Cirrus in the form of filaments, strands or hooks, not progressively invading the sky.
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.
Dense Cirrus, often in the form of an anvil, being the remains of the upper parts of Cumulonimbus.
Cirrus in the form of filaments, strands or hooks, progressively invading the sky and generally becoming dense.
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.
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.
Cirrostratus not progressively invading the sky and not completely covering the celestial dome.
Altostratus, the greater part of which is semi-transparent; through which the sun or moon may be weakly visible as through ground glass.
Altostratus, the greater part of which is sufficiently dense to hide the sun or moon, or if steady continuous precipitation, Nimbostratus.
The greater part of which is semi-transparent; the various elements of the cloud change only slowly and are all at a single level.
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.
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.
Resulting from the spreading out of cumulus or cumulonimbus.
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.
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).
Cumulus with little vertical extent and seemingly flattened, or ragged cumulus, other than of bad weather (no precipitation), or both.
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.
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.
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.
Stratocumulus not resulting from the spreading out of cumulus.
Stratus in a more or less continuous layer, or in ragged shreds, or both but no stratus fractus of bad weather (no precipitation).
Stratus fractus of bad weather or cumulus fractus of bad weather (precipitation is occurring), or both, usually below altostratus or nimbostratus.
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.
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.
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
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.
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.