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Skew-T log (p) 101

Updated: May 1, 2019

A temperature sounding is plotted on a thermodynamic chart.  There are three (3) basic thermodynamic charts used by forecasters to include the Stuve, tephigram and the Skew-T log (p) diagram.  These three thermodynamic diagrams are designed to present the environmental temperature, dewpoint temperature and wind as a function of pressure or altitude.  The Skew-T log (p) diagram is by far the most popular of the three, but all essentially depict the same information.  

Starting with the isotherms (lines of constant temperature) and isobars (lines of constant pressure), the temperature and dewpoint temperature can be plotted as a function of pressure.  Lines of constant height can also be included for convenience since standard atmospheric pressure has a distinct relationship to height above mean sea level (MSL).  The result is a partial base thermodynamic diagram that looks similar to the image above to include isobars (black), isotherms (red) and lines of constant height (blue).  

A thermodynamic diagram is also considered a nomogram.  That is, they are diagrams that contain lines showing the solutions to a set of different equations, in this case, thermodynamic equations.  The remaining three lines on the complete base diagram are the dry adiabatic lapse rate (DALR), moist adiabatic lapse rate (MALR) and the saturation mixing ratio.  These last three are constructed by solving three different thermodynamic equations as a function of temperature and pressure. The result is a complete base thermodynamic diagram that looks similar to the image below.  

You can think the base diagram as a sophisticated form of graph paper on which the environmental temperature, dewpoint temperature and wind are plotted.   Together with the plotted data the lines on the base diagram provide a means for meteorologists to calculate dozens of important thermodynamic indicies and provides a visual reference that has a high glance value.  This allows them to predict convective outbreaks, icing potential, cloud heights and turbulence just to name a few.

When looking at a Skew-T log (p) diagram, it is important to understand the origin of the environmental temperature, dewpoint temperature and wind plotted on the chart.  Such data can come from two sources.  The first is from a radiosonde launch (weather balloon) depicting actual environmental conditions - this is referred to as a RAOB (or radiosonde observation) like the one shown below for Amarillo, Texas. The data can also be plotted from the output from a numerical weather prediction model depicting an analysis or forecast.  RAOBs are usually only available twice at day at 0000 UTC and 1200 UTC at specific launch sites around the world.  On the other hand, forecast model output is available at a much higher spatial and temporal resolution covering all airports throughout the U.S. and the world (depending on the forecast model you use).

The RUC soundings tool provides an interactive utility to view the data from RAOBs and several different forecast models.  The data from the forecast model is influenced by the radiosonde launches as well as a plethora of other environmental data to include surface observations (METARs), aircraft reports, satellite and ground-based radar just to name a few.  To learn more about this diagram from a pilot's perspective, please consider purchasing the Introduction to the Skew-T log (p) Diagram premium workshop.  

Most pilots are weatherwise, but some are otherwise™

Scott Dennstaedt

Weather Systems Engineer

CFI & former NWS meteorologist  

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