A skew-T log-P diagram is one of four

thermodynamic diagrams Thermodynamic diagrams are diagrams used to represent the thermodynamic states of a material (typically fluid) and the consequences of manipulating this material. For instance, a temperature–entropy diagram (Temperature–entropy diagram, T–s ...

commonly used in

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...

analysis and forecasting. In 1947, N. Herlofson proposed a modification to the

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 ...

that allows straight, horizontal isobars and provides for a large angle between isotherms and dry adiabats, similar to that in the

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 ...

. This made the diagram useful for analysis techniques that were then being adopted by the

United States Air Force The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...

. Such a diagram has pressure plotted on the vertical axis, with a

logarithmic scale A logarithmic scale (or log scale) is a method used to display numerical data that spans a broad range of values, especially when there are significant differences among the magnitudes of the numbers involved. Unlike a linear Scale (measurement) ...

(thus the "log-P" part of the name), and the temperature plotted

skewed In probability theory and statistics, skewness is a measure of the asymmetry of the probability distribution of a real-valued random variable about its mean. The skewness value can be positive, zero, negative, or undefined. For a unimodal ...

, with isothermal lines at 45° to the plot (thus the "skew-T" part of the name). Plotting a hypothetical set of measurements with constant temperature for all altitudes would result in a line angled 45° to the right. In practice, since temperature usually drops with altitude, the graphs are usually mostly vertical (see examples linked to below). The major use for skew-T log-P diagrams is the plotting of radiosonde soundings, which give a vertical profile of the

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

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 ...

temperature throughout the

troposphere The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...

and lower

stratosphere The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...

. The isopleths on the diagram can then be used to simplify many tedious calculations involved, which were previously performed by hand or not at all. Many skew-T log-P diagrams also include a vertical representation of the wind speed and direction using wind barbs. Important atmospheric characteristics such as saturation,

atmospheric instability Atmospheric instability is a condition where the Earth's atmosphere is considered to be unstable and as a result local weather is highly variable through distance and time. Atmospheric instability encourages vertical motion, which is directly cor ...

, and

wind shear Wind shear (; also written windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical ...

are critical in severe

weather forecasting Weather forecasting or weather prediction is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather info ...

, by which skew-T log-P diagrams allow quick visual analysis. The diagrams are widely used by glider pilots to forecast the strength of thermals and the height of the base of the associated cumulus clouds.

Bibliography

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External links

Downloadable/printable skew-T log-P diagrams in PDF format

NOAA observed sounding archive

Printable blank skew-T log-P diagrams in multiple densities

NOAA Skew-T log-P page
Atmospheric thermodynamics Meteorological diagrams {{climate-stub

See also

Bibliography

External links