Entropy and Potential Temperature Questions and Answers

Answer: A

Surfaces between about 310 and 330 kelvin cut the tropopause in mid-latitudes during winter and are the standard choice for isentropic potential vorticity maps. The 290 surface intersects the ground and the 500 surface lies far up in the stratosphere. Choosing a surface that cuts the feature of interest is the first step in any isentropic analysis.

Answer: B

The whole distribution of potential temperature shifts upward in summer, so a surface that cuts the tropopause in January lies well within the troposphere in July. Standard practice uses different surfaces for the two seasons. The tropopause is in fact higher rather than lower in summer, which is why the second option is wrong on both counts.

Answer: C

The surface passes from the troposphere at low latitudes to the stratosphere at high latitudes, and the potential vorticity jumps by an order of magnitude across the crossing. That sharp gradient is the dynamical tropopause seen in plan view, and the distortions of the contour reveal the waves and the wave breaking. It is one of the most informative single fields in synoptic meteorology. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Reversible advection stretches and folds the contour but preserves the gradient, whereas breaking followed by small scale mixing genuinely destroys it and broadens the transition. The mid-latitude surf zone is where that mixing occurs. Distinguishing reversible distortion from irreversible mixing is exactly what the equivalent latitude framework was designed to do. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

The conventional reference is 1000 hectopascals, chosen because it is a round number close to typical surface pressure rather than for any physical reason. Any fixed reference would serve equally well provided it is used consistently, since only differences of potential temperature enter physical arguments. Using the actual surface pressure would make the quantity vary from place to place and would destroy the conservation property that makes it useful in the first place.

Answer: B

At the reference pressure the correction factor becomes unity and the potential temperature equals the actual temperature. This is a useful check on any calculation. The equivalent potential temperature differs by the latent heat contribution and coincides with the potential temperature only for perfectly dry air.

Answer: C

Adiabatic descent compresses and warms the parcel, but the potential temperature is defined precisely to be unchanged by that compression. The temperature rises at the dry adiabatic rate of 9.8 kelvin per kilometre. Recognising that one quantity changes while the other does not is the whole purpose of introducing the potential temperature at all.

Answer: D

Isobaric heating raises both the temperature and the potential temperature while leaving the pressure and the water content unchanged. Relative humidity falls because the saturation value rises. This is the case that separates diabatic from adiabatic processes most clearly, since the potential temperature changes only when heat is actually added.

Answer: A

Latent heat released by condensation warms the parcel relative to a dry adiabat, so its potential temperature rises. That is precisely why potential temperature is not conserved for saturated ascent and why equivalent potential temperature was introduced. Freezing releases further heat and raises it more, so the sign is the same above and below the freezing level. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Equivalent potential temperature is what a parcel would have if all its vapour were condensed and the released heat retained, then brought adiabatically to the reference pressure. It is therefore conserved for both dry and saturated adiabatic motion. Virtual temperature is a density correction and answers a quite different question about the same parcel.