Geostrophic and Gradient Wind Questions and Answers

Answer: C

The ageostrophic wind is conventionally decomposed into an isallobaric part, driven by the local time change of the pressure field, and an inertial advective part associated with acceleration. Friction adds a third component within the boundary layer. The thermal wind is a shear rather than a departure from balance, and while the ageostrophic wind is largely divergent, naming it that way describes a property rather than a cause. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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.

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.

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.

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.

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.

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.

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.

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.

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.