Thermal Wind Balance Questions and Answers

Q: Statement I: A barotropic atmosphere has no thermal wind. Statement II: A baroclinic atmosphere has no thermal wind. In the light of the above statements, choose the correct answer.

Barotropy means isobaric and isopycnal surfaces are parallel, so there is no horizontal temperature gradient on a pressure surface and the thermal wind vanishes. Baroclinicity is precisely the opposite case, in which the thermal wind is non-zero. The second statement therefore inverts the definition. Almost all interesting mid-latitude weather occurs in baroclinic conditions, which is why the barotropic case is treated mainly as a simplifying idealisation.

Q: Statement I: Thickness is proportional to layer mean virtual temperature. Statement II: Thickness is proportional to the pressure at the base of the layer. In the light of the above statements, choose the correct answer.

The hypsometric equation gives thickness as proportional to layer mean virtual temperature multiplied by the logarithm of the ratio of the bounding pressures, so the first statement is correct. The base pressure enters only through that logarithm, which is fixed once the two levels are chosen, so thickness varies with temperature alone for a given layer. That is what makes a thickness chart readable as a temperature chart.

Q: Consider the following statements about the thermal wind. 1. It blows parallel to the layer mean isotherms. 2. It is a shear rather than a wind. 3. It vanishes in a baroclinic atmosphere. Which of the statements given above is or are correct?

The first two statements are correct, while the third inverts the definition: the thermal wind vanishes in a barotropic atmosphere and is non-zero in a baroclinic one. Confusing the two terms is the commonest error in this topic. A useful check is that baroclinic instability, the source of mid-latitude weather, requires a thermal wind and therefore requires baroclinicity by definition.

Q: Consider the following statements about thickness charts. 1. Low thickness indicates cold air. 2. The 540 decametre contour is a traditional snow guide. 3. Thickness gives the temperature at each individual level within the layer. Which of the statements given above is or are correct?

Thickness is a mass-weighted mean, so it conceals the vertical distribution and cannot give level by level temperatures, which makes the third statement false. The first two are correct. This limitation is exactly why a shallow warm layer aloft can defeat the 540 decametre rule, and why forecasters examine the full sounding rather than relying on a single bulk quantity.

Q: Consider the following pairs of quantity and what it measures. Thickness - layer mean temperature Thermal wind - vertical shear Baroclinicity - intersecting pressure and density surfaces Barotropy - strong vertical shear How many of the pairs given above are correctly matched?

The last pairing is wrong: barotropy means no vertical shear of the geostrophic wind, since the thermal wind vanishes. The other three are correctly matched. Setting the four terms out together makes the logical chain visible, from intersecting surfaces through a horizontal temperature gradient to a vertical shear, which is the whole content of thermal wind balance in one line.

Q: Consider the following pairs of observation and inference. Veering wind with height - warm advection Backing wind with height - cold advection No turning with height - no thermal advection Speed increasing with height - no thermal wind How many of the pairs given above are correctly matched?

A wind whose speed increases with height without turning has a thermal wind parallel to the flow, so there is certainly a thermal wind and the last pairing fails. The other three are correct for the northern hemisphere. Working through the four cases as a set clarifies that turning indicates advection while speed change alone indicates a gradient along the flow.

Q: Which of the following is not required for the thermal wind relation to hold? Frictionless flow Geostrophic balance Hydrostatic balance A non-zero Coriolis parameter

Friction is not part of the derivation: the relation follows from combining geostrophic and hydrostatic balance, and both of those require a non-zero Coriolis parameter. Friction does of course invalidate geostrophic balance within the boundary layer, but it enters through that route rather than as a separate condition. Distinguishing what a derivation actually uses from what happens to be true alongside it is a habit worth cultivating. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Which of the following does not follow from thermal wind balance? The Ekman spiral The jet stream position Veering as warm advection Frontal shear

The Ekman spiral arises from the balance between pressure gradient, Coriolis and frictional forces within the boundary layer, and has nothing to do with any horizontal temperature gradient. The other three are all direct consequences of the thermal wind relation. Both the spiral and the thermal wind describe wind turning with height, which is why the pair makes a useful test of whether the mechanisms have been kept distinct. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: A sounding at 50 degrees north shows wind backing from westerly at 850 hectopascals to south-westerly at 500 hectopascals. What does the profile indicate?

Turning anticlockwise with height is backing, which places the thermal wind to the left of the lower wind and therefore implies cold advection in the northern hemisphere. No absolute temperatures are needed, only the sense of the turning, which is what makes this such a quick and useful check. The same profile in the southern hemisphere would indicate warm advection instead.

Q: A forecaster notes that the 1000 to 500 hectopascal thickness has fallen from 546 to 534 decametres over twelve hours. What has happened to the layer?

Thickness is proportional to layer mean virtual temperature, so a fall of 12 decametres corresponds to a cooling of roughly 6 kelvin averaged over the layer. This is a typical signature of cold advection behind a front. Humidity affects thickness only weakly through virtual temperature, and subsidence warms rather than cools, which would raise the thickness rather than lower it.

51.

Statement I: A barotropic atmosphere has no thermal wind.

Statement II: A baroclinic atmosphere has no thermal wind.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are incorrect
B.Statement I is incorrect but Statement II is correct
C.Statement I is correct but Statement II is incorrect
D.Both Statement I and Statement II are correct

Answer: C

Barotropy means isobaric and isopycnal surfaces are parallel, so there is no horizontal temperature gradient on a pressure surface and the thermal wind vanishes. Baroclinicity is precisely the opposite case, in which the thermal wind is non-zero. The second statement therefore inverts the definition. Almost all interesting mid-latitude weather occurs in baroclinic conditions, which is why the barotropic case is treated mainly as a simplifying idealisation.

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52.

Statement I: Thickness is proportional to layer mean virtual temperature.

Statement II: Thickness is proportional to the pressure at the base of the layer.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are correct
B.Both Statement I and Statement II are incorrect
C.Statement I is incorrect but Statement II is correct
D.Statement I is correct but Statement II is incorrect

Answer: D

The hypsometric equation gives thickness as proportional to layer mean virtual temperature multiplied by the logarithm of the ratio of the bounding pressures, so the first statement is correct. The base pressure enters only through that logarithm, which is fixed once the two levels are chosen, so thickness varies with temperature alone for a given layer. That is what makes a thickness chart readable as a temperature chart.

Discuss in Forum

53.

Consider the following statements about the thermal wind.

1. It blows parallel to the layer mean isotherms.

2. It is a shear rather than a wind.

3. It vanishes in a baroclinic atmosphere.

Which of the statements given above is or are correct?

A.1 and 2 only
B.1 and 3 only
C.2 and 3 only
D.1, 2 and 3

Answer: A

The first two statements are correct, while the third inverts the definition: the thermal wind vanishes in a barotropic atmosphere and is non-zero in a baroclinic one. Confusing the two terms is the commonest error in this topic. A useful check is that baroclinic instability, the source of mid-latitude weather, requires a thermal wind and therefore requires baroclinicity by definition.

Discuss in Forum

54.

Consider the following statements about thickness charts.

1. Low thickness indicates cold air.

2. The 540 decametre contour is a traditional snow guide.

3. Thickness gives the temperature at each individual level within the layer.

Which of the statements given above is or are correct?

A.1, 2 and 3
B.1 and 2 only
C.1 and 3 only
D.2 and 3 only

Answer: B

Thickness is a mass-weighted mean, so it conceals the vertical distribution and cannot give level by level temperatures, which makes the third statement false. The first two are correct. This limitation is exactly why a shallow warm layer aloft can defeat the 540 decametre rule, and why forecasters examine the full sounding rather than relying on a single bulk quantity.

Discuss in Forum

55.

Consider the following pairs of quantity and what it measures.

Thickness - layer mean temperature

Thermal wind - vertical shear

Baroclinicity - intersecting pressure and density surfaces

Barotropy - strong vertical shear

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

Answer: C

The last pairing is wrong: barotropy means no vertical shear of the geostrophic wind, since the thermal wind vanishes. The other three are correctly matched. Setting the four terms out together makes the logical chain visible, from intersecting surfaces through a horizontal temperature gradient to a vertical shear, which is the whole content of thermal wind balance in one line.

Discuss in Forum

56.

Consider the following pairs of observation and inference.

Veering wind with height - warm advection

Backing wind with height - cold advection

No turning with height - no thermal advection

Speed increasing with height - no thermal wind

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

A wind whose speed increases with height without turning has a thermal wind parallel to the flow, so there is certainly a thermal wind and the last pairing fails. The other three are correct for the northern hemisphere. Working through the four cases as a set clarifies that turning indicates advection while speed change alone indicates a gradient along the flow.

Discuss in Forum

57.

Which of the following is not required for the thermal wind relation to hold?

Frictionless flow

Geostrophic balance

Hydrostatic balance

A non-zero Coriolis parameter

A.Frictionless flow
B.Geostrophic balance
C.Hydrostatic balance
D.A non-zero Coriolis parameter

Answer: A

Friction is not part of the derivation: the relation follows from combining geostrophic and hydrostatic balance, and both of those require a non-zero Coriolis parameter. Friction does of course invalidate geostrophic balance within the boundary layer, but it enters through that route rather than as a separate condition. Distinguishing what a derivation actually uses from what happens to be true alongside it is a habit worth cultivating. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Discuss in Forum

58.

Which of the following does not follow from thermal wind balance?

The Ekman spiral

The jet stream position

Veering as warm advection

Frontal shear

A.Frontal shear
B.The Ekman spiral
C.The jet stream position
D.Veering as warm advection

Answer: B

The Ekman spiral arises from the balance between pressure gradient, Coriolis and frictional forces within the boundary layer, and has nothing to do with any horizontal temperature gradient. The other three are all direct consequences of the thermal wind relation. Both the spiral and the thermal wind describe wind turning with height, which is why the pair makes a useful test of whether the mechanisms have been kept distinct. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

59.

A sounding at 50 degrees north shows wind backing from westerly at 850 hectopascals to south-westerly at 500 hectopascals.

What does the profile indicate?

A.No advection
B.A situation that cannot be interpreted without knowing the actual temperatures at the two levels
C.Cold advection through the layer
D.Warm advection through the layer since the wind is turning towards the direction of the warmer air

Answer: C

Turning anticlockwise with height is backing, which places the thermal wind to the left of the lower wind and therefore implies cold advection in the northern hemisphere. No absolute temperatures are needed, only the sense of the turning, which is what makes this such a quick and useful check. The same profile in the southern hemisphere would indicate warm advection instead.

Discuss in Forum

60.

A forecaster notes that the 1000 to 500 hectopascal thickness has fallen from 546 to 534 decametres over twelve hours.

What has happened to the layer?

A.It has warmed substantially because a falling thickness indicates a reduction of the layer's density
B.It has become drier
C.It has been compressed by subsidence without any change to the mean temperature within the column
D.It has cooled substantially

Answer: D

Thickness is proportional to layer mean virtual temperature, so a fall of 12 decametres corresponds to a cooling of roughly 6 kelvin averaged over the layer. This is a typical signature of cold advection behind a front. Humidity affects thickness only weakly through virtual temperature, and subsidence warms rather than cools, which would raise the thickness rather than lower it.

Discuss in Forum

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