Gas Laws and Atmospheric Variables Questions and Answers

Answer: C

The screen is painted white to reflect solar radiation and has double louvred walls to allow ventilation while excluding direct and reflected sunlight. Its design has been essentially unchanged since the nineteenth century, which is valuable for the homogeneity of long climate records. Modern aspirated shields perform better but changing the instrument shelter introduces a discontinuity into the record. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

Applying the gas law with 1013.25 hectopascals and 288.15 kelvin gives 1.225 kilograms per cubic metre, about one eight hundredth of the density of water. This value appears in every aerodynamic calculation and is worth remembering. The comparison with water is a useful reminder of how little mass a large volume of air actually contains.

Answer: A

The gas law gives density as pressure divided by the product of the gas constant and temperature, so at fixed pressure the density is inversely proportional to the absolute temperature. This is the basis of buoyancy and hence of all convection. A rise of 10 kelvin from 290 reduces the density by about 3 per cent, which is enough to drive vigorous vertical motion. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Water molecules have a molecular mass of 18 against the mean of 29 for dry air, so replacing some dry air molecules with water molecules at fixed pressure and temperature lowers the density. This is why virtual temperature exceeds actual temperature. The effect is significant for convective buoyancy in the tropics and is one of the more counterintuitive results in the subject. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

With density fixed, the gas law gives pressure proportional to absolute temperature, which is Gay-Lussac's law. Doubling the absolute temperature therefore doubles the pressure. The atmosphere rarely holds volume constant, so this case arises more often in laboratory and engineering contexts than in meteorology, but the relation is part of the same single equation of state. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

Isobaric means at constant pressure, isochoric at constant volume, isothermal at constant temperature and adiabatic without heat exchange. Cooling to the dew point is an isobaric process, while a rising parcel undergoes an approximately adiabatic one. Keeping the four prefixes straight is essential because each corresponds to a different form of the first law. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Vertical motion in the atmosphere is rapid enough compared with radiative and conductive heat exchange that it is very nearly adiabatic, which is why the dry adiabatic lapse rate is such a good approximation. The parcel does work against its surroundings as it expands, and that work comes from its internal energy, which is why it cools. Neither temperature, pressure nor volume is held constant. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Potential temperature, the temperature a parcel would have if brought adiabatically to 1000 hectopascals, is conserved for dry adiabatic motion and is therefore the natural label for an air mass. Temperature itself changes as the parcel expands or compresses. Equivalent potential temperature is conserved for moist adiabatic motion too, which makes it the more general of the two. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

The conventional reference is 1000 hectopascals, chosen for arithmetic convenience rather than physical significance. Any fixed reference would serve equally well provided it is used consistently. Using the actual surface pressure would make the quantity non-conserved and would defeat the purpose of the definition entirely.

Answer: D

The Exner function is the pressure raised to the ratio of the gas constant to the specific heat at constant pressure, normalised by the reference pressure, and temperature equals potential temperature times this function. It appears whenever the thermodynamic equation is written in terms of potential temperature. Treating it as a non-dimensional pressure coordinate simplifies many expressions considerably. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.