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Vll. WALL CLOUDS AND OTHER LOWERINGS

Wall Clouds

The wall cloud is defined as an isolated cloud lowering attached to the rain-free base. The wall cloud is usually to the rear (generally south or southwest) of the visible precipitation area. Sometimes, though, the wall cloud may be to the east or southeast of the precipitation area. This is usually the case with high-precipitation supercells where the precipitation has wrapped around the western edge of the updraft. Wall clouds are usually about two miles in diameter and mark the area of strongest updraft in the storm. See figure 21 for examples of wall clouds.

Figure 21: (a, b & c) The wall cloud is an isolated lowering of the rain-free base. It marks an area of strong updraft. Photos - NWS, David Hoadley, Steve Tegrmeier.

As the storm intensifies, the updraft draws in low-level air from several miles around. Some low-level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses (at a lower altitude than the rain-free base) to form the wall cloud. This process is shown in figure 22.

Figure 22: Wall cloud formation (left). As cool, moist air is pulled from the rain area, the moisture quickly condenses to form the wall cloud (right). Photos - Alan Moller

Shelf Clouds and Roll Clouds

Shelf clouds and roll clouds are examples of "accessory clouds" that you may see beneath the cloud base of a storm. Shelf clouds are long, wedge-shaped clouds associated with the gust front (figure 23). Roll clouds are tube-shaped clouds and are also found near the gust front (figure 24).

Shelf/roll clouds can develop anywhere an area of outflow is present. Shelf clouds typically form near the leading edge of a storm or squall line. A shelf cloud can form under the rain-free base, however, and take on the appearance of a wall cloud. A shelf cloud may also appear to the southwest of a wall cloud in association with a phenomena called the rear flank downdraft (which will be discussed later).

Figure 23: Shelf cloud. The shelf cloud marks an area of thunderstorm outflow. Photo - Alan Moller.

Figure 24: Roll cloud. Similar to the shelf cloud, the roll cloud also marks an area of storm outflow. View is to the southwest. Photo - Gary Woodall.

Shelf Clouds vs. Wall Clouds

Perhaps your biggest challenge as a spotter will be to discern between shelf clouds under the rain-free base and legitimate wall clouds. Remember that shelf clouds signify an area of downdraft and outflow while wall clouds indicate an area of updraft and inflow. If a shelf cloud is observed for several minutes, it will tend to move away from the precipitation area. A wall cloud, though, will tend to maintain its relative position with respect to the precipitation area. Shelf clouds tend to slope downward away from the precipitation while wall clouds tend to slope upward away from the precipitation area. Table 3 summarizes these differences.

WALL CLOUDS VS. SHELF CLOUDS

Wall Clouds:	Shelf Clouds:
Suggest inflow/updraft	Suggest downdraft/outflow
Maintain position with respect to rain	Move away from rain
Slope upward away from precip. area	Slope downward away from precip. area

Table 3: Characteristics of Wall Clouds vs. Shelf Clouds.

Only a few of the lowerings that will be seen when spotting will be legitimate wall clouds, and only a few of these wall clouds will actually produce tornadoes. Once a wall cloud has been positively identified, the next challenge will be to determine its tornado potential. There are four main characteristics usually observed with a tornadic wall cloud. First, the wall cloud will be persistent. It may change its shape, but it will be there for 10-20 minutes before the tornado appears. Second, the wall cloud will exhibit PERSISTENT rotation. Sometimes the rotation will be very visible and violent before the tornado develops. Third, strong surface winds will blow in toward the wall cloud from the east or southeast (inflow). Usually surface winds of 25-35 miles an hour are observed near tornadic wall clouds. Fourth, the wall cloud will exhibit evidence of rapid vertical motion. Small cloud elements in or near the wall cloud will quickly rise up into the rain-free base. Not all tornadic wall clouds will have these characteristics (and some tornadoes do not form from wall clouds), but these four characteristics are good rules of thumb to follow.
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