Entropy and Potential Temperature Questions and Answers

Answer: C

Warm moist tropical air at 300 kelvin with 20 grams per kilogram of vapour has an equivalent potential temperature near 350 kelvin, the latent heat contributing about fifty kelvin. Polar air reaches only about 280. The large contrast is what makes the quantity such an effective air mass label in the tropics.

Answer: D

Repeated convection carries air along a moist adiabat and the compensating subsidence warms the environment until the profile approaches that adiabat, along which equivalent potential temperature is constant. Tropical soundings show this clearly. A decrease with height would indicate potential instability, which convection removes.

Answer: A

Saturated equivalent potential temperature uses the saturation mixing ratio in place of the actual one, so it depends on temperature and pressure alone and characterises the environment rather than the air. Comparing a parcel's equivalent potential temperature with the environment's saturated value is an elegant way of assessing conditional instability throughout a column. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

A parcel is buoyant wherever its equivalent potential temperature exceeds the saturated equivalent potential temperature of the environment, which identifies the level of free convection and the equilibrium level directly. The comparison avoids the need to construct a parcel path on a diagram. It is the form in which convective instability is usually expressed in theoretical work. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: C

The excess of the parcel's equivalent potential temperature over the environmental saturated value is proportional to the buoyancy, so integrating it over the depth gives the convective available potential energy. Where the difference is negative the parcel is negatively buoyant and the integral gives the inhibition. The formulation unifies the two quantities within a single framework. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Wet bulb potential temperature is conserved for both dry and saturated adiabatic motion, like equivalent potential temperature, but takes values between about 0 and 24 degrees Celsius which are much easier to interpret. British forecasting practice uses it routinely for air mass classification. A psychrometer measures the wet bulb temperature, from which the potential form must still be computed. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Polar maritime air over north-west Europe typically has a wet bulb potential temperature near 10 degrees, against about 16 for tropical maritime and 2 for arctic air. The bands are narrow enough that a single value identifies the air mass reliably. Because the quantity is conserved through the vertical motion the air undergoes, it survives the journey from the source region. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: B

An air mass that has ascended over a mountain range and descended again, losing its moisture, has quite a different temperature but almost the same wet bulb potential temperature. Conservation is what allows the label to survive the journey. It does depend on both temperature and humidity rather than on humidity alone, which is exactly why it captures the air mass so well. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

Turbulent mixing, radiative heating and cooling, and the removal of condensate by precipitation all alter the wet bulb potential temperature of a parcel. Adiabatic ascent and descent, whether saturated or not, leave it unchanged, and isobaric cooling to saturation conserves the mixing ratio and hence the label. Knowing exactly which processes break the conservation is what makes the quantity usable as a tracer. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

Charts of wet bulb potential temperature at 850 hectopascals are a standard British forecasting product, showing the air mass distribution and the position of the fronts as gradients within the field. Thickness charts show mean temperature and are a related but distinct product. A tephigram is a vertical rather than a horizontal display.