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The three thermodynamic diagrams in common use in operational meteorology

The three plots below represent the main thermodynamic diagrams in use around the world.

image of the three diagrams in use

 SKEW-T/LOG(P)

 STÜVE

  TEPHIGRAM

they are intended to demonstrate the differences in appearance between the three versions, rather than be a strictly accurate representation of that particular diagram. For this reason, I have deliberately left off the labels!

This table will attempt to explain the differences between each diagram, and also explain a little about each printed 'line' on the charts, with the units usually used for the variable displayed.

Line on diagram 

 units used (usually)

 SKEW-T/LOG(P)

 STÜVE

 TEPHIGRAM

 ISOTHERMS (lines of constant 'real' temperature)

 degC

 straight & parallel; angled 45deg/slope to right. Equal spacing (linear); angled circa 90deg to Dry Adiabats up to 300hPa.

 straight, parallel & vertical. Equal spacing (i.e. linear).

 straight & parallel; angled 45deg/slope to right. Equal spacing (linear); angled exactly 90deg to Dry Adiabats whole diagram.

 ISOBARS (lines of constant atmospheric pressure)

 hPa (or millibars in old money)

 straight, parallel & horizontal; increased spacing per unit pressure change with altitude.

 straight, parallel & horizontal; increased spacing per unit pressure change with altitude.

 very slightly curved upwards (i.e. convex towards top of diagram). [ not really noticeable for routine use.]; quasi-horizontal; increased spacing per unit pressure change with altitude.

 DRY ADIABATS (lines of constant potential temperature for a dry air sample [ i.e. an unsaturated air parcel path. ] )

 degC (but strictly, and sometimes found, degrees Kelvin are used)

 curved: approx. 45deg to left near 1000 hPa, decreasing to within 10deg of vertical near 100 hPa.

 straight - sharply angled to left - gently convergent to left. (meet at a theoretical point where P=0; T(K)=0)

 straight & parallel; angled 45deg/slope to left; Equal spacing (linear); angled exactly 90deg to Isotherms whole diagram.

 SATURATED ADIABATS (lines of equivalent potential temperature for a saturated [or 'wet'] air parcel path.)

 degC (but strictly, and sometimes found, degrees Kelvin are used)

 curved - but not constant; on right-hand side of diagram, curve starts right and bears left above 400 hPa; on left-hand side, curve starts left and quickly become parallel with Dry Adiabats.

 slightly curved to left with height - curve minimal left-hand side of diagram; a gently increasing left turn on right hand side.

 The only notably curved lines on this diagram: On the right-hand side, starts slightly right before curving left; on left-hand end, curve all to left. On most diagrams, not shown above about -50degC.

 SATURATED HUMIDITY MIXING RATIO (lines of constant saturation mixing ratio with respect to a plane water surface.)

 g/kg ( i.e. ratio of mass of water vapour in given volume to the mass of the dry air in that sample. )

 quasi-straight*; angled to right, at less than 45deg to the vertical; gently convergent to a point well above the top of the diagram. (* for practical work can be regarded as straight & parallel)

 quasi-straight*; angled to left, at less than 20deg to the vertical; gently convergent to a point well above the top of the diagram. (* for practical work can be regarded as straight & parallel)

 quasi-straight*; angled to right at less than 45deg to vertical, i.e. less slope than isotherm. (* for practical work can be regarded as straight & parallel)

… and diagrammatically, I have attempted to highlight the various lines here…. the diagrams are not true representations of each style or necessarily to scale, but are close enough to pick out the major differences listed above:

ISOTHERMS:

SkewT/LogP

 Stüve

 Tephigram

image of the three diagrams comparing isotherm plots


ISOBARS:

SkewT/LogP

 Stüve

 Tephigram

images of the three diagrams showing the isobars


DRY ADIABATS:

SkewT/LogP

 Stüve

 Tephigram

images of the three diagrams comparing the dry adiabats


SATURATED ADIABATS:

SkewT/LogP

 Stüve

 Tephigram

images of the three diagrams showing saturated adiabats


SATURATED MIXING RATIO ISOPLETHS:

SkewT/LogP

 Stüve

 Tephigram

images of three diagrams showing the HMR lines


Some definitions/abbreviations:- (not already given elsewhere by hyperlink)

degC 

 degrees Celsius

 Dry Adiabats

 describes the 'parcel path' on a thermodynamic diagram when that parcel is unsaturated: i.e. 'dry'. Taken to be 3degC per 1000 ft, or 9.8 degC per km.

Entropy 

 the ratio of the amount of heat absorbed by an object in undergoing a reversible thermodynamic process to the absolute temperature of the object (dQ/T) is defined to be the increase in entropy.

 g/kg

 grams (of water vapour) per kilogram (of dry air)

 hPa

 hecto Pascal … same as millibars.

 Potential temperature

 is defined as the temperature an air parcel would have, if it were moved vertically (upwards >> decreasing pressure >> expansion; downwards >> increasing pressure >> compression ), from its existing pressure and temperature to a standard level (usually defined to be 1000 hPa). Provided the parcel remains 'dry' or more strictly un-saturated, then the rate of cooling (upward motion), or warming (downward motion) occurs at the Dry Adiabatic Lapse Rate (DALR), which is 9.8 degC per km ( or 3 degC per 1000 feet ). This value is constant. Once a parcel becomes saturated (i.e., the initially un-saturated parcel is lifted until it cools to its dew-point temperature), then the subsequent release of latent heat of vaporization offsets the cooling rate, and the Saturated Adiabatic Lapse Rate (SALR) is consequently less than the DALR. (see Saturated Adiabats)

 Saturated Adiabats

 describes the 'parcel path' on a thermodynamic diagram when that parcel is saturated: Taken to be, very roughly, 1.5degC per 1000 ft, or 5 degC per km IN THE LOWEST FEW HUNDREDS OF MILLIBARS OF THE TROPOSPHERE. Towards a temperature of minus 50degC, tends to the Dry Adiabatic Lapse Rate, as air at lower temperatures holds less and less moisture, hence less offset from the latent heat release.


Advantages and disadvantages

Very much in the eye of the beholder here. The tephigram is regarded as near-perfect for strict thermodynamic calculations, and its large angle between isotherms and dry adiabats renders it the most effective as assessing degrees of stability. However, the skewT also has this property, and tephigrams aren't exactly plentiful on the net. The skewT though has curved dry adiabats, though for short vertical calculation perhaps not a major problem. Indeed, for a parcel undergoing saturation, you are going to have to cross over to the saturated adiabats anyway, which on all the diagrams are curved. The Stüve is clean and simple in that it has the most straight lines, and is perhaps intuitive in that isotherms are vertical against a pressure plot. The disadvantage is that the angle between isotherm and dry adiabat is not as great as the other two versions. Again, constant use would offset this minor difficulty. For those used to the tephigram, the skewT is most like what you are used to.