Fundamental Forces in the Atmosphere Questions and Answers

Answer: A

Greater roughness means stronger drag, a larger reduction in wind speed and hence a weaker Coriolis force, so the flow crosses the isobars at a larger angle, typically thirty degrees or more over forest against ten to fifteen over the sea. In the free troposphere friction is negligible and the angle approaches zero. Curvature changes the balance but does not by itself produce cross-isobaric flow, which requires a dissipative force.

Answer: B

Unequal solar heating creates horizontal temperature contrasts, which through the hydrostatic relation create horizontal pressure contrasts, which drive the wind. That chain is the fundamental energy pathway of atmospheric motion. Rotation shapes the resulting circulation but supplies no energy, friction dissipates it, and latent heat release, though a powerful amplifier in storms, is itself a consequence of motion that the solar heating started.

Answer: C

Continuum mechanics is valid when a volume small compared with the flow still contains enough molecules for averages to be meaningful, which holds throughout the lower and middle atmosphere where the mean free path is measured in fractions of a micrometre. It fails only in the extreme upper atmosphere. The ideal gas law and compositional uniformity are separate assumptions, and the atmosphere is emphatically compressible.

Answer: D

Combining geostrophic and hydrostatic balance shows that the geostrophic wind must change with height wherever a horizontal temperature gradient exists, which is why the jet stream sits above the strongest surface temperature contrasts. It is a diagnostic relation rather than a force balance in its own right. The other options describe geostrophic balance, hydrostatic balance and the vorticity equation respectively.

Answer: A

Over ranges of tens of kilometres a shell is deflected by hundreds of metres, and gunnery tables have carried the correction since the nineteenth century. The trade winds are also a consequence of rotation, though their explanation involves the Hadley circulation rather than a direct deflection, so the artillery case is the cleaner demonstration. Bath water has too large a Rossby number, and winds do not in fact blow directly down the gradient above the boundary layer.

Answer: B

In the southern hemisphere the sign of \( f \) is negative and the deflection is always ninety degrees to the left of the velocity, which is why southern lows circulate clockwise and highs anticlockwise. The absolute direction of the deflection depends on the direction of travel, so no fixed compass answer can be correct. The parameter certainly does not vanish away from the equator, and its magnitude grows towards the pole exactly as in the north.

Answer: C

Geopotential is the potential energy per unit mass relative to sea level, and dividing it by a standard gravity gives geopotential height, the quantity contoured on upper air charts. Using it absorbs the variation of gravity with latitude and height, so that a given geopotential height corresponds to the same energy everywhere. Pressure and temperature are different variables, and geometric height differs from geopotential height by a small but systematic amount.

Answer: D

Because geopotential height is defined through the work done against gravity, surfaces of constant geopotential height are surfaces of constant energy, and the variation of \( g \) with latitude and altitude no longer complicates the equations. Radiosondes in fact measure pressure and temperature and derive height hypsometrically, so temperature is very much required, and the two heights differ by only a small amount that changes sign with latitude.

Answer: A

Pressure gradient, gravitation and friction are all real forces and would persist, while Coriolis and centrifugal forces, being consequences of rotation, would vanish. The atmosphere would then develop a direct thermal circulation from equator to pole with no westerly jets and no Rossby waves. Posing the question this way is a useful test of whether the distinction between real and apparent forces has actually been absorbed.

Answer: B

Without rotation the equator-to-pole temperature contrast would drive a single overturning cell in each hemisphere, rising at the equator and sinking at the pole. Rotation breaks this into the observed three-cell structure by making the poleward flow unstable to Coriolis deflection beyond about thirty degrees. Rossby waves require the beta effect and so would not exist. The thought experiment isolates exactly what rotation contributes to the general circulation.