CDS (II) 2024 General Knowledge · Question 74 of 120

Correct — C, Cumulus and Altostratus clouds. That is the option marked for this paper, and both clouds it names do appear on the World Meteorological Organization's own list of clouds from which cumulonimbus may form — that list runs cumulus congestus, altocumulus castellanus, stratocumulus castellanus, altostratus and nimbostratus. Two things should be said plainly. First, the ordinary developmental path is through cumulus congestus, and the WMO says so directly: in the majority of cases the transformation into cumulonimbus passes through the cumulus congestus stage. A towering cumulus whose top glaciates into ice crystals and spreads into an anvil has become a cumulonimbus, and that is the sequence every textbook draws. Option (a) names exactly that cloud. Second, nimbostratus is on the same WMO list as altostratus, so option (b) has the same standing as option (c) and the stem offers nothing that separates them. Take from this item the physics rather than the letter: cumulonimbus is a convective cloud that grows upward from a cumulus tower until it reaches the tropopause, and it is the only cloud that produces hail, thunder and lightning.

• (b)Cumulus and Nimbostratus clouds — Nimbostratus is a thick, layered rain cloud rather than the convective tower a cumulonimbus grows from; it appears on the WMO list only as an occasional parent, alongside altostratus.
• (d)Cirrocumulus clouds — Cirrocumulus is a high, thin, ice-crystal cloud of the cirrus family, made of small white patches. It has neither the depth nor the water content to build a thunder cloud.

Clouds are classified by height and by form. Cumulus is the heaped, convective family, formed where a parcel of warm air rises, cools to its dew point and condenses. As the surface heats through the day a fair-weather cumulus can grow into cumulus congestus, a tall cauliflower tower; if the ascent continues to the tropopause the top freezes into ice crystals and is sheared flat into an anvil, and the cloud is then a cumulonimbus. That is the only cloud that gives hail, thunder and lightning, and it is the engine of the thunderstorm. Altostratus and nimbostratus, by contrast, are layered clouds of gentler, more widespread ascent, and produce steady rather than violent precipitation.

The item is a recall question with a wording problem, and it is worth being clear about that rather than papering over it. Since the answer here is unsettled, the safe preparation is to hold the physical sequence — surface heating, a rising parcel, condensation, a cumulus tower, glaciation of the top, an anvil — and the diagnostic features of the finished cloud. Those are what CDS itself tested in 2023, when it described a cloud born through convection, alone in producing hail, thunder and lightning, and shaped like a large cauliflower tower with an anvil top, and asked candidates to name it.

• The WMO lists cumulus congestus, altocumulus castellanus, stratocumulus castellanus, altostratus and nimbostratus as clouds from which cumulonimbus may form.
• The WMO adds that in most of those cases the transformation passes through the cumulus congestus stage.
• Cumulonimbus is the only cloud type that produces hail, thunder and lightning.
• Its top glaciates into ice crystals and spreads laterally into the anvil shape when the rising air meets the tropopause.
• Cirrocumulus is a high cloud of the cirrus family made of ice crystals, and altostratus and nimbostratus are layered clouds of gentle widespread ascent.

• Treating nimbostratus and cumulonimbus as the same kind of rain cloud; one gives steady drizzle, the other thunderstorms.
• Assuming any tall cloud is a cumulonimbus. The anvil and the glaciated top are what mark the transition.
• Reading cirrocumulus as a low, heaped cloud because of the word cumulus in the name.

Usually by giving the diagnostic features of a cloud — convection, hail and thunder, cauliflower towers, anvil tops — and asking you to name it.

• practice — not a real PYQ

  Which one of the following clouds is associated with hail, thunder and lightning?

  • (a)Cirrostratus
  • (b)Altocumulus
  • (c)Cumulonimbus
  • (d)Stratus

  Answer(c) Cumulonimbus — the deep convective cloud of the thunderstorm, and the only cloud type that produces all three.

• practice — not a real PYQ

  The flat, spreading top of a mature thunder cloud is called the anvil. What causes it to form?

  • (a)The rising air reaching the tropopause and spreading sideways
  • (b)Rain evaporating below the cloud base
  • (c)The Coriolis force twisting the cloud top
  • (d)Dust carried up from the surface

  Answer(a) The rising air reaching the tropopause and spreading sideways — the cloud top glaciates into ice crystals and is sheared out horizontally.