During the great war the British Government decided, in its wisdom, to establish a flying field in Scotland, at which aviators were to be trained in dropping bombs. The commission having this matter in hand chose a site on the shores of Loch Doon. In laying out the field a bog had to be drained; then a railway was constructed, hangars were erected, and other operations were carried out, entailing altogether an expenditure of half a million pounds. At a certain late stage in the proceedings the disconcerting discovery was made that the field could never be successfully used for the purpose intended, on account of the gusts and eddies produced by the surrounding hills. The undertaking was therefore abandoned. The authorities had presumably enlisted the skill of engineers from the outset of the work—but they had failed to consult a meteorologist!

A few such object lessons seem to be necessary to demonstrate the fact—which ought to be obvious—that meteorology is an indispensable and vital adjunct of aeronautics. This fact is now pretty well understood. Nearly all the activities of mankind are more or less influenced by weather, but few, if any, to such an extent as aeronautical enterprises. Hence a definite branch of applied science—Aeronautical Meteorology—is rapidly taking shape. Already it enters into the curriculum of aeronauts; it has profoundly modified the methods of the ordinary meteorological services of the world; and it is raising a crop of specialists, some of whom are now employed by the business firms that manufacture or operate aircraft.

The statement has constantly been made since the war that aeronauts are becoming “independent” of the weather. This statement has a grain of truth in it, but no more. It is a fact that, under war conditions, aviators flew in every sort of weather, and often with impunity. Even since the war commercial aircraft have negotiated adverse atmospheric conditions with remarkable success. A spectacular feat of this sort was achieved on August 28, 1919, when a passenger-carrying aeroplane on the Paris-London route, piloted by Lieutenant Shaw, flew over this route through a hurricane blowing in gusts of from 40 to over 100 miles an hour, accompanied by a torrential rainstorm and such poor visibility that the pilot was frequently obliged to fly very low in order to pick up his landmarks and make sure that he was on his course. The flight was accomplished in 1 hour and 50 minutes—about half an hour less than schedule time. It is said that “the two passengers in the cabin of the machine emerged without any appearance at all of strain”—such as they certainly would have experienced if they had made the crossing by the Channel steamer on that boisterous day. In fact the land and sea route was seriously disorganized by the storm, and the Continental trains were arriving in London hours late.

Lest hasty conclusions should be drawn from this episode it should be stated at once that the company operating the air route in question, far from considering itself independent of weather, is not content with the detailed bulletins furnished to aeronauts by the British Meteorological Office (which specializes in aeronautical meteorology more extensively than any other official weather service in the world), but maintains an elaborate weather service of its own, with an able meteorologist at the head of it.

An accurate statement of the situation would be that wind and weather are no longer the grave dangers that they once were to the aeronaut; but they are still, and will probably always be, factors of the utmost importance in the successful and profitable operation of aircraft. In order to make this matter plain it will be necessary for us, first of all, to devote a few words to some of the fundamental principles involved in aerial navigation.

The layman who sees nothing mysterious in the ascent of a balloon is, in general, somewhat puzzled by the phenomenon of a heavier-than-air machine rising from the ground. Yet, in both cases, the ascent of the vehicle depends upon the fact that air is not just empty space, but a material substance, possessing density, weight, and other properties many of which pertain also to solids. A balloon rises not because it is light, but because the air about it is heavy. In other words, gravity pushes the air under the balloon more forcibly than it pulls the balloon downward. The ability of an aeroplane to leave the ground depends upon the fact that air offers resistance to bodies moving through it.

Suppose a vertical plane (A)—such, for example, as the wind shield of an automobile—is moving horizontally through still air. The resistance of the air impedes its motion, and a part of the motive power is employed in overcoming this resistance. Now, suppose the plane (B) is nearly, but not quite, horizontal, and is propelled by a force tending to make it move in the direction indicated by the arrow. This is approximately the case of an aeroplane driven by a motor; the plane representing the wings of the machine. Only a part of the air’s resistance is now effective in impeding the forward motion of the plane. The rest of it pushes the plane upward. If you hold your hand at such an angle and move it through water you will feel an analogous upward push. Moreover, you will notice that the faster you move your hand the greater is the push. Not only does this upward pressure of a fluid upon an inclined plane moving through it vary with the speed of the latter (to be exact, as the square of the speed), but it also varies with the angle which the plane presents to the fluid in its path. If the wing of an aeroplane, for example, cuts the air nearly edgewise, the upward pressure will be slight. As it departs from an edgewise position, (with the front edge higher than the rear), the upward pressure increases, but not indefinitely; beyond a certain rather small angle it begins to diminish.

In an aeroplane the upward pressure, or “lift,” is increased by giving the wings a slightly arched shape, or “camber.” The air flows over the arched wings in such a way as to produce a suction above them which helps the push from below. The actual amount of lift for a given speed has been determined by experiments for wings of various shapes and sizes and set at various angles to the line of motion. If, when the machine is in the air, the lift is just sufficient to counterbalance the weight of the aeroplane, the latter flies horizontally. An increase in lift causes the machine to rise; a decrease in lift permits gravity to pull it down.

Now suppose the aviator is flying horizontally and wishes to climb. At the rear of the machine and forming part of its tail is a hinged horizontal flap called the “elevator,” under the control of the pilot. By giving this flap an upward tilt he causes the air to exert a downward pressure on the tail of the machine, and hence the nose of the machine is carried upward. While the inertia of the aeroplane tends to carry it along the original path, its wings now present a greater angle to the air, the lift is increased, and the machine rises. The reverse of this operation will cause the machine to descend.

A vertically hinged flap in the tail, acting on exactly the same principle as the rudder of a ship, enables the pilot to turn horizontally. Two or more small horizontal flaps, known as “ailerons,” attached to the wings, are used to preserve the lateral balance of the machine, and to give it the proper “bank,” or inclination, when making a turn.

With these few details in mind, we shall be prepared to consider, in a general way, how the behavior of an aeroplane is affected by the wind and other atmospheric phenomena.

With respect to wind there is an important difference between aircraft and marine craft. Mere strength of wind is not dangerous to an aeroplane, except when starting or landing. An aviator flying above the clouds, with no landmarks in sight by which to gauge his movements, is no more conscious of the actual wind at that level, provided it is steady, than he is of the rotation of the earth on its axis. He feels the wind produced by the motion of his machine through the air—the so-called “relative wind”—but no other. The true wind may be a mere zephyr, or a hurricane blowing 150 miles an hour; the effect is the same on his machine, so far as he is able to observe. On the other hand, a strong wind has a very different effect from a light one upon the course of the aeroplane’s flight with respect to the ground beneath. If a pilot, with no landmarks to guide him, steers by compass for a certain point, and if there is a strong cross-wind of which he is unaware, he will be carried far out of his course; a wind dead ahead or astern will merely affect the speed of his flight, so that he will arrive later or sooner at his destination than he expected.

One of the important problems of aeronautics, especially from the commercial point of view, is to prevent aircraft from being driven off their course by the wind when flying with no visible landmarks; i. e., over clouds, fog, the ocean, or an unmapped country. When this problem is solved, pilots will fly above the clouds much more commonly than they do now. The winds at high levels are generally both steadier and stronger than at low. The stronger wind is an advantage or a disadvantage, according to whether it is blowing in the direction of flight or the reverse; but as the winds at different levels generally blow in different directions, a pilot who is independent of landmarks can choose whatever level affords the winds most favorable for his intended journey.

Over established air routes quite elaborate measures are now adopted to keep pilots informed of the direction and speed of the wind at different levels, so that they can make due allowance for this factor in shaping their course. In clear weather this information is easily obtained by sighting the drift of a pilot balloon with a theodolite, or by observing in a specially designed graduated mirror or pair of mirrors the drift of the smoke cloud from a shell fired by an anti-aircraft gun and timed to burst at any desired altitude. In cloudy weather the smoke trails can often be successfully observed through small breaks in the clouds. When the sky is completely overcast, a succession of shells is fired at definite short intervals of time and the distances apart of the puffs of smoke and the direction of the line in which they lie are determined from an aeroplane flying above the spot. Another method, which was devised by the French military meteorological service during the war, is to send up small balloons loaded with bombs which burst after a certain time, the position of each burst being determined by sound-ranging from the ground.

These methods of providing information concerning the winds at flying levels have, however, their serious limitations, and aeronauts now look hopefully to the perfection of the existing systems of “directional wireless,” whereby the pilot will receive whenever desired, or at regular intervals, a wireless signal from the terminus of his route or some other known point, the direction from which the signal comes being indicated by suitable apparatus on the aeroplane. Thus aided, he should never deviate far from his course, unless he chooses to.

For long journeys, such as the crossing of the Atlantic, the air pilot will naturally make use of all available information concerning the great permanent or semipermanent wind systems of the earth, such as the trade winds of the lower atmosphere, the antitrades above them, and the fairly constant eastward drift of the atmosphere at high levels in middle latitudes. The dividend-earning capacity of commercial aircraft necessarily depends upon taking advantage of favorable winds, while adverse winds may mean not only a loss of money but the danger of prolonging a journey until the fuel supply is exhausted—a serious predicament over the ocean and also over lands remote from civilization. It is, however, a common error on the part of current writers to overrate the constancy and reliability of the winds in various parts of the world, and to lay too much stress on the value of permanent wind charts. What the aeronaut needs especially to know is the typical behavior of the winds with respect to the distribution of barometric pressure, as shown by a weather map, including their variations with altitude. The time will come when the information necessary for plotting the winds at various levels will be flashed at frequent intervals by high-powered radio stations to aerial navigators in all parts of the world—a system that is already in its initial stages, especially in Europe. A pilot making a long journey will thus be able to lay his course so as to utilize the winds that will speed him on his way. Even violent storms, such as the mariner seeks to avoid, will be turned to advantage by the airman.

We have now to consider another aspect of wind that is of much more interest to the airman than to the seaman, and that is the question of “wind structure.” The layman usually thinks of a wind as a nearly steady horizontal flow of air. Such winds exist, but they are exceptional, especially in the lower levels of the atmosphere. A wind is generally full of gusts and eddies, upcurrents and downcurrents, and it is these eccentricities that gradually develop in the aviator a sort of sixth sense—a “feel” for atmospheric fluctuations, that enables him to adjust his machine instinctively to the forces tending to disturb its equilibrium. He also learns by experience the conditions under which irregularities of a pronounced character may be expected. He becomes well acquainted with the great mound of air that drives his machine upward when passing over a hill or mountain; with the eddy that lurks in the lee of such an obstacle; with the downward tendency of the air over lakes, rivers, swamps and forests.

“The air is so sensitive,” writes the late well-known British flyer, Gustav Hamel, “that it is affected even by the color of large patches of vegetation. Whether this be entirely due to the different heat-radiating power of different colors it is impossible to say, but invariably an aeroplane on passing from grass land to a field covered with yellow flowers experiences a certain amount of air disturbances only less noticeable than the inevitable bump experienced in passing from green fields to ploughed land, or from ploughed land to meadow.”

When the wind is blowing, the air for at least a few hundred feet above the ground is nearly always in a state of turmoil. This is partly due to the friction of the moving fluid against the irregular surface of the earth, and partly to the ascending and descending currents caused by differences in temperature. The latter effect is illustrated in the rapid rise of air over a bare sunlit plain and its fall over an adjacent forest or body of water. Ascending currents are often made visible by the formation of detached cumulus clouds, each of which marks the summit of a rising column of moist air, while in the spaces between the clouds the air is generally sinking. Measurements with balloons have shown that vertical currents often attain speeds of 600 feet a minute or more, while the process of hail formation appears to indicate that in thunderstorm clouds there are violent uprushes amounting to 2,000 or 2,500 feet a minute, and possibly much more. The descending air current between clouds is sometimes so strong that an aeroplane cannot force its way up through it.

The turbulence of the lower air—a phenomenon that adds so much to the difficulties of starting and landing—extends to various heights, depending especially upon the strength of the wind. A rough rule, evolved by the Zeppelin pilots before the war, was to expect turbulent conditions up to an altitude equal to from ten to twenty times the force of the wind in meters per second. Thus, for a wind of 10 meters per second, the turbulent layer would be from 100 to 200 meters thick. A good picture of the atmospheric ups and downs encountered by the airman when flying low is furnished by the behavior of the smoke from a factory chimney with a moderate wind blowing, forming smoke waves.

These disturbances give rise to the very marked fluctuations in the force of the wind known as gusts. There are certain forms of anemometer especially designed to record the gustiness of the wind. A record of the wind’s force is traced by a pen on a moving strip of paper, and the “anemogram” thus obtained shows a continuous series of irregularities, the extent of which increases with the strength of the wind. The puffs and lulls often alternate at intervals of a few seconds or less, and the actual force of the wind at a given instant may be many times greater than its average force for, say, five minutes. An ordinary anemometer does not indicate these rapid fluctuations, but merely shows the time required for a mile of wind to flow past the instrument. Thus when such an instrument tells us that the wind is blowing at the rate of 40 miles an hour, it may actually be varying between 20 and 60 miles an hour, or between even wider limits.

Since the matter became of practical importance on account of the needs of aviation, many interesting studies have been made of the effects of different kinds of topography upon the overlying air currents. A striking example of the eccentric winds that sometimes prevail in mountain valleys has been described by Mr. B.M. Varney, of the University of California, in the “Monthly Weather Review.” From the summit of a steep cliff about 1,100 feet above the floor of Yosemite Valley the writer launched broad sheets of tissue paper, and, with the aid of powerful binoculars, followed their flight as they were carried in huge spirals, thousands of feet in diameter, finally disappearing beyond the mountains on the opposite side of the valley. The accompanying sketch shows the path of one of these papers. From its starting point at A until it passed behind the summit of Liberty Cap (B), more than a mile distant, the paper was watched for 7 minutes. The top of Liberty Cap is some 1,600 feet above the point at which the flight began. This sketch visualizes one of the ticklish problems that will some day confront the pilot of a sight-seeing or mail-carrying aeroplane in the Yosemite National Park.

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.)

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.

At great altitudes the air is always very cold, summer and winter. The low temperature may interfere with the efficient working of the engine, and it is, of course, a source of discomfort to the pilot. The formation of ice and heavy deposits of snow lead to inconveniences in both aeroplanes and airships. The pelting of hail is sometimes a painful experience for aeronauts. Lastly, lightning has hitherto left aviators unscathed, but has caused numerous disasters among balloonists.

The recent rapid development of aeronautics has laid a heavy burden of additional labor upon the meteorological services of the world, and is producing something like a revolution in their methods. The history of these changes is interesting. From the beginning of the twentieth century until a few years before the World War meteorologists were engaged in a great campaign of upper-air research, utilizing kites, captive balloons, pilot balloons, and sounding balloons to measure the winds, temperature, humidity and pressure at various levels in the atmosphere. In other words, aeronautical methods were employed in the service of meteorology, but the investigators hardly entertained the idea of reversing the relation and making meteorology the handmaiden of aeronautics. The point of view prevailing in those days is well indicated by the fact that the organization that had charge of the upper-air explorations throughout the world was known as the “International Commission for Scientific Aeronautics,” a name that it bore until the year 1919.

The plan for providing regular weather reports for the benefit of aeronauts began with some small-scale enterprises in Germany about 1909. In the summer of that year Dr. Franz Linke organized a storm-warning service in connection with the International Aeronautical Exposition at Frankfort, and at the beginning of the year 1911 an aeronautical weather bureau for the whole of Germany was established, with headquarters at the Observatory of Lindenberg. Shortly before the war a similar undertaking was launched in Italy, under Dr. Matteucci, whose service was the first one in the world to publish daily charts, based on telegraphic reports, of the winds at various levels over an entire country.

During the war the regular meteorological services of the belligerent countries and the meteorological units attached to the armies and navies maintained an almost continuous service of weather information for the great fleets of fighting aircraft. Bulletins, distributed chiefly by wireless telegraphy, supplied particulars of the current and prospective winds at the flying levels, the prevalence of fog, the degree of visibility, etc. New telegraphic weather codes, far more elaborate than those in use before the war, were devised for transmitting such information, and the whole business of observing and reporting weather became immensely more arduous than it had been in the days when the only interests served by practical meteorology were those of the land and the water.

Since the close of hostilities great efforts have been made to maintain these new operations of the meteorological establishments at something like the level attained during the war. The task is, however, beset with difficulties, on account of the great expense involved. It is being accomplished with different degrees of success in different countries.