Downdraughts are sometimes real and sometimes only apparent. In passing from an ascending current into still air the drop may be real, but the downdraught may be merely apparent. In flying from an adverse wind into a wind blowing in the direction in which the plane is moving, the drop is real but the downdraught is apparent merely.
There are actual downdraughts, however, which the airman is certain to encounter—because air going up must be balanced by air coming down. Just as water pours over a perpendicular ledge, forming thereby a cataract, so air is usually pouring over a steep scarp in a similar manner. The air over a plateau is apt to be colder than that several hundred feet below. More certainly it will be colder if it has traversed great fields of snow. When, therefore, it reaches a steep scarp it pours over the edge by virtue of its own gravity. Air-falls of this sort are common in mountainous regions, but they rarely occur in lowlands.
Billow-cloud levels may be a serious problem to the airman, not because they interfere with visibility, but because occasionally they do not do so. When billow clouds are in sight the airman may fly above them or below them. If the two wind layers have about the same degree of humidity there may be no clouds to indicate the position of the plane of contact. Once within this plane, the airman experiences a series of disconcerting bumps, due to the quick transition from one billow to another; sometimes he finds it difficult to rise to the upper layer of air.
Gusty winds, eddies and whirls occur most frequently near ground level; they rarely affect high flights. Even in low flights they are infrequent, unless the plane is within the influence of cumulus clouds. They are disconcerting in making sharp turns and they may be dangerous in making a landing.
