Identify the cloud on the basis of the following characteristics relating to it: I. They look like cotton wool. II. They exist in patches and have flat base. III. They are generally formed at a height of 4 to 7 km. Select the answer using the code given below:

Correct — B, Cumulus. Every one of the three clues is a cumulus signature. 'Cotton wool' is the standard description of the heaped, cauliflower-topped cumulus, and the flat base is its most diagnostic feature — it marks the condensation level, the height at which rising air has cooled enough for its vapour to condense, which is the same for every parcel rising off the same surface and therefore lines all the bases up at one level. 'In patches' captures the way cumulus scatters across a fair-weather sky rather than spreading as a sheet. The height, 4 to 7 km, places it as a middle-level cloud, well below the ice-crystal cirrus and well above the low grey blanket of stratus. Cumulus is the classic convection cloud of a warm afternoon.

• (a)Cirrus — Cirrus forms much higher, at about 8 to 12 km, and is thin, detached and feathery, always white. It is made of ice crystals and has no flat base and no cotton-wool bulk.
• (c)Nimbus — Nimbus is the rain cloud — black or dark grey, forming at middle levels or very near the surface, extremely dense and opaque to sunlight. It is a shapeless mass of thick vapour rather than cotton-wool patches with a flat base.
• (d)Stratus — Stratus is a low, layered, sheet-like cloud that spreads horizontally over a large part of the sky and often forms from loss of heat or from a mixing of air masses. Layers, not patches, and no flat cotton-wool base.

Clouds are classified on two axes at once — form and height. On form there are the heaped cumuliform clouds, produced by rising convection currents, and the layered stratiform clouds, produced when a whole sheet of air is lifted or cooled. On height they are grouped as high (cirrus family, roughly 8 to 12 km), middle (alto and cumulus, roughly 4 to 7 km) and low (stratus and its relatives, below about 2 km). Cirrus, cumulus, stratus and nimbus are the four basic types from which the compound names — cirrostratus, altocumulus, nimbostratus, cumulonimbus — are built.

Take the clues in the order that eliminates fastest. 'Cotton wool' rules out the layered types straight away, since stratus and nimbus spread rather than heap. 'Patches' finishes stratus off, because stratus is defined by its sheet form. The height then separates cumulus from cirrus, the other non-layered candidate: cirrus lives at two to three times this altitude. The flat base is the detail worth carrying away, because it is not decorative — it is the condensation level made visible, and it is why a field of fair-weather cumulus looks as if it has been sliced along a ruler.

• Cumulus clouds look like cotton wool, exist in patches, have a flat base, and generally form between about 4,000 and 7,000 metres.
• Cirrus clouds form at about 8,000 to 12,000 metres, are thin, detached and feathery, and are always white.
• Stratus clouds are layered, spread over a wide stretch of sky, and often result from loss of heat or the mixing of air masses.
• Nimbus clouds are black or dark grey, form at middle levels or very near the surface, are shapeless masses of thick vapour and are opaque to the rays of the Sun.
• The flat base of a cumulus marks the condensation level of the rising air beneath it, which is why the bases across a cumulus field line up at one height.

• Reading 'nimbus' as a height class; it names a rain-bearing cloud, not a level of the atmosphere.
• Mixing up the descriptions of cirrus and cumulus — feathery and white against heaped and cotton-like.
• Assuming that all clouds with a flat base are the same type; the flat base marks a condensation level and can appear on several cumuliform clouds.

As a three-clue identification of a named cloud, as an ascending-order-of-height sequence, or as a match-list pairing each cloud form with its characteristic.

• practice — not a real PYQ

  The flat base commonly seen on cumulus clouds marks which one of the following?

  • (a)The tropopause
  • (b)The condensation level of the rising air
  • (c)The top of the planetary boundary layer
  • (d)The freezing level in the atmosphere

  Answer(b) The condensation level of the rising air — every parcel rising from the same surface condenses at about the same height, so the bases line up.

• practice — not a real PYQ

  Which one among the following clouds is composed almost entirely of ice crystals and forms at a height of about 8,000 to 12,000 metres?

  • (a)Stratus
  • (b)Cumulus
  • (c)Cirrus
  • (d)Nimbostratus

  Answer(c) Cirrus — thin, detached and feathery, always white, and formed high enough that its water is frozen.