Stable and unstable airmasses – what does all this mean?
A. First, to visualise what ‘stable’ and ‘unstable’ states mean in a physical sense, stand a round pencil on end on a level surface. From Newton’s First Law of Motion, it will remain upright until a force is applied. Once displaced, the pencil falls over, failing to pass through its original (upright) position. This is the UNSTABLE state. Now lay the pencil on its side, at the bottom of an incline. Displace the pencil slightly up the incline, then remove the force of displacement. The pencil will return to its original position. This is the STABLE state. In the atmosphere, whether air that is displaced does so in an unstable or stable environment depends upon the vertical temperature profile of the air — its lapse rate — and upon the moisture content of the parcel. These differences are fundamental to understanding why clouds take up the form they do. In the atmosphere, when a ‘parcel’ of air moves vertically upwards (or downwards), it cools (upward motion), or warms (downward motion
