Consider the following statements about a cloud form : 1. These are made up of ice crystals rather than droplets of water. 2. They are thin, stringy, white clouds that trail like feathers across the sky. 3. These clouds are formed at very high altitudes - normally between 6000 metres and 10000 metres. Which one among the following cloud forms is described above?

The correct answer is A, Cirrus. Cirrus clouds are thin, feathery clouds made of ice crystals that form high in the troposphere.

At heights of roughly 6,000 to 10,000 metres the air is far below freezing, so cirrus clouds hold ice crystals instead of water droplets. Strong winds at that height draw the crystals out into wispy streaks, sometimes called mares' tails. Cirrus clouds bring no rain, but a sky filling with them often signals an approaching warm front or a change in weather. Clouds are grouped by form and height into four basic types: cirrus, cumulus, stratus and nimbus, and combinations such as cirrostratus and cumulonimbus.

B is wrong because stratus clouds are low, grey, layered sheets that cover the sky, often bringing drizzle. C is wrong because cumulus clouds are puffy, cotton-like clouds of water droplets with flat bases at lower heights. D is wrong because nimbus clouds are dark, thick rain clouds that form low, near the ground.

Exam tip: cirrus = high, ice, feathery; stratus = low layers; cumulus = cotton heaps; nimbus = dark rain clouds.