Sky Watcher Chart

High Clouds

H1 Filaments of Cirrus

Cirrus in the form of filaments, strands or hooks, not progressively invading the sky.


H2 Dense Cirrus

Dense cirrus in patches or entangled sheaves, which usually do not increase or cirrus with sproutings in the form of small turrets or battlements or having the appearance of cumuliform tufts.


H3 Dense Cirrus from Cumulonimbus

Dense Cirrus, often in the form of an anvil, being the remains of the upper parts of Cumulonimbus.


H4 Cirrus progressively invading the sky

Cirrus in the form of filaments, strands or hooks, progressively invading the sky and generally becoming dense.


H5 Cirrostratus increasing but below 45° elevation

Cirrostratus progressively invading the sky and generally becoming denser, but the continuous veil does not reach 45° above the horizon. Note: H5-H8 clouds are the same clouds. The different codes refer to how much of the sky is covered by Cirrostratus and whether the clouds are changing in thickness and/or sky cover.


H6 Cirrostratus increasing and above 45° elevation

Cirrostratus progressively invading the sky and generally becoming denser, the continuous veil extends more than 45° above the horizon without the sky being totally covered.


H7 Cirrostratus covering the whole sky

Veil of Cirrostratus covering the celestial dome.


H8 Cirrostratus not invading or covering the sky

Cirrostratus not progressively invading the sky and not completely covering the celestial dome.


H9 Cirrocumulus

Patches or sheet of very small elements in the form of grains or ripples, never showing shading.

Mid Clouds

M1 Thin Altostratus

Altostratus, the greater part of which is semi-transparent; through which the sun or moon may be weakly visible as through ground glass.


M2 Thick Altostratus or Nimbostratus

Altostratus, the greater part of which is sufficiently dense to hide the sun or moon, or if steady continuous precipitation, Nimbostratus.


M3 Semi-transparent Altocumulus

The greater part of which is semi-transparent; the various elements of the cloud change only slowly and are all at a single level.


M4 Altocumulus continually changing shape

Patches (often in the form of almonds or fish) of altocumulus, the greater part of which is semi-transparent; the clouds occur at one or more levels and the elements are continually changing in appearance.


M5 Altocumulus progressively invading the sky

Semi-transparent altocumulus in bands, or altocumulus, in one or more continuous layer (semi-transparent or opaque), [both of which are] progressively invading the sky; these generally thicken as a whole.


M6 Altocumulus from spreading out of Cumulus

Resulting from the spreading out of cumulus or cumulonimbus.


M7 Altocumulus in Layers or with Altostratus or Nimbostratus

Altocumulus in two or more layers, usually opaque in places, and not progressively invading the sky; or opaque layer of Altocumulus, not progressively invading the sky; or Altocumulus together with Altostratus or Nimbostratus.


M8 Altocumulus with tufts or sproutings

Altocumulus with sproutings in the form of small towers or battlements (Altocumulus Castellanus (Accas)), or Altocumulus having the appearance of cumuliform tufts often accompanied by fall streaks (Altocumulus Floccus).


M9 Altocumulus of a chaotic sky

Altocumulus of a chaotic sky, generally at several levels.

Low Clouds

L1 Cumulus with little vertical extent

Cumulus with little vertical extent and seemingly flattened, or ragged cumulus, other than of bad weather (no precipitation), or both.


L2 Cumulus of moderate/strong development

Cumulus of moderate or strong vertical extent, generally with protuberances in the form of domes or towers, either accompanied or not by other cumulus or stratocumulus, all having bases at the same level.


L3 Cumulonimbus without anvil

Cumulonimbus, the summits of which, at least partially, lack sharp outlines but are neither clearly fibrous (cirriform) nor in the form of an anvil; cumulus, stratocumulus or stratus may also be present.


L4 Stratocumulus from spreading out of Cumulus; Cumulus may also be present

Stratocumulus most often results from the spreading out of Cumulus as in the process of its vertical development, reaches a stable layer in the atmosphere and therefore spreads. The bases of each cloud may be at the same OR at different levels.


L5 Stratocumulus not from spreading out of Cumulus (Cu)

Stratocumulus not resulting from the spreading out of cumulus.


L6 Stratus

Stratus in a more or less continuous layer, or in ragged shreds, or both but no stratus fractus of bad weather (no precipitation).


L7 Stratus fractus and/or Cumulus fractus of bad weather

Stratus fractus of bad weather or cumulus fractus of bad weather (precipitation is occurring), or both, usually below altostratus or nimbostratus.


L8 Cumulus and Stratocumulus at different heights

Cumulus and stratocumulus other than that formed from the spreading out of cumulus; the base of the cumulus is at a different level from that of the stratocumulus.


L9 Cumulonimbus with anvil

Cumulonimbus, the upper part of which is clearly fibrous (cirriform) often in the form of an anvil; either accompanied or not by cumulonimbus without anvil or fibrous upper part, by cumulus, stratocumulus, stratus or 'scud'

Other Phenomena

Mammatus Clouds

While associated with thunderstorms, they are not necessarily an indicator of severe weather. Mammatus results from the sinking of moist air into dry air. It is in essence an upside down cloud. The sharp boundary of Mammatus is much like the sharp boundary of a rising cumulonimbus cloud before an anvil has formed.


Tornadoes

A tornado is a violently rotating (usually counterclockwise in the northern hemisphere) column of air descending from a thunderstorm and in contact with the ground. Although tornadoes are usually brief, lasting only a few minutes, they can sometimes last for more than an hour and travel several miles causing considerable damage


Wall Cloud

A cloud formation associated with thunderstorms. It is a definite lowering of the cloud base typically beneath the rain-free portion of a cumulonimbus cloud, and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than that of the ambient cloud base. Sometimes, the wall cloud will often be seen to be rotating. A rotating wall cloud is the area of the thunderstorm which is most likely to produce tornadoes, and the vast majority of intense tornadoes.


Shelf Cloud

A shelf cloud is a low, horizontal wedge-shaped cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). A rising cloud motion often can be seen in the leading part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.


Wave Clouds

Wave clouds are essentially the same as the M4 clouds. These are altocumulus clouds that, for the most part, remain stationary as the wind blows though the cloud. Wave cloud result from atmospheric standing waves. These waves are created as stable air flows over a mountain range.