State whether the following is true or false.
Condensation in the atmosphere requires a surface on which the vapour can condense.
Answer: A
Condensation without any nucleus needs supersaturations of several hundred per cent, which never occur in the atmosphere. Aerosol particles serving as cloud condensation nuclei are effectively always present in sufficient numbers.
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Consider the following statements about atmospheric supersaturation.
1. Supersaturation in natural clouds rarely exceeds about one per cent.
2. Condensation nuclei allow droplets to form at such low supersaturations.
3. Supersaturations of several hundred per cent are common in clouds.
Answer: B
The first two are correct and follow from the abundance of nuclei. The third describes the requirement for droplet formation in perfectly clean air, a condition the real atmosphere never approaches. Nuclei are abundant enough that saturation is exceeded only very slightly.
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Advection fog forms when
Answer: C
The air is chilled from below until it reaches saturation, and the fog can persist while the flow continues. The last option describes radiation fog instead, which forms by a quite different route. Wind that would disperse radiation fog is what sustains the advection kind.
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All of the following are ways in which air can reach saturation EXCEPT
Answer: D
Descending air warms and moves further from saturation rather than towards it. The other three each either lower the temperature towards the dew point or raise the dew point towards the temperature. Saturation is reached when the two meet, by whichever of the routes gets there.
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Cloud droplets remain suspended rather than falling immediately because
Answer: A
A typical cloud droplet of about twenty micrometres falls at only a centimetre or so per second, which even a weak updraught exceeds. Growth to a much larger size is required before a droplet can leave the cloud as precipitation.
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State whether the following is true or false.
Cloud droplets are lighter than the air around them, which is why clouds float.
Answer: B
Liquid water is very much denser than air and the droplets are falling continuously. They appear suspended because their fall speed is tiny and gentle updraughts are usually sufficient to offset it. A cloud is better pictured as droplets falling slowly through rising air.
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Three of the following are described as adiabatic processes in cloud formation. Which is the odd one out?
Answer: C
Contact cooling transfers heat out of the air and is therefore diabatic by definition. The other three are all statements of the same adiabatic process seen from different directions. Adiabatic means only that no heat crosses the boundary of the parcel.
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