A thermodynamic diagram is a chart on which pressure, density, temperature and water vapor are plotted for a point on the Earth up through the atmosphere. By having temperature against pressure, one can get a feel for many variables at a glance without performing lengthy calculations. The data is gathered by an instrument package attached to a balloon called a radiosonde launched from hundreds of sites around the world.

One specific type of thermodynamic diagram is a skew-T log-p, or skew-t, that tilts the lines of constant temperature (isotherms) and compresses the pressure scale logarithmicly.

Elements of a Skew-T

Annotated diagram of lines on a skew-t

Isotherms - Lines of equal temperature. They define the x-axis of the chart and are labeled at the bottom. They run at a 45 degree angle towards the upper left (hence skew-T).

Isobars - Lines of equal pressure. They define the y-axis of the chart and are labled on the left side. While actually pressure, they are also proportional to height. The lines have more spacing between them at the same interval as it is a log scale (hence log-p). This spacing makes it similar to the atmosphere where there is a larger gradient of pressure near the ground.

Dry adiabats (isentropes) - Lines of constant potential temperature. Slightly curved and sloped up and to the left. They describe change in temperature in an unsaturated parcel that is moved up and down, as it is compressed or expanded by the change in pressure.

Moist adiabats (pseudoadiabats) - Lines of constant equivalent potential temperature. They are curved lines also running up and to the left. They describe change in temperature in a saturated parcel that is moved up, due to expansion and latent heat release.

Saturation mixing ratio lines (isohumes) - Lines of constant saturation mixing ratio. They are dashed lines that are sloped towards the upper left. For a given temperature and pressure, they tell you the maximum water vapor that can present.

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Unisys
Ohio State
UCAR
GOES Sounding
Texas A&M
College of DuPage

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