Fundamental Forces in the Atmosphere Questions and Answers

Answer: A

Since the Coriolis parameter is proportional to the sine of the latitude, its horizontal effect grows from zero at the equator to a maximum at the poles. The second statement inverts this. The consequence is that tropical dynamics is governed by balances quite different from those of mid-latitudes, and that tropical cyclones require a minimum distance from the equator before rotation can organise them into a coherent vortex at all.

Answer: B

Turbulent stress transmits surface drag upward only through the lowest kilometre or so, and above that the free atmosphere behaves as very nearly inviscid on the timescales of weather. The first statement is therefore wrong. This confinement is what makes the boundary layer a distinct object of study and what allows free tropospheric dynamics to be treated with frictionless equations to a good approximation.

Answer: C

Both Coriolis and centrifugal forces are consequences of describing motion in a rotating frame and vanish in an inertial description, so the first two statements hold. The Coriolis force does no work because it is perpendicular to the velocity, so the third fails. The centrifugal force is in any case absorbed into effective gravity in the standard formulation, which removes it from explicit consideration in the horizontal equations.

Answer: D

The first two statements are correct: the force is directed down the gradient and its size is set by how rapidly pressure changes with distance. The third is false and identifies the essential distinction between the pressure gradient force and every other term in the horizontal equation, since Coriolis, centrifugal and frictional forces all vanish when the air is at rest. That is why the pressure gradient force is the primary driver of all atmospheric motion.

Answer: A

The last pairing fails: the centrifugal force, like Coriolis, is an artefact of the rotating frame and has no source in any physical interaction. The other three are correctly classified. In the standard formulation the centrifugal force of the Earth's own rotation is folded into effective gravity, which is why it seldom appears explicitly, though the centrifugal effect of curved flow does appear in gradient wind balance.

Answer: B

Only the final pairing is wrong: hydrostatic balance concerns the vertical momentum equation, not the horizontal one. The other three correctly associate each balance with the regime in which it applies. Learning the four balances as a set, each with its neglected term and its characteristic Rossby number, gives a compact framework into which almost every result in this subject can be placed.

Answer: C

Buoyancy arises from the vertical equation as the residual left when hydrostatic balance is subtracted, and it has no horizontal counterpart. Pressure gradient, Coriolis and friction are the three terms of the horizontal equation once the centrifugal force has been absorbed into effective gravity. Keeping the horizontal and vertical force inventories separate prevents a common confusion when the equations are first written out in full.

Answer: D

The pressure gradient force depends only on the distribution of mass and acts on air whether it is moving or not, which is why it is the force that initiates motion. Coriolis and centrifugal forces are proportional to velocity and to the square of velocity respectively, and friction opposes an existing motion, so all three are zero in still air. This single observation organises the whole force inventory.

Answer: A

The parcel first accelerates down the pressure gradient towards the north, and as it gains speed the Coriolis force turns it to the right until it is flowing eastward with low pressure on its left. It then oscillates about the geostrophic state at the inertial frequency. Coriolis cannot act on a parcel at rest, so no instantaneous balance is possible, and the southward option reverses the pressure gradient force.

Answer: B

The geostrophic wind is proportional to the pressure gradient, so halving the isobar spacing doubles the wind. Surface roughness affects the actual wind by adding friction, but the geostrophic value is defined by the pressure field alone. Temperature enters only weakly through density, and on a constant pressure chart it does not enter at all. Reading wind speed from isobar spacing is one of the oldest and most useful skills in practical meteorology.