Thermodynamic Diagrams Questions and Answers

Q: Which parcel suits an assessment of elevated nocturnal convection?

Where a stable nocturnal layer separates the surface from the convecting layer, the relevant parcel is the one with the greatest equivalent potential temperature aloft. Elevated thunderstorms over a shallow cold pool are the classic case. Choosing a surface parcel there would show a stable atmosphere when in fact vigorous convection is possible from above the inversion. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which measurement does a planimeter make on such a diagram?

A planimeter traces the boundary of a region and reports its area, which on an area preserving diagram converts directly to energy per unit mass. Before computers this was how convective available potential energy was obtained from a plotted ascent. The technique is obsolete operationally but explains why the area property was considered so important when the diagrams were designed. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which index is obtained by lifting a parcel to 500 hectopascals?

The lifted index is the environmental temperature at 500 hectopascals minus the temperature of a parcel lifted there from the surface or from the mixed layer, so negative values indicate instability. The Showalter index performs the same operation from 850 hectopascals, which makes it a close relative rather than a wrong answer. Both are quick to evaluate by eye on a plotted sounding. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which starting level does the Showalter index use?

Showalter lifts a parcel from 850 hectopascals to 500 and compares it with the environment there, which avoids the influence of a shallow surface layer that may be unrepresentative. That makes it useful where a nocturnal inversion or a shallow moist layer would mislead a surface based index. Its weakness is that it misses instability confined below 850 hectopascals. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which quantity does the total totals index combine?

Total totals adds the vertical totals, which is the 850 minus 500 temperature difference, to the cross totals, which is the 850 dew point minus the 500 temperature. Values above about 50 indicate a thunderstorm risk. Combining a lapse rate measure with a low level moisture measure is what gives it more skill than either component alone.

Q: Which limitation do all such single level indices share?

An index evaluated at fixed levels says nothing about the depth of the unstable layer or whether a cap prevents the parcel from reaching it, so two soundings with identical indices can behave quite differently. This is why convective energy and inhibition, which integrate over depth, have largely displaced them. They remain useful as quick summaries and for comparison with historical records. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which display accompanies the thermodynamic curves on an operational sounding?

Wind barbs are plotted alongside the temperature and dew point curves, and their variation with height gives the shear that determines storm organisation. A hodograph is often drawn as an inset from the same data. Reading the thermodynamic and kinematic information together is what allows storm type as well as storm intensity to be anticipated.

Q: Which convention does a wind barb use for a full feather?

A full feather represents ten knots, a half feather five and a solid pennant fifty, so a barb with a pennant and two full feathers indicates seventy knots. The shaft points in the direction from which the wind is blowing. The convention survives from an era when knots were universal in meteorology and remains in use for aviation.

Q: Which direction does the shaft of a wind barb point?

Meteorological convention names and draws the wind by the direction it comes from, so a northerly wind blows from the north and its barb points north. Oceanographic convention is the opposite, naming currents by where they are going, which is a trap when the two disciplines meet. The reason is historical, since the origin of the wind indicates the air mass it brings. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

21.

Which parcel is normally lifted for an afternoon convective assessment?

A.One with the mean properties of the mixed layer
B.The parcel observed at the surface at the time of the morning ascent, before any heating has taken place
C.A parcel from 500 hectopascals
D.The parcel at the level of the strongest wind, since that is where the storm will draw most of its inflow from

Answer: A

By the afternoon the boundary layer is well mixed, so a parcel with the average temperature and humidity of the lowest hundred hectopascals better represents what will actually rise than the surface values alone. Mixed layer and most unstable parcel methods are both standard. Using an unrepresentative parcel is the commonest way of reaching a wrong conclusion from a correct sounding. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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22.

Which parcel suits an assessment of elevated nocturnal convection?

A.A parcel taken from the level at which the dew point depression reaches its smallest value in the sounding
B.The most unstable parcel within the lowest few hundred hectopascals
C.The surface parcel, which is the natural starting point for any assessment of convective potential at all
D.The mixed layer parcel

Answer: B

Where a stable nocturnal layer separates the surface from the convecting layer, the relevant parcel is the one with the greatest equivalent potential temperature aloft. Elevated thunderstorms over a shallow cold pool are the classic case. Choosing a surface parcel there would show a stable atmosphere when in fact vigorous convection is possible from above the inversion. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

23.

Which measurement does a planimeter make on such a diagram?

A.The lapse rate
B.The height of the tropopause, determined from the level at which the temperature curve changes its slope
C.The area enclosed between two curves
D.The temperature at a chosen pressure level, read directly from the intersection of the curve with the isobar

Answer: C

A planimeter traces the boundary of a region and reports its area, which on an area preserving diagram converts directly to energy per unit mass. Before computers this was how convective available potential energy was obtained from a plotted ascent. The technique is obsolete operationally but explains why the area property was considered so important when the diagrams were designed. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

24.

Which index is obtained by lifting a parcel to 500 hectopascals?

A.The K index, which combines temperatures and dew points at three fixed levels rather than lifting any parcel
B.The total totals index
C.The Showalter index, which lifts a parcel from 850 hectopascals rather than from the surface of the ground
D.The lifted index

Answer: D

The lifted index is the environmental temperature at 500 hectopascals minus the temperature of a parcel lifted there from the surface or from the mixed layer, so negative values indicate instability. The Showalter index performs the same operation from 850 hectopascals, which makes it a close relative rather than a wrong answer. Both are quick to evaluate by eye on a plotted sounding. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

25.

Which starting level does the Showalter index use?

A.850 hectopascals
B.The surface, which makes it identical to the lifted index except for a difference in the sign convention
C.700 hectopascals
D.The level of the lifting condensation level of the surface parcel, wherever that happens to fall in the ascent

Answer: A

Showalter lifts a parcel from 850 hectopascals to 500 and compares it with the environment there, which avoids the influence of a shallow surface layer that may be unrepresentative. That makes it useful where a nocturnal inversion or a shallow moist layer would mislead a surface based index. Its weakness is that it misses instability confined below 850 hectopascals. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

26.

Which quantity does the total totals index combine?

A.The convective available potential energy with the deep layer wind shear through the lowest six kilometres
B.The 850 temperature and dew point with the 500 temperature
C.The temperatures at 850, 700 and 500 hectopascals with no reference to the humidity of the column at all
D.The lifted index and the K index

Answer: B

Total totals adds the vertical totals, which is the 850 minus 500 temperature difference, to the cross totals, which is the 850 dew point minus the 500 temperature. Values above about 50 indicate a thunderstorm risk. Combining a lapse rate measure with a low level moisture measure is what gives it more skill than either component alone.

Discuss in Forum

27.

Which limitation do all such single level indices share?

A.They apply only in the tropics
B.They require the wind profile in addition to the temperature and humidity observations at each level
C.They ignore the vertical distribution of buoyancy and inhibition
D.They cannot be computed without a radiosonde ascent, so they are unavailable between the synoptic hours

Answer: C

An index evaluated at fixed levels says nothing about the depth of the unstable layer or whether a cap prevents the parcel from reaching it, so two soundings with identical indices can behave quite differently. This is why convective energy and inhibition, which integrate over depth, have largely displaced them. They remain useful as quick summaries and for comparison with historical records. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

28.

Which display accompanies the thermodynamic curves on an operational sounding?

A.The pressure tendency measured at the surface station during the three hours preceding the release of the balloon
B.The cloud cover
C.A time series of the temperature observed at the station during the twenty four hours before the ascent began
D.Wind barbs at each reporting level

Answer: D

Wind barbs are plotted alongside the temperature and dew point curves, and their variation with height gives the shear that determines storm organisation. A hodograph is often drawn as an inset from the same data. Reading the thermodynamic and kinematic information together is what allows storm type as well as storm intensity to be anticipated.

Discuss in Forum

29.

Which convention does a wind barb use for a full feather?

A.Ten knots
B.Five knots, which is instead the value represented by the half feather at the end of the shaft of the barb
C.Fifty knots
D.One metre per second, which is the unit in which the wind is reported in most scientific applications now

Answer: A

A full feather represents ten knots, a half feather five and a solid pennant fifty, so a barb with a pennant and two full feathers indicates seventy knots. The shaft points in the direction from which the wind is blowing. The convention survives from an era when knots were universal in meteorology and remains in use for aviation.

Discuss in Forum

30.

Which direction does the shaft of a wind barb point?

A.Along the direction of the isobars at the level for which the barb has been plotted upon the chart
B.Towards the direction from which the wind is blowing
C.Towards the direction in which the wind is blowing, in the same way that an arrow would indicate the flow
D.Always northward

Answer: B

Meteorological convention names and draws the wind by the direction it comes from, so a northerly wind blows from the north and its barb points north. Oceanographic convention is the opposite, naming currents by where they are going, which is a trap when the two disciplines meet. The reason is historical, since the origin of the wind indicates the air mass it brings. 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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