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CONDITIONS FOR FORMATION OF CONVECTIVE CLOUDS
1.1. CRITERIA OF AIR STABILITY
A convective cloud forms in the atmosphere as a result of updrafts. The ascent of bodies of air may be highly localized in a homogeneous air mass (intra air-mass processes), or it may extend over considerable territories, incorporating air masses possessing properties other than those of the original one (frontal processes).
There are two principal methods of treating the ascent of air masses; the first, the "parcel" method, determines whether ascent is at all possible, while the "slice" method considers the actual ascent. The parcel method considers the vertical adiabatic displacement of an air "parcel, " that is to say, an elementary volume of air, unmixed with the surrounding air which is in static equilibrium. If the forces acting on the parcel after displacement tend to return it to its former position, the air is in stable equilibrium. If the forces accelerate the vertical motion of the parcel, the air is in unstable equilibrium; and if the net total force acting on the parcel is zero one has a state of neutral equilibrium. Since friction comes into play only after the onset of motion, it may be neglected to a first approximation in stability calculations. The assumption that the environment is in static equilibrium implies that the vertical components of the force of gravity and the pressure gradient are in equilibrium, according to the equation