1 The condition of a slope just before the initiation of mass movement. See (2008) Geomorph. 94, 3–4, special issue.

2 The condition of a parcel of air which has positive buoyancy, and thus a tendency to rise through the atmosphere. It is the temperature of the parcel relative to the ambient air which is critical, since this affects densities. A parcel remains unstable if it cools more slowly than the surrounding, stationary air.

Absolute instability occurs when an air parcel, displaced vertically, is hastened in the direction of the displacement; such a move will end when the temperature of the parcel is at one with its surroundings. The potential instability of a parcel of air can be calculated using a tephigram. Conditional instability occurs when a parcel of air would become unstable if lifted by some other force, such as a mountain, the moistening of lower-level air, or when a lower, stable parcel of air is overlain by an unstable layer. Convective instability is a tendency of an air parcel towards instability when it has been lifted bodily until completely saturated, which often occurs via the inevitable stirrings in the atmosphere. The atmosphere is said to be unstable when the fall of temperature with height of the environmental air is more rapid than that experienced by a rising air parcel, so that the parcel continues to be less dense than the ambient air. See E. Fedorovich et al. (2004).