Thermal Wind Balance Questions and Answers

Answer: C

Routine comparison of radiosonde wind shear with the temperature gradient computed from neighbouring stations shows agreement to within observational error throughout the extratropical free troposphere. That is the direct empirical test. Cross-isobaric surface flow demonstrates friction, the pressure profile demonstrates hydrostatic balance, and clear air turbulence is a consequence of the shear rather than a test of its relation to temperature.

Answer: D

A convective updraught has a Rossby number of order a hundred and vertical accelerations comparable with the residual of gravity and the pressure gradient, so both geostrophic and hydrostatic balance fail and the thermal wind relation has no validity. Environmental shear around a storm still obeys the relation, which is why storm-relative helicity is computed from the larger scale profile rather than from within the storm itself. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Storm relative helicity integrates the streamwise vorticity of the environmental shear, and that shear is set by the thermal wind of the surrounding baroclinic zone. It is the standard measure of the potential for rotating updraughts. Convective available potential energy, the lifted index and precipitable water are all thermodynamic quantities derived from the vertical temperature and moisture profile rather than from the wind. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Supercells require both a deep-layer shear of roughly 20 metres per second and enough buoyancy to sustain a vigorous updraught, since shear without instability gives no storm and instability without shear gives only short-lived pulse cells. The shear ultimately comes from the thermal wind of the baroclinic environment, which is how this large-scale relation reaches down into the forecasting of individual storms. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

Severe storm outbreaks occur where a jet streak and its associated baroclinic zone overlie a warm moist boundary layer, supplying shear from the thermal wind and instability from the surface conditions. Anticyclones and barotropic regions supply no shear by definition. The pairing of large-scale dynamics with local thermodynamics is what makes severe weather forecasting a synthesis rather than a single calculation. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

Mars has a thin but genuine atmosphere, a rotation period close to that of Earth and a strong seasonal temperature contrast, and its observed winter jets satisfy the thermal wind relation well. The Moon and Mercury lack atmospheres capable of supporting balanced flow. Jupiter does have thermal winds, but its banded structure owes much to internal heat and deep convection, which makes it a less clean illustration of the relation. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

The derivation uses geostrophic balance, which needs rotation, and hydrostatic balance, which needs gravity, together with the gas law relating density to temperature. Nothing else is required, so the relation applies to any rotating planetary atmosphere. The gas constant appears in the coefficient but changes nothing structural, and neither oceans nor a particular vertical temperature structure is needed for the relation to hold. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: B

Log-pressure height is defined as minus a scale height times the logarithm of pressure divided by a chosen reference pressure, conventionally 1000 hectopascals. The choice is arbitrary and affects only the origin of the coordinate, not any physical result. The tropopause and mean sea level are physical surfaces rather than coordinate references, and the level of no motion belongs to the oceanographic dynamic method where an absolute velocity is needed rather than a mere shear.

Answer: C

Log-pressure height increases linearly with geometric height in an isothermal atmosphere, so vertical derivatives look familiar, while the horizontal pressure gradient force retains the simple geopotential gradient form of pressure coordinates. It is therefore the natural coordinate for middle atmosphere dynamics. Temperature is still required for thickness, the Coriolis term is untouched, and the continuity equation acquires a density weighting rather than losing one.

Answer: D

Since the thermal wind is proportional to the local horizontal temperature gradient, a reversal of that gradient reverses the sign of the shear at the same level. This is exactly what happens at the tropopause, where westerly shear below gives way to easterly shear above and the wind consequently reaches a maximum. Cancellation would occur only in the integrated wind change, and then only if the two contributions happened to be equal.