Meteorology: The Science of the Atmosphere

by Charles Fitzhugh Talman

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.)

To make matters worse, different parts of the sustaining surface of the machine may receive different impulses. One wing, for example, may graze a violent uprush of air not encountered by the other, giving the aeroplane a tilt to one side, or the tail of the machine may be driven in one direction and the nose in the other. Again, the whole machine may suddenly enter an air current of quite different speed and direction from the one in which it has been flying. To take an extreme case, it may run into a stream of air flowing just as fast, and in the same direction, as the machine itself, with the result that the relative wind becomes zero, and the machine, deprived of all lift, drops like a stone until it acquires a velocity with respect to its new environment.

When such conditions prevail, the pilot is kept busy with his “controls”; now moving his elevator to adjust his fore-and-aft balance, and now his ailerons to set him on an even keel laterally, and occasionally turning his rudder to offset the effects of horizontal gusts. The elevator and the ailerons are worked with a single lever, colloquially called the “joy-stick,” and the rudder with a bar which the pilot operates with his feet. Ordinary adjustments of this kind are performed automatically by the trained aviator, but violent disturbances call for the exercise of skill and judgment. Generally speaking, no amount of atmospheric turbulence causes any serious trouble to the trained pilot, except when he is flying close to the ground, as in starting and landing.

Before we leave the subject of wind it will be well to emphasize once more the fact, which the average layman has difficulty in grasping, that the only movements of the air that affect the safety and comfort of flight are the movements relative to the machine, and not those relative to the ground. To the aviator, when he is once clear of the ground, a steady wind of any speed is merely a mass of calm air. Hence an aviator will sometimes have perfectly smooth flying when the wind, as measured on the earth, is blowing 40 or 50 miles an hour; and again he will describe the air as rough and bumpy when flags are hanging limp from their staffs and dwellers on terra firma declare that not a breath of air is stirring. In the early days of flying aviators themselves were afraid of a strong wind. Thus Wilbur Wright, during his pioneer exhibition flights in France, would never go up unless the smoke from his cigarette rose in a straight line, and until about the end of 1909 no aviator attempted to fly in a wind of 20 miles an hour.

At the present time the only atmospheric condition that seriously hampers flying is fog or low cloud. An aviator flying in a fog or cloud is not only liable to wander far from his course, on account of the unknown leeway of his machine, but he is often in great doubt as to his proximity to the ground. One of the curious effects of such a situation is that the airman loses his sense of the vertical. On land our sense of up and down is determined by the force of gravity, pulling us toward the earth. When riding in a terrestrial vehicle, we are conscious of other pushes and pulls; such, for example, as the jolt that pitches us forward when a train stops suddenly, or the outward thrust that we feel when swinging around a curve. Again, in descending in a lift we seem to lose weight, as if gravity had suddenly grown weaker. On earth all such impressions are corrected by the sight of objects around us; but the aviator enveloped in mist has no such guides, and he often becomes quite confused about the direction of the ground. A turn, which involves banking, increases his confusion. Eventually he may be flying almost upside down without being aware of the fact. Professor Melville Jones, who has been through such experiences, says of the pilot’s confusion:

“His first indication that something is wrong is, as a rule, either an increase or a decrease of speed that is not counteracted by the accustomed movements of the controls. A period of wild suspense and utter bewilderment now follows, during which the pilot makes violent efforts to recover control, but without success. The next thing that he realizes, if he realizes anything at all, is that he is either on his back or spinning, and the next thing he knows is that he is out of the clouds with the earth standing up at a ridiculous angle and spinning round like a drunken dinner plate. Happy is he that has plenty of air room under these circumstances.”

Spirit-levels and similar instruments are affected by the same disturbances that mislead the pilot in his estimation of the vertical; but fortunately there are certain other devices, due to the exigencies of the war, during which cloud flying was a part of the tactics of the military aviator, which have virtually solved this problem, though their use has not yet become general.

The outstanding difficulty of a fog is the problem of landing. In the case of a forced landing, at a distance from a regular landing-ground, the pilot must simply trust to luck. He may descend in the water or the treetops, or on rough ground that will wreck his machine, but he has no choice. The only solution of this difficulty is the installation of a reserve engine, or some other expedient that will obviate the necessity of forced landings. The task of finding a landing ground in a fog and descending to it in safety will, in the near future, be comparatively simple. Most fogs, though by no means all, are so shallow that it is possible to tether a kite-balloon so that it will float above the fog and indicate the position of the aerial harbor. Several such balloons, flying tandem, would afford sufficient lift to support a series of electric lanterns along the cable, for use at night. Searchlights and “star shells” have been employed for the same purpose. Directional wireless and the wireless telephone seem likely, however, to be the chief dependence of the future aeronaut seeking port in a fog. These devices will also be the means of averting collisions in a fog or cloud along crowded airways, and especially in the congestion that will prevail in the vicinity of important air ports. Last but not least, the artificial dispersion of fog by means of electrical discharges, although still in the experimental stage, holds out possibilities of being the ultimate solution of the fog problem, not only for the aeronaut, but also for the mariner, the railway manager, and everybody else who is incommoded by a misty atmosphere.

Even when he is not flying in clouds or fog the aviator by no means always enjoys a clear view of distant objects. A slight haze impairs visibility, while a heavy rainstorm or snowstorm may obstruct the aeronaut’s view as badly as a fog.

Of the meteorological elements that affect aeronautics, other than those we have mentioned, the most important is the density of the atmosphere—generally expressed in terms of barometric pressure. The air diminishes in density upward, and the rarefied atmosphere of high levels has several effects on aircraft. Its decreased buoyancy imposes a limit upon the ascent of balloons; its decreased resistance makes it necessary for an aeroplane to fly at greater speed in order to get the same lift; it diminishes the power of gasoline engines, on account of the reduced supply of air; and it has various unpleasant and even dangerous effects on the aeronaut, similar to “mountain sickness.” The level that a given aeroplane cannot exceed owing to the combined effect of reduced lift and reduced engine power is known as its “ceiling.” Different types of aeroplane have very different ceilings.