Thermodynamic diagrams
Thermodynamics |
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Thermodynamic diagrams are diagrams used to represent the
Overview
Especially in
By assuming the
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.
Types of thermodynamic diagrams
General purpose diagrams include:
- PV diagram
- T–s diagram
- h–s (Mollier) diagram
- Psychrometric chart
- Cooling curve
- Indicator diagram
- Saturation vapor curve
- Thermodynamic surface
Specific to weather services, there are mainly three different types of thermodynamic diagrams used:
All three diagrams are derived from the physical P–alpha diagram which combines pressure (P) and specific volume (alpha) as its basic coordinates. The P–alpha diagram shows a strong deformation of the grid for atmospheric conditions and is therefore not useful in
Not a thermodynamic diagram in a strict sense, since it does not display the energy–area equivalence, is the
But due to its simpler construction it is preferred in education.[citation needed]
Another widely-used diagram that does not display the energy–area equivalence is the θ-z diagram (Theta-height diagram), extensively used boundary layer meteorology.
Characteristics
Thermodynamic diagrams usually show a net of five 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 vapor
- mixing ratio = lines representing the dewpoint of a rising parcel
The lapse rate, dry adiabatic lapse rate (DALR) and moist adiabatic lapse rate (MALR), are obtained. 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.
Example
The path or series of states through which a system passes from an initial equilibrium state to a final equilibrium state[1] and can be viewed graphically on a pressure-volume (P-V), pressure-temperature (P-T), and temperature-entropy (T-s) diagrams.[2]
There are an infinite number of possible paths from an initial point to an end point in a process. In many cases the path matters, however, changes in the thermodynamic properties depend only on the initial and final states and not upon the path.[3]
Consider a gas in cylinder with a free floating piston resting on top of a volume of gas V1 at a temperature T1. If the gas is heated so that the temperature of the gas goes up to T2 while the piston is allowed to rise to V2 as in Figure 1, then the pressure is kept the same in this process due to the free floating piston being allowed to rise making the process an isobaric process or constant pressure process. This Process Path is a straight horizontal line from state one to state two on a P-V diagram.
It is often valuable to calculate the work done in a process. The work done in a process is the area beneath the process path on a P-V diagram. Figure 2 If the process is isobaric, then the work done on the piston is easily calculated. For example, if the gas expands slowly against the piston, the work done by the gas to raise the piston is the force F times the distance d. But the force is just the pressure P of the gas times the area A of the piston, F = PA.[4] Thus
- W = Fd
- W = PAd
- W = P(V2 − V1)
Now let’s say that the piston was not able to move smoothly within the cylinder due to
Many engineers neglect friction at first in order to generate a simplified model.[1] For more accurate information, the height of the highest point, or the max pressure, to surpass the static friction would be proportional to the frictional coefficient and the slope going back down to the normal pressure would be the same as an isothermal process if the temperature was increased at a slow enough rate.[4]
Another path in this process is an
See also
References
- ^ ISBN 0-07-068280-1
- ISBN 978-0470-49590-2
- ^ Philip E. Bloomfield, William A. Steele, "Thermodynamic processes," in AccessScience, ©McGraw-Hill Companies, 2008, http://www.accessscience.com
- ^ ISBN 0-13-672627-5
- The Physics of Atmospheres by John Houghton, Cambridge University Press 2002. Especially chapter 3.3. deals solely with the tephigram.
- German version of Handbook of meteorological soaring flight from the Organisation Scientifique et Technique Internationale du Vol à Voile (OSTIV) (chapter 2.3)
Further reading
- Handbook of meteorological forecasting for soaring flight WMO Technical Note No. 158. ISBN 92-63-10495-6especially chapter 2.3.
External links
- 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'.