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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
Fluid

A fluid is defined as a substance that continually deforms under an applied shear stress. All liquids and all gases are fluids. Fluids are a subset of the Phase and include liquids, gas, Plasma physics and, to some extent, plasticity ....
) and the consequences of manipulating this material. For instance, a temperature-entropy
Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
 diagram (T-s diagram) may be used to demonstrate the behavior of a fluid as it is changed by a compressor.

cially in meteorology
Meteorology

Meteorology is the interdisciplinary scientific study of the Earth's atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century....
 they are used to analyze the actual state of the atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
 derived from the measurements of radiosonde
Radiosonde

A radiosonde is a unit for use in weather balloons that measures various Earth's atmosphere parameters and transmits them to a fixed receiver....
s, usually obtained with weather balloons.






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Thermodynamic diagrams are diagrams used by scientists and engineers to represent the thermodynamic states of a material (typically fluid
Fluid

A fluid is defined as a substance that continually deforms under an applied shear stress. All liquids and all gases are fluids. Fluids are a subset of the Phase and include liquids, gas, Plasma physics and, to some extent, plasticity ....
) and the consequences of manipulating this material. For instance, a temperature-entropy
Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
 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

Meteorology is the interdisciplinary scientific study of the Earth's atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century....
 they are used to analyze the actual state of the atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
 derived from the measurements of radiosonde
Radiosonde

A radiosonde is a unit for use in weather balloons that measures various Earth's atmosphere parameters and transmits them to a fixed receiver....
s, usually obtained with weather balloons. In such diagrams, temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
 and humidity
Humidity

Humidity is the amount of water vapor in the air. In daily language the term "humidity" is normally taken to mean relative humidity. Relative humidity is defined as the ratio of the partial pressure of water vapor in a Air parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature....
 values (represented by the dew point
Dew point

The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to Condensation into water....
) are displayed with respect to pressure
Atmospheric pressure

Atmospheric pressure is sometimes defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere....
. Thus the diagram gives at a first glance the actual atmospheric stratification
Atmospheric stratification

Atmospheric stratification is the division of the Earth's atmosphere into distinct layers, each with specific properties such as temperature or humidity....
 and vertical water vapor distribution. Further analysis gives the actual base and top height of convective clouds
Cumulus cloud

Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. These are often described as "puffy" or "cotton-like" in appearance, cumulus clouds may appear alone, in lines, or in clusters....
 or possible instabilities in the stratification.

By assuming the energy
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 amount due to solar radiation it is possible to predict the 2 m (6.6 ft) temperature, humidity, and wind during the day, the development of the boundary layer
Planetary boundary layer

The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface....
 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

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

  • Skew-T log-P diagram
    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 which allows straight, horizontal isobars, and provides for a large angle between isotherms and dry adiabats, similar to that in the tephigram....
  • Tephigram
    Tephigram

    A tephigram is one of four thermodynamic diagrams commonly used in weather analysis and forecasting. The name evolved from the original name "T--gram" to describe the axes of temperature and entropy used to create the plot....
  • Emagram
    Emagram

    An emagram is one of four thermodynamic diagrams used to display profiles of temperature and moisture in the atmosphere. The emagram has axes of temperature and pressure ....


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 atmospheric sciences
Atmospheric sciences

Atmospheric sciences is an umbrella term for the study of the Earth's atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems....
. 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
    Stüve diagram

    A St?ve diagram is one of four thermodynamic diagrams commonly used in weather analysis and forecasting. Usually, temperature and dew point data from radiosonde are plotted on these diagrams to allow calculations of convective stability or Convective Available Potential Energy....


Due to its simpler construction it is preferred in education.

Thermodynamic diagrams usually show a net of five different lines:

  • isobar
    Isobar

    Isobar may refer to:* a contour line of equal or constant pressure in meteorology* two nuclides with the same mass number in nuclear physics* a heat pipe...
    s     = lines of constant pressure
  • isotherm
    Isotherm

    An isotherm may refer to:*A type of contour line or surface connecting points of equal temperature*An isothermal process in a thermodynamic cycle....
    s     = lines of constant temperature
  • dry adiabats
    Heat capacity ratio

    The heat capacity ratio or adiabatic index or ratio of specific heats, is the ratio of the heat capacity at constant pressure to heat capacity at constant volume ....
     = 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
    Dew point

    The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to Condensation into water....
     of a rising parcel


The lapse rate
Lapse rate

The lapse rate is defined as the negative of the rate of change in an atmospheric variable, usually temperature, with height in an atmosphere. While typically applied to Earth's atmosphere, the concept can be extended to any gravitationally supported ball of gas....
, 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
Lifted condensation level

The lifted condensation level or lifting condensation level is formally defined as the height at which the relative humidity of an air parcel will reach 100% when it is cooled by dry adiabatic lifting....
, level of free convection
Level of free convection

The level of free convection is the altitude in the Earth's atmosphere where the temperature of the environment decreases faster than the moist adiabatic lapse rate of a saturated air parcel at the same level....
, onset of cloud formation. etc. can be derived from the soundings.

Further reading

  • Handbook of meteorological forecasting for soaring flight WMO Technical Note No. 158. ISBN 92-63-10495-6 especially chapter 2.3.


External links

  • A PowerPoint presentation with a very good mathematical description about the derivation of the different diagrams.
  • A very large technical manual (164 pages) how to use the diagrams.
  • A course on how to use diagrams at the 'Cooperative Program for Operational Meteorology, Education and Training'.
  • RAOB analysis software for radio soundings.
  • a PowerPoint presentation about the use of the Stuve diagram (in German).
  • Blank colour and monochrome tephigrams as pdf from .
  • and Blank b&w and color Tephigrams as pdf from School of Mathematical sciences in Dublin.
  • Radio soundings from all over the world from the at the University of Cologne/Germany
  • Radio soundings from all over the world from the at the University of Wyoming/USA