Figure 1-14.—Cumulonimbus mamma on mid-etage altostratus cloud layer.

spissatus) will continue to thicken downward enough to be classified altostratus. Occasionally, rounded bulges may develop, protruding from the base of the dense cirrus blow-off. Although found on the dense cirrus or altostratus blow-off from a cumulonimbus cell, these rounded bulges are called cumulonimbus mamma, or simply mamma (fig. 1-14). Mamma is a strong indicator that conditions are favorable for severe weather to occur under and near the cumulonimbus base. Mamma should NOT be interpreted as developing funnel clouds. During the dissipation stage, there are more pronounced downdrafts within the cloud than updrafts. The remaining updrafts may become more concentrated and originate from the rear portion of the cloud cell. Severe weather, such as strong or gusty surface winds, heavy rain, hail, and tornadoes, usually is confined to the dissipating stage. The beginning of the dissipation stage may be marked by the sudden onset of strong, cold downdrafts, known as a downrush, exiting the base of the cloud. These winds are deflected by the ground to create very gusty, sometimes dangerously strong winds blowing outward from under the cloud cell. The leading edge and upward boundary of the bubble of gusty winds form a distinct boundary known as the gust front or the outflow boundary. The outflow boundary—a sharply defined separation between the wind flowing toward the base of the cloud cell and the strong, cold, outward flowing wind-typically is associated with low-level wind shear (LLWS), a dangerous phenomenon that has caused many fatal aircraft crashes. A particularly strong, concentrated downrush from the base of the cloud is known as a microburst. A microburst produces concentrated, extremely strong straight-line winds blowing outward from the base of the CB cell. Typically, these winds produce great damage and are often initially and incorrectly reported by the public as a tornado. A roll cloud (arcus) may form along the leading edge of the cumulonimbus cloud base during a downrush. The ragged bottom edge of the roll cloud is usually much lower than the more uniform base of the Figure 1-14.—Cumulonimbus mamma on mid-etage altostratus cloud layer. 1-14