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| Wall cloud (Murus) | |
|---|---|
 A rain-free base with a wall cloud lowering in the foreground and precipitation in the background. Taken inNeedmore, Texas. | |
| Abbreviation | Cb mur. |
| Symbol |  |
| Genus | Cumulonimbus (heap, rain) |
| Species |
|
| Variety | None |
| Altitude | 500-16,000 m (2,000-52,000 ft) |
| Classification | Family C (Low-level) |
| Appearance | A dark cloud feature that protrudes from the base of a cumulonimbus more popularly known as a wall cloud. |
| Precipitation | Very common nearby, but not under :Rain,Snow,Snow pellets orHail, heavy at times |
A wall cloud (murus[1] orpedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of acumulonimbus cloud and from whichtornadoes sometimes form.[2] It is typically beneath the rain-free base (RFB)[3] portion of athunderstorm, and indicates the area of the strongestupdraft within a storm. Rotating wall clouds are an indication of amesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do rotate; however, some do not.[4][5]
Wall clouds are formed by a process known asentrainment, when aninflow of warm, moist air rises andconverges, overpowering wet,rain-cooled air from the normally downwinddowndraft. As the warm air continues to entrain the cooler air, the air temperature drops, and thedew point increases (thus thedew point depression decreases). As this air continues to rise, it becomes moresaturated with moisture, which results in additionalcloudcondensation, sometimes in the form of a wall cloud. Wall clouds may form as a descending of thecloud base or may form as risingscud comes together and connects to the storm's cloud base.
Wall clouds can be anywhere from a fraction of 1 mi (1.6 km) wide to over 5 mi (8 km) across. Wall clouds form in the inflow region, on the side of the storm coinciding with the direction of the steering winds (deep layer winds through the height of the storm). In the Northern Hemisphere wall clouds typically form at the south or southwest end of a supercell. This is in the rear of the supercell near the main updraft and most supercells move in a direction with northeasterly components, for supercells forming in northwest flow situations and moving southeastward, the wall cloud may be found on the northwest or back side of such storms. Rotating wall clouds are visual evidence of a mesocyclone.
Some wall clouds have a feature similar to an "eye", as in amesoscale convective vortex.
Attached to many wall clouds, especially in moist environments, is acauda[1] (tail cloud), a tail-like band of cloud extending from the wall cloud toward theprecipitation core.[6] It can be thought of as an extension of the wall cloud in that the tail cloud is connected to the wall cloud and condensation forms for a similar reason.[citation needed] Cloud elements may be seen to be moving into the wall cloud, as it is also an inflow feature.[6] Most movement is horizontal, but some rising motion is also often apparent at the junction between the tail cloud and the wall cloud.[6]
Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base; this feature is acollar cloud.[7]
Another accessory cloud is theflumen, commonly known as thebeaver's tail.[1] It is formed by the warm, humidinflow of a strong thunderstorm, and is often mistaken for tornadoes.[1] Although the presence of a flumen is associated with tornado risk,[citation needed] the flumen does not rotate.[1]
Many storms containshelf clouds, which are often mistaken for wall clouds since an approaching shelf cloud appears to form a wall made of clouds and may contain turbulent motions.[5] Wall clouds are inflow clouds and tend to slope inward, or toward the precipitation area of a storm. Shelf clouds, on the other hand, areoutflow clouds that jut outward from the storm, often asgust fronts. Also, shelf clouds tend to move outward away from the precipitation area of a storm.
Shelf clouds most often appear on the leading edge of a thunderstorm as they are formed by condensation from the cool outflow of the storm that lifts warmer air in the ambient environment (at theoutflow boundary). When present in a supercell thunderstorm these shelf clouds on the leading edge of a storm are associated with theforward flank downdraft (FFD). Shelf clouds in supercells also form with therear flank downdraft (RFD), although these tend to be more transitory and smaller than shelf clouds on the forward side of a storm.[8][9] A wall cloud will usually be at the rear of the storm, though small, rotating wall clouds (a feature of amesovortex) can occur within the leading edge (typically of a quasi-linear convective system (QLCS) orsquall line) on rare occasion.[5]
The wall cloud feature was first identified byTed Fujita and as associated with tornadoes in tornadic storms following a detailed site investigation of the1957 Fargo tornado.[10][11] In the special case of asupercell thunderstorm, but also occasionally with intensemulticellular thunderstorms such as the QLCS above, the wall cloud will often be seen to be rotating. Arotating wall cloud is the area of the thunderstorm that is most likely to produce tornadoes, and the vast majority ofintense tornadoes.
Tornadogenesis is most likely when the wall cloud is persistent with rapid ascent and rotation. The wall cloud typically precedes tornadogenesis by ten to twenty minutes but may be as little as one minute or more than an hour. Often, the degree of ascent and rotation increase markedly shortly before tornadogenesis, and sometimes the wall cloud will descend and "bulk" or "tighten". Tornadic wall clouds tend to have strong, persistent, and warm inflow air. This should be sensible at the surface if one is in the inflow region; in the Northern Hemisphere, this is typically to the south and southeast of the wall cloud. Large tornadoes tend to come from larger, lower-wall clouds closer to the back of the rain curtain (providing less visual warning time to those in the path of an organized storm).
Although it is rotating wall clouds that contain moststrong tornadoes, many rotating wall clouds do not produce tornadoes. Absent the co-position of a low-level boundary with an updraft, tornadoes very rarely occur without a sufficientlybuoyant rear flank downdraft (RFD), which usually manifests itself visually as a drying out of clouds, called aclear slot ornotch. The RFD initiates the tornado,occludes around the mesocyclone, and when it wraps completely around, cuts off the inflow causing death of the low-level mesocyclone (or "tornado cyclone") and tornadolysis. Therefore, in most cases, the RFD is responsible for both the birth and the death of a tornado.
Usually, but not always, the dry slot occlusion is visible (assuming one's line of sight is not blocked by precipitation) throughout the tornado life cycle. The wall cloud withers and will often be gone by the time the tornado dissipates. If conditions are favorable, then, often even before the original tornado lifts, another wall cloud and occasionally a new tornado may form downwind of the old wall cloud, typically to the east or the southeast in the Northern Hemisphere (east or northeast in the Southern Hemisphere). This process is known as cyclic tornadogenesis and the resulting series of tornadoes as atornado family.
The rotation of wall clouds is usuallycyclonic;anticyclonic wall clouds may occur with anti-mesocyclones or with mesovortices on the leading edge of a QLCS (Again, this relationship is reversed in the Southern Hemisphere).[12]
The dense cumulonimbus cloud cover of theeyewall of an intensetropical cyclone may also be referred to as a wall cloud or eyewall cloud.[13]
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