Meteorology: The Science of the Atmosphere

by Charles Fitzhugh Talman

The United States Army and the United States Navy both established meteorological services not long after this country entered the war. The former was attached to the Signal Corps, and was partly officered and recruited from the Weather Bureau. A training school for army meteorologists was opened at College Station, Texas. Upward of 300 men were given instruction in this school, and most of them were sent overseas. The naval meteorological service was headed by the director of Blue Hill Observatory, and the junior officers received special training at that institution.

The varied activities carried on by these war-time units were so different from the traditional duties of meteorologists that they may be said to mark the advent of a new branch of applied science—Military Meteorology. They were, moreover, as we shall see, extremely fruitful of effects upon the science of meteorology in general.

In the principal battle zones the military weather men maintained a dense network of observation stations, the reports from which, combined with those received from the regular peace-time weather stations of the Allied and neutral countries, enabled the forecasters at headquarters to keep closely in touch with atmospheric changes. Observations of both surface and upper-air conditions were made at frequent intervals, and radiotelegraphy was largely used to insure prompt transmission of the reports. In general, weather maps were drawn four times a day. Information was distributed locally to the fighting units by telephone and otherwise.

The vast fleet of aircraft called into being by the war would, of itself, have imposed upon the military meteorologists the necessity of paying a great amount of attention to the upper air. Pilot balloons were sent up so frequently and at so many points that the aviators generally knew just what winds they would encounter aloft. Special arrangements were made to follow the progress across the country of the thundersqualls which constituted a serious danger to the “sausages,” or observation balloons, as well as to aeroplanes on the ground, and to hangars.

There was, however, another urgent reason for keeping a close watch of the winds and other atmospheric conditions at various levels above the earth’s surface. Experience acquired early in the war proved that old-fashioned methods of correcting the aim of artillery for meteorological disturbances were extremely inadequate for modern guns, the projectiles of which rise to altitudes of from 10,000 to 20,000 feet and encounter conditions quite different from those prevailing at the surface. The flight of a projectile is affected by the force and direction of the wind, and the density of the air through which it passes. Some modern projectiles remain in the air as long as 70 seconds, and a moderate wind blowing across the path of such a projectile may easily cause it to fall half a mile away from the point at which it would strike if fired in still air. One of the routine duties of the army weather service was to observe the winds and compute the air-densities at different heights wherever such information was required by the artillery. In order to facilitate the application of such data by the gunners ingenious methods were developed for computing what is known as the “ballistic wind.” This is a fictitious wind which, if affecting the projectile throughout its flight, would produce the same total deflective effect and effect on range as the various winds that the projectile actually encounters.

Meteorological observations were also of great importance in connection with the new process of locating distant guns known as “sound-ranging.” This process consists, briefly, in determining the exact instant of arrival at several points of the sound waves propagated through the air from the gun that is being located. If sound traveled at a uniform speed, these observations would show the exact distance of each of the observing points from the gun, and a simple geometrical construction would indicate the position of the latter. The speed of sound waves in the air is, however, affected by both wind and temperature. Accordingly, allowance had to be made for these varying factors, and the necessary data were supplied by the meteorological units.

The observation and prediction of winds favoring the use of poisonous gases by friend or foe was one of the most delicate tasks allotted to the army meteorologists. The flow of such gases is determined by the winds close to the surface of the earth, and these are greatly affected by topography. Local air currents controlled by the slope of the ground were especially utilized for gas attacks. Strong winds were unfavorable, because they quickly dissipated the gas cloud. The meteorologists not only advised their own troops when to use gas, but also gave warning when the atmospheric conditions were such that gas was likely to be used by the enemy. The use of gas shells was less dependent for its success upon the wind than the liberation of gas clouds, but even when shells were used the wind and weather at the objective point were factors of importance.

The exigencies of warfare developed several new features of meteorological practice, the utility of which did not cease with the war. Thus it became customary to measure the degree of “visibility” of distant objects, for the benefit of aviators and gunners, and this element was included in the routine weather reports. Scales of visibility, ranging from “very bad” to “excellent,” etc., were adopted, and eventually certain forms of apparatus (“visibility meters”) were devised for getting fairly precise measurements of this weather factor.

Another novel practice that deserves to be perpetuated was the plan adopted by the military forecasters of adding to their predictions a statement as to their probable accuracy; this was expressed on a numerical scale of “odds,” instead of by use of the vague terms “probably” and “possibly,” which have generally served the purpose of the dubious forecaster.

The war brought about many improvements in the instruments and methods used in sounding the upper air; and the intensive campaign of pilot-balloon observations carried out at the military stations provided a body of data for study quite unparalleled in the history of meteorology.