highs, under the left entrance / right exit
region of jets) or on the periphery of convective cells.
• Radiation Inversion
- where would you expect a radiation inversion to develop?
and when?
• Frontal Inversion
- transition zone in between the cold and warm airmass.
Subsidence Inversion
Suppose air
from 200 –
300hPa layer
is subsiding
Lapse rate >0
500 hPa:
Close to
Below 500hPa:
Stronger and
almost 10degC
warmer above
Subsidence Inversion
Potter and Coleman, 2003a
Radiation Inversion
Radiation Inversion
Potter and Coleman, 2003a
Frontal Inversion
Potter and Coleman, 2003a
The Tropopause
The last item we’ll be concerned with for today is the
tropopause, since it’s the upper limit of what we ordinarily
consider “weather.” Is this always the case? There are other
important reasons for knowing where the tropopause is that
we’ll get to later in the course.
There’s a long, technical definition given by the WMO, but in
general the tropopause is identified by an abrupt change in
lapse rate toward more stable (sometimes even inverted)
Finding the Tropopause
• Hess, 1959: Introduction to Theoretical Meteorology, Holt,
Rinehart and Winston, 1959
• Petty, G (2008). A First Course in Atmospheric
Thermodynamics, Sundog Publishing.
• Potter and Coleman, 2003a: Handbook of Weather, Climate
and Water: Dynamics, Climate, Physical Meteorology,
Weather Systems and Measurements, Wiley, 2003