Thermodynamic diagrams

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Thermodynamic diagrams are diagrams used by scientists and engineers to represent the thermodynamic states of a material (typically fluid) and the consequences of manipulating this material. For instance, a temperature-entropy diagram (T-s diagram for short) may be used to demonstrate the behavior of a fluid as it is changed by a compressor.

They are also used in meteorology to analyse radiosonde-soundings of the atmosphere. Temperature and humidity values (represented by the dewpoint) are displayed with respect to pressure. With thermodynamic diagrams it is possible to predict the 2m temperature, humidity and wind during the day, the development of the boundary layer of the atmosphere, the occurrence and development of clouds and the conditions for soaring flight during the day.

The main feature of thermodynamic diagrams is the equivalence between the area in the diagram and energy. When air changes pressure and temperature during a process and prescribes a closed curve within the diagram the area enclosed by this curve is proportional to the energy which has been gained or released by the air.

There are mainly three different types of thermodynamic diagrams used by the different weather services:

These all are derived from the physical P-alpha-diagram which combines pressure and specific volume (alpha) as basic coordinates. The P-alpha-diagram shows a strong deformation of the grid for atmospheric conditions and is therefore not useful in Meteorology. The three diagrams are constructed from the P-alpha-diagram by using appropriate coordinate transformations.

Not a thermodynmic diagram in a strict sense is the

Stüve diagram

since it does not display the energy - area equivalence.

Thermodynamic diagrams usually show five nets of different lines:

isobars = lines of constant pressure
isotherms = lines of constant temperature
dry adiabats = lines of constant potential temperature representing the temperature of a rising parcel of dry air
saturated adiabats or pseudoadiabats = lines representing the temperature of a rising parcel saturated with water vapour
mixing ratio = lines representing the dewpoint of a rising parcel

With the help of these lines, parameters such as cloud condensation level, level of free convection, onset of cloud formation etc. can be derived from the soundings.

[edit] Literature

'The Physics of Atmospheres' by John Houghton, Cambridge University Press 2002

Especially chapter 3.3. deals solely with the tephigram.

'Handbook of meteorological forecasting for soaring flight' WMO Technical Note No. 158. ISBN 92-63-10495-6 / ASIN B000IZXKQI

This book gives a very deep insight into the use of the diagrams with respect to soaring. It has been prepared by OSTIV ('Organisation Scientifique et Technique Internationale du Vol à Voile' (French: 'international scientific and technical organisation on soaring flight')). You can get the book via Amazon or this OSTIV page. There is a German translation of the book on the web here.

[edit] External links

radarmet.atmos.colostate.edu/../AT620_ch4.ppt A PowerPoint presentation with a very good mathematical description about the derivation of the different diagrams.
www.met.tamu.edu/../aws-tr79-006.pdf A very large technical manual (164 pages) how to use the diagrams.
www.comet.ucar.edu/../sld010.htm A course on how to use diagrams at comet the 'Cooperative Program for Operational Meteorology, Education and Training'.
www.raob.com RAOB analysis software for radio soundings.
www.ultx.de/thermik/60Tempanalyse.ppt a PowerPoint presentation about the use of the Stuve diagram (in German).
maths.ucd.ie/../blank_tephigram.pdf .
maths.ucd.ie/../Colour-Tephigram.pdf