Although, on an average, the air is much steadier at high levels than near the ground, very unsteady currents are sometimes found at all altitudes attainable by aircraft. Thunderclouds, thousands of feet above the earth, are always the seat of violent turmoil, but such clouds can, as a rule, be avoided by the airman. When a stratum of air glides over another differing sharply from it in density--and distinct strata of this sort are not uncommon in the atmosphere--friction between the strata sets up waves like those produced in water by wind blowing over it. If the two streams are moving in the same direction, but at different speeds, the waves are long and regular; when they are more or less crossed, the waves are short and choppy. The moisture at the crests of these waves may be cooled to such an extent as to condense into visible clouds, arranged in long continuous rolls or rows of detached patches; forms frequently assumed by cirro-cumulus and alto-cumulus. More often, however, the waves of air remain invisible, because the conditions of moisture and temperature are not right for the production of cloud. Recalling, now, what has been said above about the way in which the lift of an aeroplane varies with the angle at which the wings meet the air and also with the speed of the machine relative to the air, it will be easy to understand some of the difficulties experienced in maintaining one’s equilibrium when flying in a turbulent atmosphere. Waves, eddies, vertical currents and other features of wind structure cause abrupt changes in the attitude and the speed of the machine with respect to the air stream. The sudden increases and decreases of lift thus produced have much the same effect upon the machine as if it were running over a solid obstacle on the one hand or plunging into a vacuous space in the atmosphere on the other, and hence are aptly described by aviators as “bumps” and “holes in the air,” respectively. The latter term, which seems to have become firmly rooted in all languages (French, _trou d’air_; German, _Luftloch_; etc.), has had the unfortunate effect of keeping alive in the public mind the idea that the aviator occasionally runs into a vacuum or semivacuum, such as could not exist in the atmosphere. (The nearest approach to such a thing is the rarefaction in the core of a tornado or waterspout, due to the enormous centrifugal force of the vortex; something that no aviator has yet encountered.)