Begin by deciding if the cloud is a layer-type (Group 1) or a Heap-type (Group 2).

Ignore all entries under the heading you have NOT chosen.

Under the heading you HAVE chosen, find the description at the next level of indentation that best fits the cloud. Finally move to the third level of indentation, if present.

For example, if you see a layer of cloud (Group 1), very high and white (1.1), with a halo around the moon (1.1.2), you should be led by the key to the name Cirrostratus.

If you see a heap cloud (Group 2) with its base at low levels (2.3), bubbling up strongly with a sharp-edged white top, and also producing a shower of rain (2.3.3), then this is a Cumulus Congestus.

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1. - Layer or Sheet of Cloud - featureless base, edges often indistinct, no puffiness.

1.1 - Clean white, high level (same levels as jet contrails), partly transparent, no precipitation reaches ground.

1.1.1 - Sky partly covered with delicate streaks or fibrous patches, "mares tails" or "Christmas angel hair".
CIRRUS (Ci)

1.1.2 - Continuous thin veil, usually produces "ring" around the sun or moon.
CIRROSTRATUS (Cs)

1.2 - Dirty white or gray, middle troposphere (below contrails but above light aircraft), sun visible only as hazy patch ("ground glass sun"), usually no precipitation.
ALTOSTRATUS (As)

1.3 - Uniform dull gray layer with base at low level (same levels as light aircraft).

1.3.1 - Overcast, thick, dark gray, usually covering whole sky and completely hiding sun, continuous or intermittent rain or snow.
NIMBOSTRATUS (Ns)

1.3.2 - Very low, light gray, often thin and "burns off" during the day, only light drizzle or no precipitation.
STRATUS (St)

2. - Heaps, Rolls, Puffs, or Waves of Cloud - edges of rolls and puffs often sharply defined.

2.1 - Clean white, high level (same levels as aircraft contrails), patches of small distinct puffs or ripples.
CIRROCUMULUS (Cc)

2.2 - Dirty white or gray, middle troposphere (below contrails but above light aircraft), patches of distinct rolls or puffs or waves.
ALTOCUMULUS (Ac)

2.3 - Flat gray, bases at lower levels (same levels as light aircraft).

2.3.1 - Large low rolls with puffy edges but very little vertical extent. Sometimes organized into "streets" with clear avenues between, no precipitation in summer but light snow flurries in winter.
STRATOCUMULUS (Sc)

2.3.2 - Puffy "cauliflower" clouds of fair weather, no precipitation, usually covering less than half the sky and having only a little vertical extent, bright white tops with sharp edges.
CUMULUS (Cu)

2.3.3 - Rapidly growing heap clouds with considerable vertical extent and sharply rounded white tops, likely to produce vigorous brief showers or snow flurries.
CUMULUS CONGESTUS (Cu+)

2.3.4 - Very large convective cloud with great vertical extent, rising to aircraft contrail levels with fibrous anvil top. Produces heavy showers, thunder, lightning and hail.
CUMULONIMBUS (Cb)

 

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