Skip to main content
Geosciences LibreTexts

5.5: Guide for Quick Identification of Thermo Diagrams

  • Page ID
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)

    If you receive a thermo diagram via the internet without any labels identifying the diagram type, use the following procedure to figure it out:

    1. if there are some lines from lower left to upper right, then either Skew-T, Tephigram, or θ-Z .
      1. if the isobars are exactly horizontal and straight, then Skew-T,
      2. else if the strongly curved lines (moist adiabats) asymptotically approach vertical near the top of the diagram, then θ-Z,
      3. else Tephigram,
    2. else either Emagram or Stüve .
      1. if dry adiabats are slightly curved, then Emagram,
      2. else Stüve (including Pseudoadiabatic version of Stüve).

    When you encounter an unfamiliar thermo diagram, you can take steps to help segregate and identify the various lines. First, identify the isobars, which are horizontal and straight, or nearly so, on virtually all diagrams. Isobars are usually spaced roughly logarithmically with height, and are labeled as either 100, 90, 80, 70 kPa etc., or as 1000, 900, 800, 700 mb or hPa etc. These serve as a surrogate measure of height.

    Next, identify the dry and moist adiabats. These are the lines that converge and become parallel at cold temperatures and high altitudes. In all the diagrams, the moist adiabats are the most curved. The adiabats are spaced and labeled roughly linearly, such as 0, 10, 20, 30 °C etc, or often in Kelvin as 273, 283, 293, 303 K, etc. While the dry adiabats are always labeled as potential temperature θ, the moist adiabats can be labeled as either equivalent potential temperature θe , liquid-water potential temperature θL , or wet-bulb potential temperature θw.

    An alternative for identifying adiabats is to look at any labeled temperature at the bottom of the graph. Usually at 100 kPa (1000 mb or hPa), you will find that T , θw , and θ lines all radiate from the same points. For all diagrams, the T line is rightmost, θ is leftmost, and θw is in the middle.

    Finally, identify the isotherms and isohumes. These lines cross each other at a small angle, and extend from the bottom to the top of the graph. The isohumes always tilt to the left of the isotherms with increasing height. The isotherms increment linearly (e.g., 0, 10, 20 , 30, °C etc.), while the isohumes usually increment logarithmically (e.g., 1, 2, 5, 10, 20 g kg–1, etc.). Isotherms are either vertical, or tilt upward to the right.

    Sample Application

    For each thermo diagram numbered below, identify its name.

    Screen Shot 2020-02-12 at 11.36.59 AM.png

    Screen Shot 2020-02-12 at 11.37.30 AM.png

    Screen Shot 2020-02-12 at 11.37.57 AM.png

    CAUTION: Thermo diagrams printed or published by other agencies do NOT follow any standard format of line style or color. That is the reason these thermo diagrams are shown only with black lines in this sample application — so you don’t use the colors as a crutch.]

    Screen Shot 2020-02-12 at 11.42.54 AM.png

    Screen Shot 2020-02-12 at 11.43.46 AM.png

    Find the Answer: Use the quick guide.

    1. Stüve, because satisfies item 2b in the guide.
    2. θ-Z diagram, because satisfies item 1b.
    3. Tephigram, because satisfies item 1c.
    4. Emagram, because satisfies item 2a.
    5. Skew-T, because satisfies item 1a.

    Check: Reasonable, agrees with Figure 5.3.

    Exposition: For practice, pick any one of the diagrams above, and without looking back in Figure 5.3, try to identify which lines are which.

    [Hint: In this book, all thermo diagrams use the same format for the same types of lines: isotherms and isobars are thin solid green, isohumes are thin dotted blue, dry adiabats are thick solid dark-orange, moist adiabats are thick dashed dark-orange, and heights are thin dashed grey (if plotted at all).]

    5.5: Guide for Quick Identification of Thermo Diagrams is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Roland Stull via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.