Gas Laws and Atmospheric Variables Questions and Answers

Answer: B

Clausius and Clapeyron derived the relation from the equality of the Gibbs functions of the two phases, giving a saturation vapour pressure that increases roughly exponentially with temperature. The rate is about 7 per cent per kelvin near terrestrial temperatures. It is the single most important relation in moist thermodynamics and underlies projections of increasing extreme rainfall in a warming climate. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

The saturation vapour pressure rises about 7 per cent for each kelvin near terrestrial temperatures, which corresponds to a doubling for roughly every 10 kelvin. That single number underlies the water vapour feedback, the intensification of extreme rainfall and the shape of the moist adiabat. It varies slowly with temperature but 7 per cent is a good working value across the relevant range. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

Below zero degrees the saturation vapour pressure over ice is lower than over supercooled water, with the difference reaching a maximum near minus 12 degrees. This is the basis of the Bergeron process, in which ice crystals grow at the expense of surrounding droplets. Without that difference, precipitation formation in mid-latitude clouds would be far less efficient. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

In a mixed phase cloud the air is supersaturated with respect to ice while being only just saturated with respect to water, so ice crystals grow rapidly while droplets evaporate. This is the dominant precipitation mechanism in mid-latitude clouds. Collision and coalescence dominates in warm tropical clouds containing no ice, so both mechanisms operate but in different environments. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

The two curves coincide at 0 degrees and converge again at very low temperatures, with the maximum difference near minus 12 degrees. That is why the Bergeron process operates most efficiently in that temperature range, and why the layer between about minus 10 and minus 20 degrees is so important for precipitation formation. At minus 40 all supercooled water has frozen and no mixed phase cloud remains. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

A chilled mirror hygrometer cools a polished surface until condensation is detected optically, and the surface temperature at that moment is the dew point. It is a fundamental measurement requiring no calibration against a humidity standard. Capacitive polymer sensors are the common radiosonde instrument and gravimetric methods are laboratory references. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Capacitive polymer sensors are small, fast responding and cheap enough to be expendable, which is what a radiosonde requires. Their weakness is a slow response and dry bias at very low temperatures, which has required careful correction in climate records. Chilled mirror instruments are used as reference standards on research flights but are far too expensive for routine use. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

Sensor response slows dramatically in the cold upper troposphere, producing a dry bias that differs between manufacturers and has changed as instruments have been redesigned. Correcting for it is essential before using the records for trend analysis. The problem is one of response time rather than outright failure, and readings do continue to be produced. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

At fixed temperature and pressure the number of molecules per unit volume is fixed, so replacing heavier nitrogen and oxygen molecules with lighter water molecules reduces the density. The reason therefore explains the assertion completely. This is why virtual temperature exceeds actual temperature and why humid air is buoyant, a result that runs counter to the everyday intuition that humid air feels heavy.

Answer: C

The reason gives the correct definition, and it is precisely that definition which makes the assertion false: the saturation value depends strongly on temperature, so the same air can have very different relative humidities at different temperatures. Mixing ratio and dew point are conserved measures of water content and are preferred when the actual amount matters.