Cloud Physics Questions and Answers

Answer: A

Cooling reduces the saturation vapour pressure until the air already holds more vapour than it can retain. The excess condenses onto available particles, and it is those droplets rather than the vapour itself that are visible.

Answer: B

Both are true and the reason gives the mechanism, and this adiabatic cooling is what makes ascent the dominant route to cloud formation in the atmosphere. No external refrigeration is involved at any point in the process.

Answer: C

The dry adiabatic lapse rate is close to 9.8 degrees Celsius per kilometre and applies to any unsaturated parcel regardless of how much vapour it carries. It is a consequence of expansion alone rather than of the humidity of the air.

Answer: D

Once condensation begins, the latent heat released partly offsets the cooling from expansion. The saturated rate is therefore variable, being near 4 degrees per kilometre in warm moist air and approaching the dry rate in very cold air.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. The saturated rate is the dry rate offset by the latent heat released as vapour condenses, and warm saturated air holds far more vapour than cold, so it releases far more heat per unit of lifting. The rate is therefore about 4 degrees per kilometre in warm tropical air and approaches the dry 9.8 in very cold air, where there is almost nothing left to condense.

Answer: B

Correct answer: a rising parcel first becomes saturated

A rising parcel cools at the dry adiabatic rate while its dew point falls only slowly, so the two converge; the height at which they meet is the lifting condensation level, and condensation begins there. It is the cloud base for any parcel lifted from the surface, which is what the next item is about.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Over uniform terrain the parcels rising from different points start with much the same temperature and dew point, so they all reach saturation at the same height and every cloud base forms at that level. The flat common base of a field of fair-weather cumulus is therefore a direct reading of the lifting condensation level.

Answer: D

Correct answer: 1250 metres

The dry adiabatic rate is about 10 degrees per kilometre and the dew point falls about 2, so the two close at roughly 8 degrees per kilometre - giving the rule of thumb \[ h \approx 125\,(T - T_d) = 125 \times (25-15) = 1250\ \mathrm{m}. \] The 2500 m offered uses the temperature depression twice over, and 125 m drops a factor of ten.

Answer: A

Correct answer: A-2, B-4, C-1, D-3

Four ways of lifting air, and everything else about the cloud follows from which one is at work. Orographic lifting forces the air over a mountain barrier. Convective lifting comes from surface heating making a parcel buoyant. Frontal lifting has warm air ascending over a denser air mass along a boundary. And convergence piles air up horizontally so that it has nowhere to go but up.

Answer: B

Much of the moisture is lost as precipitation on the windward ascent, and the descending air then warms adiabatically. It arrives on the far side both drier in absolute terms and further from saturation than before it began.