Clouds and precipitation

Correct answer: C. A visible cloud contains suspended droplets or ice crystals, but visibility of the cloud does not prove that precipitation reaches the ground. If droplets begin falling into a dry layer of air below the cloud, evaporation can remove some or all of their liquid water before they arrive at the surface. Radar or visual observation may show falling precipitation aloft, while a ground rain gauge records no measurable rainfall. This phenomenon is often described as virga when the precipitation evaporates before reaching the ground. A is wrong because very large, heavy droplets are more likely to reach the ground. B is wrong because cold, saturated lower air is less favourable for rapid evaporation. D is unrelated; flooding does not explain why falling drops disappear before measurement. Memory cue: visible cloud plus dry air below can mean “rain that vanishes.”

Correct answer: D. A mixed-phase cloud contains ice crystals together with supercooled liquid droplets, which remain liquid below the normal freezing point. In such an environment, water vapour can preferentially deposit onto ice crystals, while supercooled droplets may freeze when they contact ice. The ice particles can therefore grow larger and heavier. As their fall speed and weight increase, they may become precipitation particles that descend from the cloud as snow, ice pellets, or, after melting, rain, depending on the temperature layers below. A is wrong because mixed-phase processes can enhance particle growth rather than make precipitation impossible. B is unrelated, and C is wrong because water vapour is redistributed, not simply made to disappear. Memory cue: in a mixed cloud, ice often acts as a growth centre.

A is correct. Condensation nuclei provide surfaces on which water vapour can condense. When nuclei are numerous and moisture is limited, the available water may be shared among many droplets, so the average droplet can remain small. Such droplets generally need further growth before they can fall as rain.

C is correct. In many warm clouds, droplets collide and coalesce to become larger. If these processes are weak, droplets grow more slowly and fewer may become large enough to fall as rain, so rainfall may be delayed or reduced.

D is correct. Supercooled droplets are liquid droplets below 0°C. When they strike an ice particle, they can freeze onto it. Repeated accretion can build up layers of ice and enlarge the particle into hail, particularly in a cumulonimbus with strong updrafts.

Answer: A. Turbulence creates irregular motions that bring cloud droplets into contact more often. Some collisions make droplets merge, so their size increases and they may move closer to becoming precipitation. However, strong impacts or unstable drops can also cause larger droplets to break into smaller ones. The net effect depends on droplet size, liquid content, temperature and the strength of the turbulent motion; it is not always one-directional. B and C contradict the fact that cloud droplets are water. D is too absolute: turbulence can either support growth or cause breakup, but it does not always stop precipitation. Memory cue: turbulence mixes, collides and can both build and break droplets.

A is correct. When moist air cools to saturation, water vapour condenses into tiny droplets or ice crystals that form clouds. For precipitation to fall, these particles generally need to grow large enough. Whether they reach the ground also depends on conditions in the air below, including its temperature and moisture; consequently, not every cloud produces rain at the surface.