State whether the following is true or false.
Ventilation effects are negligible for cloud droplets of a few micrometres.
Answer: C
Such droplets fall so slowly that the surrounding air is essentially still relative to them. The correction becomes important only once a drop is large enough to fall at an appreciable speed. Raindrops of a millimetre gain substantially from the effect.
Please log in or register to report this question.
Supersaturation in a rising parcel is determined by the balance between
Answer: D
Ascent cools the parcel and generates supersaturation, while growing droplets remove vapour and destroy it. The peak occurs shortly above cloud base where the second term first overtakes the first. Everything above that level grows in near balance between the two.
Please log in or register to report this question.
Assertion (A): Supersaturation reaches its maximum just above cloud base rather than at the cloud top.
Reason (R): Newly activated droplets begin consuming vapour faster than ascent can supply it.
Answer: A
Both are true and the reason gives the turning point, and the peak value fixes how many nuclei activate in that cloud. Above it the supersaturation settles to a small quasi-steady value. Droplets continue to grow there, but no new ones are created.
Please log in or register to report this question.
Consider the following statements about updraught speed and condensation.
1. A faster updraught produces a higher peak supersaturation.
2. That activates more nuclei and yields more droplets.
3. Updraught speed has no bearing on the droplet concentration.
Answer: B
The first two are correct and link the dynamics to the microphysics. The third is contradicted by both observation and theory, since the aerosol population alone does not fix the droplet number. Dynamics and aerosol together determine what a cloud contains.
Please log in or register to report this question.
Three of the following appear in the diffusional growth equation for a droplet. Which is the odd one out?
Answer: C
Collision efficiency belongs to the coalescence problem and has no place in the diffusional treatment. The other three each appear directly as terms in the growth expression. Diffusion of vapour and conduction of heat are the two limiting processes.
Please log in or register to report this question.
Roughly how long does condensation alone take to grow a droplet to about twenty micrometres in radius in a typical cloud?
Answer: D
Correct answer: tens of minutes to an hour
Growing to about twenty micrometres by condensation takes something of the order of half an hour, and beyond that radius the process becomes hopelessly slow because of the inverse dependence on radius. Yet warm cumulus clouds are observed to produce rain within twenty minutes or so, which is precisely the gap that collision and coalescence must fill - condensation builds the droplets and collection makes the drops.
Please log in or register to report this question.
Assertion (A): A droplet reaching one micrometre does so very quickly compared with the later stages of its growth.
Reason (R): The growth rate in radius is inversely proportional to the radius already attained.
Answer: A
Both are true and the reason gives the dependence, and the first micrometre is covered in a matter of seconds. Each successive micrometre takes progressively longer than the one before. The last few micrometres account for most of the elapsed time.
Please log in or register to report this question.
The quasi-steady supersaturation well above cloud base is typically
Answer: B
Correct answer: a small fraction of the peak value near cloud base
The peak occurs just above cloud base, before the newly activated droplets have grown enough to consume the vapour being made available by cooling. Once they have, the supersaturation falls to a quasi-steady value at which the supply from ascent and the uptake by the droplets balance - typically well under a tenth of the peak. That is why activation is essentially confined to the base of the cloud.
Please log in or register to report this question.
All of the following slow the condensational growth of a droplet EXCEPT
Answer: C
Greater diffusivity carries vapour to the droplet faster and so accelerates growth. The other three each either reduce the driving gradient or spread the same flux over a larger surface. Growth is always the ratio of supply to the area it must cover.
Please log in or register to report this question.
State whether the following is true or false.
Two droplets of different initial size approach one another in size as they grow by condensation.
Answer: D
The smaller grows faster in radius because of the inverse dependence, so the gap narrows. This is the spectral narrowing that makes subsequent collision growth harder rather than easier. A spread of fall speeds is what coalescence actually requires.
Please log in or register to report this question.
