Geostrophic and Gradient Wind Questions and Answers

Q: Match each balance with the force it omits. List I A. Geostrophic B. Cyclostrophic C. Inertial List II P. centrifugal Q. Coriolis R. pressure gradient Select the correct match.

Geostrophic balance omits the centrifugal term, cyclostrophic balance omits the Coriolis term, and inertial motion omits the pressure gradient force. Gradient wind balance, which omits none of the three, completes the family. Presented as a table of omissions the four balances become a single coherent scheme rather than four separate facts to be memorised independently.

Q: Which measurement is needed to compute the geostrophic wind from a chart?

Contour spacing gives the geopotential gradient and the latitude gives the Coriolis parameter, and those two quantities determine the geostrophic wind completely. Density has vanished because the chart is drawn on a pressure surface, so temperature is not required. The surface wind and the pressure tendency give other information: the former reflects friction and the latter is the source of the isallobaric wind.

Q: Which instrument arrangement gives the pressure gradient at a point?

A pressure gradient is a spatial derivative, so it requires simultaneous readings from several stations rather than a time series from one. A barograph gives the local tendency, which is a quite different quantity used for the isallobaric wind. A radiosonde gives a vertical profile at one location. Inferring the gradient from the wind reverses the usual logic and would be circular if the balance is what is being tested. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Which effect does an error in the analysed height field have on the derived geostrophic wind?

Differentiation amplifies small-scale error, so a height analysis accurate to a few decametres can give a geostrophic wind in error by several metres per second. This is why modern analysis systems assimilate wind observations directly rather than deriving the wind from the mass field. Both speed and direction are affected. The general point, that derivatives amplify noise, recurs throughout the diagnosis of atmospheric fields. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which advantage does direct wind observation have over deriving the wind from pressure?

Because the geostrophic relation involves a spatial derivative, small errors in the mass field produce large errors in the derived wind, whereas a direct measurement carries only its own instrumental error. This is why wind profilers, aircraft reports and satellite-derived winds are so valuable in data assimilation. Cost and latitude are not the relevant considerations, and pressure observations are in fact widely available. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Q: Which situation makes the geostrophic approximation poor even in mid-latitudes?

A small radius of curvature makes the centrifugal term comparable with the other two, so the Rossby number rises and gradient wind balance must replace geostrophic balance. Broad weak systems and straight flow are precisely where geostrophy works best. Alignment of contours with isotherms says something about the thermal wind rather than about the validity of the geostrophic approximation at a single level.

Q: Which quantity measures how far a flow departs from gradient wind balance?

Evaluating the three terms and taking their sum gives the net unbalanced force, which is what accelerates the parcel. The Rossby number indicates whether balance is likely rather than measuring the departure in a particular case. The Richardson number concerns stratification and shear, and the ageostrophic ratio measures departure from geostrophic rather than from gradient balance, which is a different question.

Q: Which process restores gradient wind balance after it is disturbed?

An unbalanced state radiates inertia-gravity waves that carry the imbalance away, leaving a balanced residual on a timescale of hours. This geostrophic adjustment is why the atmosphere is observed to remain close to balance almost everywhere despite continual forcing. Friction and diabatic heating act far more slowly, and baroclinic instability generates rather than removes departures from the zonal state.

Q: Which conclusion follows from the speed of geostrophic adjustment?

Because imbalance is radiated away within hours while the balanced flow evolves over days, the atmosphere spends almost all of its time close to balance. This justifies balanced models such as the quasi-geostrophic system and explains why unbalanced initial conditions ruined Richardson's forecast. Gravity waves carry the adjustment but little of the energy, and friction remains important on its own longer timescale.

81.

Match each balance with the force it omits.

List I

A. Geostrophic

B. Cyclostrophic

C. Inertial

List II

P. centrifugal

Q. Coriolis

R. pressure gradient

Select the correct match.

A.A-Q, B-P, C-R
B.A-R, B-P, C-Q
C.A-P, B-R, C-Q
D.A-P, B-Q, C-R

Answer: D

Geostrophic balance omits the centrifugal term, cyclostrophic balance omits the Coriolis term, and inertial motion omits the pressure gradient force. Gradient wind balance, which omits none of the three, completes the family. Presented as a table of omissions the four balances become a single coherent scheme rather than four separate facts to be memorised independently.

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82.

Match each wind with what it describes.

List I

A. Geostrophic

B. Thermal

C. Ageostrophic

List II

P. vertical shear of the geostrophic wind

Q. two-force balanced flow

R. departure from balance

Select the correct match.

A.A-Q, B-P, C-R
B.A-P, B-Q, C-R
C.A-R, B-P, C-Q
D.A-Q, B-R, C-P

Answer: A

The geostrophic wind is the two-force balanced flow, the thermal wind is the vertical shear of that flow implied by a horizontal temperature gradient, and the ageostrophic wind is the departure of the actual wind from balance. The three together give a complete description of the horizontal wind at synoptic scales, with the small third member carrying the vertical motion and the evolution.

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83.

Which measurement is needed to compute the geostrophic wind from a chart?

A.The rate at which the pressure at a fixed point is changing over the course of the last three hours
B.The contour spacing and the latitude
C.The temperature of the layer together with the pressure at the level for which the chart is drawn
D.The wind speed at the surface

Answer: B

Contour spacing gives the geopotential gradient and the latitude gives the Coriolis parameter, and those two quantities determine the geostrophic wind completely. Density has vanished because the chart is drawn on a pressure surface, so temperature is not required. The surface wind and the pressure tendency give other information: the former reflects friction and the latter is the source of the isallobaric wind.

Discuss in Forum

84.

Which instrument arrangement gives the pressure gradient at a point?

A.A radiosonde
B.An anemometer, from which the pressure gradient may be inferred by assuming geostrophic balance holds
C.A network of barometers
D.A single barograph recording the change in pressure at that point over a period of several hours

Answer: C

A pressure gradient is a spatial derivative, so it requires simultaneous readings from several stations rather than a time series from one. A barograph gives the local tendency, which is a quite different quantity used for the isallobaric wind. A radiosonde gives a vertical profile at one location. Inferring the gradient from the wind reverses the usual logic and would be circular if the balance is what is being tested. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Discuss in Forum

85.

Which effect does an error in the analysed height field have on the derived geostrophic wind?

A.It is reduced because taking a gradient averages over the errors at neighbouring points on the chart
B.It has no effect
C.It affects only the direction of the derived wind and leaves its magnitude essentially unchanged
D.It is amplified because the wind depends on a gradient

Answer: D

Differentiation amplifies small-scale error, so a height analysis accurate to a few decametres can give a geostrophic wind in error by several metres per second. This is why modern analysis systems assimilate wind observations directly rather than deriving the wind from the mass field. Both speed and direction are affected. The general point, that derivatives amplify noise, recurs throughout the diagnosis of atmospheric fields. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

86.

Which advantage does direct wind observation have over deriving the wind from pressure?

A.It avoids the amplification of analysis error by differentiation
B.It gives the wind at a level where no pressure observations happen to be available at that moment
C.It is cheaper
D.It removes the need to know the latitude at which the observation has been taken by the instrument

Answer: A

Because the geostrophic relation involves a spatial derivative, small errors in the mass field produce large errors in the derived wind, whereas a direct measurement carries only its own instrumental error. This is why wind profilers, aircraft reports and satellite-derived winds are so valuable in data assimilation. Cost and latitude are not the relevant considerations, and pressure observations are in fact widely available. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Discuss in Forum

87.

Which situation makes the geostrophic approximation poor even in mid-latitudes?

A.A region where the height contours are parallel to the isotherms so that no thermal wind is present
B.Strongly curved flow around a small intense low
C.A broad slow-moving anticyclone in which the pressure gradient is weak and the isobars widely spaced
D.Uniform straight flow

Answer: B

A small radius of curvature makes the centrifugal term comparable with the other two, so the Rossby number rises and gradient wind balance must replace geostrophic balance. Broad weak systems and straight flow are precisely where geostrophy works best. Alignment of contours with isotherms says something about the thermal wind rather than about the validity of the geostrophic approximation at a single level.

Discuss in Forum

88.

Which quantity measures how far a flow departs from gradient wind balance?

A.The Richardson number
B.The ratio of the ageostrophic wind to the geostrophic wind at the level being examined on the chart
C.The residual of the three-force balance
D.The Rossby number, which compares the inertial acceleration against the Coriolis acceleration exactly

Answer: C

Evaluating the three terms and taking their sum gives the net unbalanced force, which is what accelerates the parcel. The Rossby number indicates whether balance is likely rather than measuring the departure in a particular case. The Richardson number concerns stratification and shear, and the ageostrophic ratio measures departure from geostrophic rather than from gradient balance, which is a different question.

Discuss in Forum

89.

Which process restores gradient wind balance after it is disturbed?

A.Frictional dissipation, which removes the excess energy of the disturbance over a period of days
B.Baroclinic instability
C.Diabatic heating, which alters the mass field until it is once again consistent with the wind field
D.The radiation of gravity waves

Answer: D

An unbalanced state radiates inertia-gravity waves that carry the imbalance away, leaving a balanced residual on a timescale of hours. This geostrophic adjustment is why the atmosphere is observed to remain close to balance almost everywhere despite continual forcing. Friction and diabatic heating act far more slowly, and baroclinic instability generates rather than removes departures from the zonal state.

Discuss in Forum

90.

Which conclusion follows from the speed of geostrophic adjustment?

A.Balanced flow is the natural state of the large-scale atmosphere
B.Gravity waves must be the dominant carriers of energy within the mid-latitude troposphere at all times
C.Friction is unimportant
D.The pressure field must always be computed before the wind field in any numerical analysis scheme

Answer: A

Because imbalance is radiated away within hours while the balanced flow evolves over days, the atmosphere spends almost all of its time close to balance. This justifies balanced models such as the quasi-geostrophic system and explains why unbalanced initial conditions ruined Richardson's forecast. Gravity waves carry the adjustment but little of the energy, and friction remains important on its own longer timescale.

Discuss in Forum

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