Thermodynamic Diagrams

Graphical tools that plot atmospheric soundings against thermodynamic reference lines to analyse stability, moisture and convective potential.

Definition

A thermodynamic diagram is a chart on which atmospheric temperature and humidity profiles are plotted against thermodynamic coordinate lines to allow rapid graphical analysis of atmospheric processes.

Scope

Covers the major aerological diagrams including the skew-T log-P diagram, the tephigram and the Stuve diagram; the families of reference lines for isotherms, isobars, dry and moist adiabats and saturation mixing ratio; and the graphical determination of quantities such as the lifting condensation level, convective available potential energy and parcel paths.

Core questions

• How are temperature, pressure and moisture represented on aerological diagrams?
• How can stability and convective energy be read graphically from a sounding?
• What distinguishes the skew-T, tephigram and Stuve diagrams?

Key theories

Area-proportional thermodynamic diagrams

Diagrams are constructed so that areas enclosed by parcel and environment curves are proportional to energy, allowing convective energy and inhibition to be estimated by inspection.

Mechanisms

On a skew-T log-P diagram pressure decreases logarithmically upward and isotherms are skewed to spread out the troposphere. A measured sounding of temperature and dew point is plotted against reference dry adiabats, moist adiabats and mixing-ratio lines. Lifting a parcel along these lines locates the lifting condensation level and level of free convection, and the area between the parcel path and the environmental temperature curve gives the convective available potential energy and convective inhibition.

Clinical relevance

Aerological diagrams are standard tools in operational forecasting for assessing instability, predicting cloud bases, and diagnosing severe-weather potential from radiosonde data.

History

The tephigram was introduced by Napier Shaw in the early twentieth century, and the skew-T log-P diagram, developed by Herlofson, became the standard in operational meteorology for its convenient layout of the troposphere.

Key figures

• Napier Shaw
• Julio Iribarne

Seminal works

• iribarne1981

Frequently asked questions

Why are isotherms drawn slanted on a skew-T diagram?

Skewing the isotherms spreads the data over a larger area and makes the lapse rate, stability and the angle between parcel and environment curves easier to read.