6.4: Cloud Classification

Clouds are classified by their shapes (i.e., their morphology). The classification method introduced in 1803 by Luke Howard is still used today, as approved by the World Meteorology Organization (WMO).

WMO publishes an International Cloud Atlas with photos to help you identify clouds. As mentioned, you can easily find similar photos for free by pointing your web search engine at “cloud classification”, “cloud identification”, “cloud types”, or “International Cloud Atlas”.

The categories and subcategories of clouds are broken into:

• genera - main characteristics of clouds
• species - peculiarities in shape & structure
• varieties - arrangement and transparency
• supplementary features and accessory clouds - attached to other (mother) clouds
• mother clouds - clouds with attachments
• meteors - precipitation (ice, water, or mixed) Most important are the genera.

6.4.1. Genera

The ten cloud genera are listed in Table 6-2, along with their official abbreviations and symbols as drawn on weather maps. These genera are mutually exclusive; namely, one cloud cannot have two genera. However, often the sky can have different genera of clouds adjacent to, or stacked above, each other.

Table 6-2. Cloud genera
Genus	Abbreviation	WMO Symbol	USA Symbol
cirrus	Ci
cirrostratus	Cs
cirrocumulus	Cc
altostratus	As
altocumulus	Ac
nimbostratus	Ns
stratus	St
stratocumulus	Sc
cumulus	Cu
cumulonimbus	Cb

6.4.2. Species

Subdividing the genera are cloud species. Table 6-3 lists the official WMO species. Species can account for:

• forms (clouds in banks, veils, sheets, layers, etc.)
• dimensions (horizontal or vertical extent)
• internal structure (ice crystals, water droplets, etc.)
• likely formation process (orographic lift, etc.)

Species are also mutually exclusive. An example of a cloud genus with specie is “Altocumulus castellanus (Ac cas)”, which is a layer of mid-level lumpy clouds that look like castle turrets.

Table 6-3. WMO cloud species. (Ab. = abbreviation)
Specie	Ab	Description
calvus	cal	the top of a deep Cb that is starting to become fuzzy, but no cirrus (ice) anvil yet
capillatus	cap	Cb having well defined streaky cirrus (ice) anvil
castellanus	cas	small turrets looking like a crenellated castle. Turret height > diameter. Can apply to Ci, Cc, Ac, Sc.
congestus	con	very deep Cu filling most of troposphere, but still having crisp cauliflower top (i.e., no ice anvil). Often called Towering Cu (TCU).
fibratus	fib	nearly straight filaments of Ci or Cs.
floccus	flo	small tufts (lumps) of clouds, often with virga (evaporating precip.) falling from each tuft. Applies to Ci, Cc, Ac, Sc.
fractus	fra	shredded, ragged, irregular, torn by winds. Can apply to Cu and St.
humilis	hum	Cu of small vertical extent. Small flat lumps.
lenticularis	len	having lens or almond cross section. Called mountain wave clouds. Applies to Cc, Ac, Sc.
mediocris	med	medium size Cu
nebulosus	neb	Cs or St with veil or layer showing no distinct details
spissatus	spi	thick Ci that looks grey.
stratiformis	str	spreading out into sheets or horizontal layers. Applies to Cc, Ac, Sc.
uncinus	unc	hook or comma shaped Ci
volutus	vol	low horizontal tube-shaped roll cloud caused by a soliton wave or undular bore. For Ac, Sc.

6.4.3. Varieties

Genera and species are further subdivided into varieties (Table 6-4), based on:

• transparency (sun or moon visible through cloud)
• arrangement of visible elements

These varieties are NOT mutually exclusive (except for translucidus and opacus), so you can append as many varieties to a cloud identification that apply. For example, “cumulonimbus capillatus translucidus undulatus” (Cb cap tr un).

Table 6-4. WMO cloud varieties. Ab. = Abbreviation.
Variety	Ab.	Description
duplicatus	du	superimposed cloud patches at slightly different levels. Applies to Ci, Cs, Ac, As, Sc.
intortus	in	Ci with tangled, woven, or irregularly curved filaments
lacunosus	la	honeycomb, chessboard, or regular arrangement of clouds and holes. Applies to Cc, Ac, and Sc.
opacus	op	too thick for the sun or moon to shine through the cloud layer or patch. Applies to Ac, As, Sc, St.
perlucidus	pe	a layer of clouds with small holes between elements. Applies to Ac, Sc.
radiatus	ra	very long parallel bands of clouds that, due to perspective, appear to converge at a point on the horizon. Applies to Ci, Ac, As, Sc, and Cu.
translucidus	tr	layer or patch of clouds through which sun or moon is somewhat visible. Applies to Ac, As, Sc, St.
undulatus	un	cloud layers or patches showing waves or undulations. Applies to Cc, Cs, Ac, As, Sc, St.
vertebratus	ve	Ci streaks arranged like a skeleton with vertebrae and ribs, or fish bones.

6.4.4. Supplementary Features

Table 6-5. WMO cloud supplementary features, and the mother clouds to which they are attached.
Feature	Ab.	Description
arcus	arc	a dense horizontal roll cloud with tattered bottom edges, close to the ground, along and above the leading edge of Cb outflow gust-fronts. Usually attached to the Cb, but can separate from it as the gust front spreads. Often has an arch shape when viewed from above or below.
asperitas	asp	well defined chaotic waves on the underside of Ac or Sc.
cauda	cau	tail cloud at low altitude from a precipitation region toward (& attached to) a Cb wall cloud.
cavum	cav	a large circular or linear hole in a Cc or Ac layer, caused by ice-nuclei seeding of supercooled cloud drops
fluctus	flu	Kelvin-Helmholtz curls or breaking wave clouds with Ci, Ac, Sc, St, Cu.
flumen	flm	inflow cloud bands at height of Cb base. Can look like a beaver tail.
incus	inc	the upper portion of a Cb, spread out into an (ice) anvil with smooth, fibrous or striated appearance.
mamma	mam	hanging protuberances like udders on the underside of Ci, Cc, Ac, As, Sc, and Cb anvil clouds.
murus	mur	wall cloud. An isolated lowering of a Cb cloud base.
praecipitatio	pra	precipitation falling from a cloud and reaching the ground. Mother clouds can be As, Ns, Sc, St, Cu, Cb.
tuba	tub	a cloud column or tornado funnel cloud protruding from a cloud base, and indicating an intense vortex. Usually attached to Cb, but sometimes to Cu.
virga	vir	visible precipitation trails that evaporate before reaching the ground. They can hang from Cc, Ac, As, Cu, Ns, Sc, Cu, or Cb clouds.

6.4.5. Accessory Clouds

Smaller features attached to other clouds.

Table 6-6. WMO accessory clouds, and the mother clouds to which they are attached. (Ab.=abbreviation.)
Name	Ab.	Description
pannus	pan	ragged shreds, sometimes in a continuous layer, below a mother cloud and sometimes attached to it. Mother clouds can be As, Ns, Cu, Cb.
pileus	pil	a smooth thin cap, hood, or scarf above, or attached to, the top of rapidly rising cumulus towers. Very transient, because the mother cloud quickly rises through and engulfs it. Mother clouds are Cu con or Cb.
velum	vel	a thin cloud veil or sheet of large horizontal extent above or attached to Cu or Cb, which often penetrate it.