Thermodynamic diagrams are diagrams used to represent the

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...

states of a material (typically

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

) and the consequences of manipulating this material. For instance, a temperature–

entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

diagram ( T–s diagram) may be used to demonstrate the behavior of a fluid as it is changed by a compressor.

Overview

Especially in

meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...

, they are used to analyze the actual state of the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

derived from the measurements of

radiosonde A radiosonde is a battery-powered telemetry instrument carried into the atmosphere usually by a weather balloon that measures various atmospheric parameters and transmits them by radio to a ground receiver. Modern radiosondes measure or calculat ...

s, usually obtained with weather balloons. In such diagrams,

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

and

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

values (represented by the

dew point The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the ...

) are displayed with respect to

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

. Thus the diagram gives at a first glance the actual atmospheric stratification and vertical water vapor distribution. Further analysis gives the actual base and top height of convective clouds or possible instabilities in the stratification. By assuming the

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

amount due to

solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...

it is possible to predict the 2 m (6.6 ft) temperature, humidity, and wind during the day, the development of the

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a Boundary (thermodynamic), bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces ...

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.

Types of thermodynamic diagrams

General purpose diagrams include: * PV diagram * T–s diagram * h–s (Mollier) diagram *

Psychrometric chart Psychrometrics (or psychrometry, ; also called hygrometry) is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures. History With the inventions of the hygrometer and thermometer, the theori ...

Cooling curve A cooling curve is a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid. The independent variable (X-axis) is time and the dependent variable (Y-axis) is temperature.Garland, Nibler, ...

Indicator diagram An indicator diagram is a chart used to measure the thermal, or cylinder, performance of Reciprocating engine, reciprocating steam and Internal combustion engine, internal combustion engines and compressors. An indicator chart records the press ...

* Saturation vapor curve * Thermodynamic surface Specific to weather services, there are mainly three different types of thermodynamic diagrams used: *

Skew-T log-P diagram A skew-T log-P diagram is one of four thermodynamic diagrams commonly used in weather analysis and forecasting. In 1947, N. Herlofson proposed a modification to the emagram that allows straight, horizontal isobars and provides for a large angle ...

Tephigram A tephigram is one of a number of thermodynamic diagrams commonly used in weather analysis and forecasting. The name evolved from the original name "T-\phi-gram" to describe the axes of temperature (T) and entropy (\phi) used to create the plo ...

Emagram An emagram is one of four thermodynamic diagrams used to display temperature lapse rate and moisture content profiles in the atmosphere. The emagram has axes of temperature (T) and pressure (p). In the emagram, the dry adiabats make an angle of ab ...

* Stüve diagram All four 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

atmospheric sciences Atmospheric science is the study of the Earth's atmosphere and its various inner-working physical processes. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather forecasting. Climatology is the study ...

. The three diagrams are constructed from the P–alpha diagram by using appropriate coordinate transformations. Not a thermodynamic diagram in a strict sense, since it does not display the energy–area equivalence, is the * Stüve diagram But due to its simpler construction it is preferred in education. 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 The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the ...

of a rising parcel The

lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'' (in its "becoming less" sense, not its "interruption" sense). In dry air, ...

, 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 The level of free convection (LFC) is the altitude in the atmosphere where an air parcel lifted adiabatically until saturation becomes warmer than the environment at the same level, so that positive buoyancy can initiate self-sustained 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 stateThermodynamics (Third Edition), Kenneth Wark, McGraw-Hill Inc, 1977, and can be viewed graphically on a pressure-volume (P-V), pressure-temperature (P-T), and temperature-entropy (T-s) diagrams. There are an infinite number of possible paths from an initial point to an end point in a

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management * Business process, activities that produce a specific s ...

. 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.Philip E. Bloomfield, William A. Steele, "Thermodynamic processes," in AccessScience, ©McGraw-Hill Companies, 2008, http://www.accessscience.com Gax expanding doing work on a piston in a cylinder

Gax expanding doing work on a piston in a cylinder

Consider a gas in cylinder with a free floating piston resting on top of a volume of gas at a temperature . If the gas is heated so that the temperature of the gas goes up to while the piston is allowed to rise to 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 In thermodynamics, an isobaric process is a type of thermodynamic process in which the pressure of the Thermodynamic system, system stays constant: Δ''P'' = 0. The heat transferred to the system does work (thermodynamics), work, but a ...

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 Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an ani ...

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''.Physics – Principles with Applications (Second Edition), Douglas C, Giancoli, Printice Hall, Inc., 1985, Thus *''W'' = ''Fd'' *''W'' = ''PAd'' *''W'' = ''P''(''V''₂ − ''V''₁) P-V diagram of expansion of a gas under a free floating piston with friction

P-V diagram of expansion of a gas under a free floating piston with friction

Now let’s say that the piston was not able to move smoothly within the cylinder due to

static friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

with the walls of the cylinder. Assuming that the temperature was increased slowly, you would find that the process path is not straight and no longer isobaric, but would instead undergo an

isometric process In thermodynamics, an isochoric process, also called a constant-volume process, an isovolumetric process, or an isometric process, is a thermodynamic process during which the volume of the closed system undergoing such a process remains constant. ...

till the force exceeded that of the frictional force and then would undergo an

isothermal process An isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a change in the sy ...

back to an equilibrium state. This process would be repeated till the end state is reached. See ''figure 3''. The work done on the piston in this case would be different due to the additional work required for the resistance of the friction. The work done due to friction would be the difference between the work done on these two process paths. Many engineers neglect friction at first in order to generate a simplified model. 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. Another path in this process is an

. This is a process where volume is held constant which shows as a vertical line on a P-V diagram. ''Figure 3'' Since the piston is not moving during this process, there is not any work being done.

References

* ''The Physics of Atmospheres'' by John Houghton, Cambridge University Press 2002. Especially chapter 3.3. deals solely with the tephigram. * German version o
''Handbook of meteorological soaring flight''
from th
Organisation Scientifique et Technique Internationale du Vol à Voile (OSTIV)
(chapter 2.3)

External links

{{Commons category, Thermodynamic diagrams
www.met.tamu.edu/../aws-tr79-006.pdf
A very large technical manual (164 pages) how to use the diagrams.

A course on how to use diagrams at Comet, the 'Cooperative Program for Operational Meteorology, Education and Training'.

diagrams A diagram is a symbolic representation of information using visualization techniques. Diagrams have been used since prehistoric times on walls of caves, but became more prevalent during the Enlightenment. Sometimes, the technique uses a three- ...

Diagrams