Geostrophic and Gradient Wind Questions and Answers

Q: Which component of the ageostrophic wind is driven by a changing pressure field?

Where the pressure field is falling rapidly, an isallobaric wind blows towards the region of greatest fall, which is why convergence develops ahead of a deepening low. The name comes from isallobars, contours of pressure tendency. The other components arise from acceleration, from friction and from curvature respectively, and separating them helps in attributing an observed cross-isobaric flow to its cause.

Q: Which practical use does the isallobaric wind have?

Because the isallobaric wind blows towards falling pressure, a region of rapid fall attracts inflow and therefore convergence and ascent, which is a useful early indication of development. Pressure tendency also relates to system movement, but through a different argument. Friction and boundary layer depth are separate matters entirely, measured by other means.

Q: Assertion (A): The geostrophic wind blows parallel to the isobars. Reason (R): In geostrophic balance the Coriolis force exactly opposes the pressure gradient force. Select the correct answer from the options below.

The Coriolis force is perpendicular to the velocity and the pressure gradient force is perpendicular to the isobars, so if the two are antiparallel the velocity must lie along the isobars. The reason therefore explains the assertion completely. The argument also shows why any observed cross-isobaric flow is evidence of a third force, which in the free atmosphere means the flow is accelerating and in the boundary layer means friction is acting.

Q: Assertion (A): There is no dynamical limit on how deep a low pressure centre may become. Reason (R): The gradient wind equation has a real solution for cyclonic curvature at any pressure gradient. Select the correct answer from the options below.

The quadratic gradient wind equation always yields a real root for cyclonic curvature, whereas for anticyclonic curvature a real root exists only below a limiting pressure gradient. The reason is therefore exactly why the assertion holds. The consequence is visible in the observed record, where the extremes of low pressure lie much further from the mean than the extremes of high pressure do.

Q: Assertion (A): The geostrophic approximation can be used at the equator. Reason (R): The Coriolis parameter is proportional to the sine of the latitude. Select the correct answer from the options below.

The reason is correct and is precisely why the assertion fails: with \( f \) vanishing at the equator, the geostrophic wind would be unbounded for any non-zero pressure gradient. In practice tropical pressure gradients are extremely weak and other balances apply. This is the recurring pattern in which a true reason establishes the falsity of the assertion, and it rewards reading both statements before choosing.

Q: Assertion (A): A tornado must rotate cyclonically. Reason (R): Cyclostrophic balance involves the square of the tangential wind speed. Select the correct answer from the options below.

The reason is a correct statement of the balance, and because the wind appears squared the equation is satisfied by either sense of rotation, so the assertion is false. Anticyclonic tornadoes are documented, though rare, because most descend from cyclonically rotating mesocyclones. The example shows how the mathematical form of a balance can settle a question that intuition gets wrong.

Q: Statement I: The thermal wind is an actual wind experienced by a parcel. Statement II: The thermal wind is the vector difference between geostrophic winds at two levels. In the light of the above statements, choose the correct answer.

The thermal wind is a shear rather than a wind: no parcel ever experiences it, and it exists only as the difference between the geostrophic winds at the top and bottom of a layer. The name is unfortunate and misleads many people on first meeting. Keeping the definition firmly in mind prevents the error of trying to add the thermal wind to an observed wind as though it were a physical flow.

Q: Consider the following statements about geostrophic balance. 1. It is a diagnostic relation. 2. It holds best for large-scale mid-latitude flow. 3. It can be used to predict how the flow will evolve. Which of the statements given above is or are correct?

Geostrophic balance contains no time derivative, so it relates the wind and pressure fields at one instant and cannot generate an evolution, which makes the first two statements true and the third false. Prediction requires the ageostrophic residual retained at the next order of the scale expansion. That distinction, between a diagnostic and a prognostic relation, is fundamental to everything that follows in this subject. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Consider the following statements about the gradient wind. 1. It includes the centrifugal term. 2. It is subgeostrophic around a low. 3. It applies only in the northern hemisphere. Which of the statements given above is or are correct?

The first two statements are correct, while the third is false since the balance applies in both hemispheres with the appropriate sign of the Coriolis parameter. In the southern hemisphere the circulation senses reverse but the subgeostrophic character of cyclonic flow does not, because both the Coriolis and centrifugal forces still act outward around a low. Checking a rule in both hemispheres is a good test of whether it has been understood or merely memorised.

61.

Which component of the ageostrophic wind is driven by a changing pressure field?

A.The inertial advective component, which arises from the acceleration of a parcel along its trajectory
B.The frictional component
C.The curvature component, which appears wherever the trajectory of the parcel departs from a straight line
D.The isallobaric component

Answer: D

Where the pressure field is falling rapidly, an isallobaric wind blows towards the region of greatest fall, which is why convergence develops ahead of a deepening low. The name comes from isallobars, contours of pressure tendency. The other components arise from acceleration, from friction and from curvature respectively, and separating them helps in attributing an observed cross-isobaric flow to its cause.

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62.

Which practical use does the isallobaric wind have?

A.It indicates where low level convergence is developing
B.It gives the speed at which a pressure system will move across the chart over the next twelve hours
C.It measures friction
D.It determines the depth of the boundary layer through which the surface stress is being transmitted

Answer: A

Because the isallobaric wind blows towards falling pressure, a region of rapid fall attracts inflow and therefore convergence and ascent, which is a useful early indication of development. Pressure tendency also relates to system movement, but through a different argument. Friction and boundary layer depth are separate matters entirely, measured by other means.

Discuss in Forum

63.

Assertion (A): The geostrophic wind blows parallel to the isobars.

Reason (R): In geostrophic balance the Coriolis force exactly opposes the pressure gradient force.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

The Coriolis force is perpendicular to the velocity and the pressure gradient force is perpendicular to the isobars, so if the two are antiparallel the velocity must lie along the isobars. The reason therefore explains the assertion completely. The argument also shows why any observed cross-isobaric flow is evidence of a third force, which in the free atmosphere means the flow is accelerating and in the boundary layer means friction is acting.

Discuss in Forum

64.

Assertion (A): There is no dynamical limit on how deep a low pressure centre may become.

Reason (R): The gradient wind equation has a real solution for cyclonic curvature at any pressure gradient.

Select the correct answer from the options below.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

Answer: C

The quadratic gradient wind equation always yields a real root for cyclonic curvature, whereas for anticyclonic curvature a real root exists only below a limiting pressure gradient. The reason is therefore exactly why the assertion holds. The consequence is visible in the observed record, where the extremes of low pressure lie much further from the mean than the extremes of high pressure do.

Discuss in Forum

65.

Assertion (A): The geostrophic approximation can be used at the equator.

Reason (R): The Coriolis parameter is proportional to the sine of the latitude.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Answer: D

The reason is correct and is precisely why the assertion fails: with \( f \) vanishing at the equator, the geostrophic wind would be unbounded for any non-zero pressure gradient. In practice tropical pressure gradients are extremely weak and other balances apply. This is the recurring pattern in which a true reason establishes the falsity of the assertion, and it rewards reading both statements before choosing.

Discuss in Forum

66.

Assertion (A): A tornado must rotate cyclonically.

Reason (R): Cyclostrophic balance involves the square of the tangential wind speed.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: A

The reason is a correct statement of the balance, and because the wind appears squared the equation is satisfied by either sense of rotation, so the assertion is false. Anticyclonic tornadoes are documented, though rare, because most descend from cyclonically rotating mesocyclones. The example shows how the mathematical form of a balance can settle a question that intuition gets wrong.

Discuss in Forum

67.

Statement I: The gradient wind around a low is subgeostrophic.

Statement II: The gradient wind around a high is supergeostrophic.

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

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

Answer: B

Both statements are correct: around a low two outward forces oppose the inward pressure gradient so a smaller wind balances, while around a high only one inward force opposes two outward ones so a larger wind is required. Setting the two cases side by side is the clearest way to remember which is which, and the asymmetry between them is what limits the intensity of anticyclones while leaving cyclones unconstrained.

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68.

Statement I: The thermal wind is an actual wind experienced by a parcel.

Statement II: The thermal wind is the vector difference between geostrophic winds at two levels.

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

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

Answer: C

The thermal wind is a shear rather than a wind: no parcel ever experiences it, and it exists only as the difference between the geostrophic winds at the top and bottom of a layer. The name is unfortunate and misleads many people on first meeting. Keeping the definition firmly in mind prevents the error of trying to add the thermal wind to an observed wind as though it were a physical flow.

Discuss in Forum

69.

Consider the following statements about geostrophic balance.

1. It is a diagnostic relation.

2. It holds best for large-scale mid-latitude flow.

3. It can be used to predict how the flow will evolve.

Which of the statements given above is or are correct?

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

Answer: D

Geostrophic balance contains no time derivative, so it relates the wind and pressure fields at one instant and cannot generate an evolution, which makes the first two statements true and the third false. Prediction requires the ageostrophic residual retained at the next order of the scale expansion. That distinction, between a diagnostic and a prognostic relation, is fundamental to everything that follows in this subject. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

70.

Consider the following statements about the gradient wind.

1. It includes the centrifugal term.

2. It is subgeostrophic around a low.

3. It applies only in the northern hemisphere.

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 is false since the balance applies in both hemispheres with the appropriate sign of the Coriolis parameter. In the southern hemisphere the circulation senses reverse but the subgeostrophic character of cyclonic flow does not, because both the Coriolis and centrifugal forces still act outward around a low. Checking a rule in both hemispheres is a good test of whether it has been understood or merely memorised.

Discuss in Forum

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