Pressure and Heights

This reading provides a little more detail to reading and interpreting a Skew-T thermodynamic diagram. As an example, a thermodynamic diagram from Greensboro, NC, taken October 19, 2000, will be used. The thermodynamic diagram is shown at right (click on the image to see full-sized). This particular thermodynamic diagram comes from the University of Wyoming (if you go there, click on the "Back" button to return to the reading).

Pressure

Pressure lines, or isobars, represent the foundation of any thermodynamic diagram. The isotherms are shown running up the left-hand side of the chart (in blue, beginning at the surface at 1013 mb. The isotherms are drawn logarithmically, with the spacing between each 100 mb of pressure getting larger as one approaches 100 mb.

Heights

It is sometimes useful to be able to determine the height, or altitude (normally in kilometers) of a particular isotherm. To find the altitude, one would use a table of Standard Atmospheres, the most recent being the 1976 Table of Standard Atmospheres. A Standard Atmosphere Table and Calculator is provided to you for determining the height of the atmosphere. The calculator provided here determines pressure, temperature, air density, and speed of sound as a function of altitude, but you can use the calculator to "hone in" on the height as a function of any one of these four variables.

On most thermodynamic diagrams, this one being no exception, the heights in meters are plotted on the isotherm lines. For example, looking at the Greensboro diagram, we see the value of 5790 meters at the 500-mb isobar.