Entropy and Potential Temperature Questions and Answers

Answer: C

The potential temperature is constructed by undoing the compression, bringing the parcel to a reference pressure, so it cannot be changed by that compression. The reason therefore explains the assertion by construction. The actual temperature rises at the dry adiabatic rate during the descent, which is the change the definition was designed to remove.

Answer: D

Because the quantity survives the ascent and descent an air mass undergoes on its journey, including any condensation and evaporation, it retains the value acquired in the source region. The reason therefore explains the assertion completely. Mixing and diabatic heating do erode the label eventually, which is why air masses gradually lose their identity far from their source.

Answer: A

Both statements are correct and together they define how isentropic analysis works and where it fails. Adiabatic motion is confined to a surface, so trajectories are two dimensional, and heating moves the parcel across surfaces, which is why trajectories through cloud must be computed diabatically. The two statements bound the applicability of the whole technique.

Answer: B

The first two statements are correct, but latent heat release during saturated ascent raises the potential temperature. Equivalent potential temperature is the quantity conserved in that case. Knowing exactly which processes each conserved variable survives is what allows the right one to be chosen for a given problem.

Answer: C

Virtual temperature is the temperature dry air would need in order to have the same density as the moist sample, which is not a process applied to the parcel but a comparison with a different one. The other three correctly describe the thought experiment defining each quantity. Setting them out as processes rather than as formulae makes their conservation properties much easier to recall. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

Latent heat release raises the potential temperature during saturated ascent, which is precisely why the equivalent and wet bulb forms were introduced. Both of those and the total water content are conserved in the reversible process. In the pseudo-adiabatic idealisation the condensed water is removed, so the total water content is not conserved there either, which is a refinement worth noting.

Answer: A

A conserved air mass label taking the same value at two places indicates that the air has a common origin and has undergone similar processing, which is what defines a single air mass. Surface temperature may differ considerably because of local heating and elevation. A front would appear as a sharp gradient between the two rather than as identical values.

Answer: B

Potential temperature is conserved during dry adiabatic motion and is proportional to the exponential of the specific entropy, which is what makes isentropic and constant potential temperature surfaces identical. Virtual, equivalent and wet bulb temperatures are defined by quite different thought experiments on the same parcel and answer different questions about it. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

For a warm moist parcel near the surface the temperature is lowest, the virtual temperature exceeds it by a few kelvin, the potential temperature exceeds that if the parcel is above 1000 hectopascals, and the equivalent potential temperature is highest of all, giving 2, 4, 3, 1. The ordering is a useful check on any set of computed values.

Answer: D

Potential temperature removes the pressure dependence, virtual temperature accounts for the density effect of the water vapour, and equivalent potential temperature adds the latent heat that the vapour would release. Each modifies the plain temperature to answer a specific question, and knowing which question each answers prevents them being confused.