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Show 3 When conditions are not favorable for wedge accretion, the cornice roof can often build upwards but without horizontal extension. In this case, the accretion face ( Fig. 2B) remains approximately vertical. CIinqinq Mechanism. Photos indicate wedge accretion ( Fig. 2A) to be most commonly brought about by the mechanical clinging of either rimed or unrimed new stellar snow flakes under a wide range of temperature, wind, and humidity. The crystalline appendages can adhere mechanically to one another along the leading edge until sintering takes place and hardens the mass. Seligman ( 1936) believed this was the only mechanism which could account for horizontal growth of cornices. Granular Cornice Growth. Although wedge accretion most commonly forms from stellar snow, macro photographs taken of granular snow during cornice growth suggest that this form of snow can also cause wedge accretion. The latter type of growth has been observed only during conditions of high relative humidity, slightly below freezing temperatures, and moderate surface wind speeds of from 15 to 35 mph ( 7 - 15 m./ sec). Although very difficult to measure quantitatively, rounded saltating grains apparently adhere and sinter nearly instantaneously at the tip of the leading edge. The edge is thus extended particle by particle outward and upward. The mechanism for this very rapid sintering process is unclear at present. Discussion of Adhesion Causes. It is possible that riming and electrostatic effects are important in grain adhesion prior to sintering. However, it should be noted that wedge growth by accretion of granular particles has been observed both on clear days and on days when riming was occuring. Other possible mechanisms to explain such adhesion and rapid sintering are now under study. These include the measurement of possible |