If microscopic particles of dust, salt and smoke are absent from the atmosphere, what direct impact will this have on cloud formation?

Correct answer: Water vapour will lack condensation surfaces, making cloud formation much more difficult

Correct answer: B. Dust, sea-salt and smoke particles can act as cloud-condensation nuclei. Water vapour usually condenses more readily on such tiny particles than it would form droplets directly in perfectly clean air. As the vapour collects on the nuclei, small cloud droplets develop and can grow. If these particles are absent, ordinary cloud formation would be greatly hindered and would require stronger supersaturation; the safest conclusion is that it becomes much more difficult, rather than claiming that all clouds can never exist. Option A concerns colour and is unsupported. Option C is the opposite of the expected effect. Option D concerns groundwater and has no direct connection with atmospheric nuclei. Cue: nuclei provide the ‘seeds’ on which cloud droplets grow.