The Skew T, Log P Diagram is a thermo-dynamic diagram on which information obtained from upper-air soundings is plotted and analyzed. Several procedures have been developed that allow rapid graphical computations from data plotted on a Skew T chart. In this lesson we explain the more important parameters of the Skew T diagram, and then cover the more frequently used computations that you will need to know to analyze the Skew T. These procedures include computations of derived measurements, cloud formation parameters, expected temperatures, cloud layer criteria, convective weather guidelines, indices for severe weather, contrail formation guidelines, the freezing level, and icing. We will also discuss frontal analysis on the Skew T. Before going on, you should review Unit 3- Lesson 3 of the AG3 training manual for plotting procedures and AG2, Volume 2, Unit 2 - Lesson 4, for stability theory. Note: The figures referred to in the text are in black and white, while the Skew T diagram is printed in black, brown, light green, and dark green. You may wish to obtain an actual Skew T, Log P Diagram (DOD WPC 9-16) in order to follow the text more accurately.

The Skew T diagram represents a graphical presentation of the relationship between many parameters in the thermodynamic equation. In the AG3 manual you studied the isobars, isotherms, height scales, and wind scale, while learning to plot the temperature curve, dew point curve, pressure altitude curve, and wind reports. Now you need to learn what the other lines are and how to use them.

The dry adiabats represent the rate at which nonsaturated air will cool as it moves upward, or warm as it moves downward in the atmosphere. This rate of cooling or warming is called the dry adiabatic lapse rate. On the diagram, the dry adiabats are drawn as thin, slightly curved brown lines extending diagonally upward from right to left. These lines have a spacing, or interval, of 2C, and are labeled across the top, bottom, and along the sides of the chart in diagonal bold brown numbers. The light brown numbers within parentheses below the bold numbers at the top of the diagram are the values for the dry adiabats in the 100- to 25-millibar pressure range. The values of the dry adiabats and the isotherms are the same at the 1,000-millibar level. These values represent potential temperature in degrees Celsius.

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