Thermodynamic diagrams
Encyclopedia
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) may be used to demonstrate the behavior of a fluid as it is changed by a compressor.
they are used to analyze the actual state of the atmosphere
derived from the measurements of radiosonde
s, usually obtained with weather balloons. In such diagrams, temperature
and humidity
values (represented by the dew point
) are displayed with respect to pressure
. 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
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
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.
Specific to weather services, there are mainly three different types of thermodynamic diagrams used:
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
. 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
Due to its simpler construction it is preferred in education.
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.
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
) and the consequences of manipulating this material. For instance, a temperature-entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
diagram (T-S diagram) may be used to demonstrate the behavior of a fluid as it is changed by a compressor.
Overview
Especially in meteorologyMeteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
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, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the 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 atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required...
s, usually obtained with weather balloons. In such diagrams, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
values (represented by the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
) are displayed with respect to pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
. 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
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They 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 an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
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. On Earth it usually responds to changes in surface forcing in an hour or less...
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:- P-V diagram
- T-s diagram
- H-S (Mollier) diagram
- Psychrometric chart
- Cooling curveCooling curveA 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 is time and the dependent variable is temperature...
- Indicator diagram
- Saturation vapor curveSaturation Vapor CurveThe saturation vapor curve usually states for the separation curve between the two-phase state and the vapor state in the T-s diagram. When used in a power cycle, the fluid expansion depends strongly on the nature of this saturation curve :...
Specific to weather services, there are mainly three different types of thermodynamic diagrams used:
- Skew-T log-P diagramSkew-T log-P diagramA 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...
- TephigramTephigramA tephigram is one of four 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 and entropy used to create the plot...
- EmagramEmagramAn 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 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 atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather...
. 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 diagramStüve diagramA Stüve diagram is one of four thermodynamic diagrams commonly used in weather analysis and forecasting. Usually, temperature and dew point data from radiosondes 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.
Characteristics
Thermodynamic diagrams usually show a net of five different lines:- isobars = lines of constant pressure
- isotherms = lines of constant temperature
- dry adiabatsHeat capacity ratioThe 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 . It is sometimes also known as the isentropic expansion factor and is denoted by \gamma or \kappa . The latter symbol kappa is...
= 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 dewpointDew pointThe dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
of a rising parcel
The lapse rate
Lapse rate
The lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise. The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and...
, 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 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
- 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.ulweb.de/thermik/60Tempanalyse.ppt a PowerPoint presentation about the use of the Stuve diagram (in German).
- Tephigram page, University of Reading Blank colour and monochrome tephigrams as pdf from the Department of Meteorology, University of Reading.
- maths.ucd.ie/../blank_tephigram.pdf and .../Colour-Tephigram.pdf Blank b&w and color Tephigrams as pdf from UCD School of Mathematical sciences in Dublin.
- http://www.meteo.uni-koeln.de/meteo.php?show=En_We_We Radio soundings from all over the world from the 'Institut für Geophysik und Meteorologie' at the University of Cologne/Germany
- weather.uwyo.edu/upperair/sounding.html Radio soundings from all over the world from the Department of Atmospheric sciences at the University of Wyoming/USA