Thermodynamic Diagrams Questions and Answers

Q: Which quantities do the axes of a tephigram represent?

The name tephigram comes from T and phi, the symbol once used for entropy, since the axes are temperature and the logarithm of potential temperature which is proportional to entropy. Because the axes are temperature and entropy, an area on the diagram is heat, which gives the required energy property. The whole diagram is then rotated through about forty five degrees so that the isobars run approximately horizontally and pressure decreases upward, which makes a plotted sounding look natural to the eye.

Q: Which rotation is applied to the tephigram in practice?

Rotating the diagram places the isobars almost horizontally and the dry adiabats at about forty five degrees, which produces the familiar appearance of a plotted ascent. Without the rotation the sounding would run diagonally across the page and be much harder to read. The rotation changes nothing about the thermodynamics, since areas are preserved under rotation, and it is purely a matter of presentation. The isotherms then run diagonally upward to the right, which is the feature that distinguishes a tephigram at a glance.

Q: Which lines run diagonally on a skew-T diagram?

On a skew-T log-p diagram the isotherms are skewed to run at about forty five degrees while the isobars remain straight and horizontal with a logarithmic spacing. Skewing the isotherms spreads a typical sounding across the diagram rather than compressing it into a narrow band, which is the whole purpose of the design. The dry adiabats curve gently to the left and the saturated adiabats curve more strongly, converging towards the dry adiabats at low temperature.

Q: Which diagram plots temperature against the logarithm of pressure without skewing?

The emagram has straight vertical isotherms and horizontal isobars logarithmically spaced, which preserves the area property while keeping the layout simple. Its disadvantage is that typical soundings occupy only a narrow diagonal band, which is exactly what the skew-T modification was introduced to correct. The name derives from energy per unit mass diagram. It remains in use across much of continental Europe. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which diagram sacrifices the area property for simplicity?

The Stuve diagram plots temperature against pressure raised to the power kappa, which makes both the isotherms and the dry adiabats straight lines but destroys the equal area property. It is therefore convenient for illustration and teaching but cannot be used to measure convective energy by planimetry. Rotation and skewing preserve areas, so neither the tephigram nor the skew-T loses the property despite their distorted appearance. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which family of lines shows the temperature a parcel follows on dry ascent?

Dry adiabats are lines of constant potential temperature, and an unsaturated parcel moves along one as it is lifted or lowered. They are straight on a tephigram, being lines of constant ordinate before rotation, and gently curved on a skew-T. Following the correct family for the state of the parcel is the essential skill in using any of these diagrams, since a parcel switches from the dry to the saturated family the moment it reaches its condensation level.

Q: Which family shows the path of a saturated ascending parcel?

Saturated adiabats, also called moist adiabats or pseudoadiabats, show the temperature of a parcel in which condensation is occurring and the latent heat is offsetting part of the cooling. They are steeply curved, lying close to the dry adiabats at very low temperature where there is little water to condense and diverging markedly from them in warm moist air. Lines of constant wet bulb potential temperature are the same family. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: Which family shows the water vapour content of a parcel?

Saturation mixing ratio lines, usually drawn dashed, give the mixing ratio of saturated air at each point, so following one from the dew point at the surface gives the mixing ratio of the actual air. Their intersection with the dry adiabat through the surface temperature locates the lifting condensation level. They are the least obtrusive family on the diagram and are also the most frequently overlooked by beginners. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: Which construction locates the lifting condensation level?

Following the dry adiabat upward from the surface temperature and the saturation mixing ratio line upward from the surface dew point, the two meet at the lifting condensation level. That construction encodes the fact that potential temperature and mixing ratio are both conserved during dry ascent. It is the first step in every parcel analysis and is worth practising until it becomes automatic. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Which property must a usable thermodynamic diagram possess?

A.Area on the diagram must be proportional to energy
B.The isotherms must be vertical so that the temperature at each level can be read straight off the horizontal axis
C.Pressure must be linear
D.The saturated adiabats must be straight lines so that a saturated ascent can be followed without a curved template

Answer: A

The defining requirement is that an area enclosed on the diagram represents work or energy per unit mass, which is what allows convective available potential energy to be measured directly as the area between two curves. Diagrams achieving this are said to be area preserving or equal area. The tephigram, skew-T and emagram all have the property while the Stuve does not, which is why the Stuve is chosen only where simplicity matters more than quantitative work. No diagram can make every family of lines straight while preserving the area property, so each design trades one convenience against another.

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Which quantities do the axes of a tephigram represent?

A.Temperature and mixing ratio, so that the saturation curve becomes a single straight line across the whole chart
B.Temperature and the logarithm of potential temperature
C.Temperature and pressure directly, with the pressure axis inverted so that height increases up the diagram
D.Pressure and specific volume

Answer: B

The name tephigram comes from T and phi, the symbol once used for entropy, since the axes are temperature and the logarithm of potential temperature which is proportional to entropy. Because the axes are temperature and entropy, an area on the diagram is heat, which gives the required energy property. The whole diagram is then rotated through about forty five degrees so that the isobars run approximately horizontally and pressure decreases upward, which makes a plotted sounding look natural to the eye.

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Which rotation is applied to the tephigram in practice?

A.None at all
B.A rotation that varies with latitude so that the isobars remain horizontal at the station where it is used
C.About forty five degrees so that isobars run nearly horizontally
D.Ninety degrees, so that the temperature axis becomes vertical and the entropy axis becomes horizontal instead

Answer: C

Rotating the diagram places the isobars almost horizontally and the dry adiabats at about forty five degrees, which produces the familiar appearance of a plotted ascent. Without the rotation the sounding would run diagonally across the page and be much harder to read. The rotation changes nothing about the thermodynamics, since areas are preserved under rotation, and it is purely a matter of presentation. The isotherms then run diagonally upward to the right, which is the feature that distinguishes a tephigram at a glance.

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Which lines run diagonally on a skew-T diagram?

A.The isobars, which are drawn at an angle so that the plotted sounding runs approximately vertically up the page
B.The mixing ratio lines only
C.The dry adiabats, which are skewed relative to the vertical so that the saturated adiabats become straight
D.The isotherms

Answer: D

On a skew-T log-p diagram the isotherms are skewed to run at about forty five degrees while the isobars remain straight and horizontal with a logarithmic spacing. Skewing the isotherms spreads a typical sounding across the diagram rather than compressing it into a narrow band, which is the whole purpose of the design. The dry adiabats curve gently to the left and the saturated adiabats curve more strongly, converging towards the dry adiabats at low temperature.

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Which diagram plots temperature against the logarithm of pressure without skewing?

A.The emagram
B.The skew-T, whose isotherms are deliberately inclined so that the plotted sounding spreads across the page
C.The tephigram
D.The Stuve diagram, which uses pressure raised to the power kappa so that the dry adiabats become straight

Answer: A

The emagram has straight vertical isotherms and horizontal isobars logarithmically spaced, which preserves the area property while keeping the layout simple. Its disadvantage is that typical soundings occupy only a narrow diagonal band, which is exactly what the skew-T modification was introduced to correct. The name derives from energy per unit mass diagram. It remains in use across much of continental Europe. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

Which diagram sacrifices the area property for simplicity?

A.The skew-T log-p diagram, in which the skewing of the isotherms distorts the areas enclosed on the chart
B.The Stuve diagram
C.The tephigram, whose rotation destroys the proportionality between the area enclosed and the energy involved
D.The emagram

Answer: B

The Stuve diagram plots temperature against pressure raised to the power kappa, which makes both the isotherms and the dry adiabats straight lines but destroys the equal area property. It is therefore convenient for illustration and teaching but cannot be used to measure convective energy by planimetry. Rotation and skewing preserve areas, so neither the tephigram nor the skew-T loses the property despite their distorted appearance. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

Which family of lines shows the temperature a parcel follows on dry ascent?

A.The isotherms
B.The mixing ratio lines, along which the water vapour content of the parcel remains constant during the ascent
C.The dry adiabats
D.The saturated adiabats, which apply once the parcel has become saturated and condensation has begun within it

Answer: C

Dry adiabats are lines of constant potential temperature, and an unsaturated parcel moves along one as it is lifted or lowered. They are straight on a tephigram, being lines of constant ordinate before rotation, and gently curved on a skew-T. Following the correct family for the state of the parcel is the essential skill in using any of these diagrams, since a parcel switches from the dry to the saturated family the moment it reaches its condensation level.

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Which family shows the path of a saturated ascending parcel?

A.The dry adiabats, which continue to apply because the parcel remains adiabatic even after condensation begins
B.The mixing ratio lines
C.The isotherms, since the release of latent heat exactly offsets the cooling produced by the expansion above
D.The saturated adiabats

Answer: D

Saturated adiabats, also called moist adiabats or pseudoadiabats, show the temperature of a parcel in which condensation is occurring and the latent heat is offsetting part of the cooling. They are steeply curved, lying close to the dry adiabats at very low temperature where there is little water to condense and diverging markedly from them in warm moist air. Lines of constant wet bulb potential temperature are the same family. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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Which family shows the water vapour content of a parcel?

A.The saturation mixing ratio lines
B.The saturated adiabats, along which the parcel loses water continuously as condensation proceeds during ascent
C.The isobars
D.The dry adiabats, along which the potential temperature of the parcel remains fixed throughout its ascent

Answer: A

Saturation mixing ratio lines, usually drawn dashed, give the mixing ratio of saturated air at each point, so following one from the dew point at the surface gives the mixing ratio of the actual air. Their intersection with the dry adiabat through the surface temperature locates the lifting condensation level. They are the least obtrusive family on the diagram and are also the most frequently overlooked by beginners. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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10.

Which construction locates the lifting condensation level?

A.The point at which the environmental curve crosses the moist adiabat drawn through the surface temperature
B.The intersection of the dry adiabat with the saturation mixing ratio line
C.The intersection of the temperature curve with the dew point curve at the level where the two happen to meet
D.The intersection of two saturated adiabats

Answer: B

Following the dry adiabat upward from the surface temperature and the saturation mixing ratio line upward from the surface dew point, the two meet at the lifting condensation level. That construction encodes the fact that potential temperature and mixing ratio are both conserved during dry ascent. It is the first step in every parcel analysis and is worth practising until it becomes automatic. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Discuss in Forum

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