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 FLOW OF AIR OVER TWO RIDGES