The purpose of the “aërological flight” or “weather hop,” the name applied to the daily altitude flight made by naval aërological units, is understood partially throughout aviation activities, but to the general service this work is practically unknown. In order that the service at large may understand the significance of these morning flights, the following discussion will attempt to bring upper-air soundings down to earth.
Meteorologists have long recognized that surface weather is controlled by constant changes that take place in the laboratory of the atmosphere aloft. Consequently they began their first explorations of the upper air by observing cloud movements and formations, using the various cloud types as a basis for anticipating weather changes. Observation stations were established later on a few isolated mountain peaks, but the information thus collected was insufficient for a proper analysis of the changes going on in the upper or free air. Kites were then introduced, and later exploration was conducted by the method of the balloon sonde or registering balloon. It was by the use of these balloon soundings in 1898 that Teisserene de Bort and Lawrence Rotch discovered the recently popularized member of the meteorological family—the stratosphere. It was not until man learned to fly, however, that it was possible for scientists to observe, regularly and more exactly, the important changes that are going on continuously in the upper air.
Atmospheric changes, commonly called weather, are caused by the unequal distribution of the sun’s heat over the surface of the earth. The air masses over the tropical regions are warmer and therefore less dense than the masses either farther north or south, and consequently the colder and denser surrounding air masses in the higher latitudes, acted upon by gravity, seek to displace the warmer, less dense air over the tropical regions. Many factors, principally the rotation of the earth, prevent this regional air mass movement from being a simple flow. The actual transfer and mixing of the warm and cold air is accomplished by the rotation of large air masses, commonly called cyclones (Lows) or anticyclones (Highs), which are associated with fairly definite types of weather.
For many years, weather forecasting has been based upon a system of plotting these high and low pressure areas as they trek irregularly across certain portions of the globe. This map or synoptic method has many limitations since surface reports, particularly overland, are subject to inaccuracies because of the influence of topography and other local conditions. During the World War, however, when Norwegian forecasters were unable to obtain meteorological reports from England and the Continent necessary to prepare weather advices for their extensive fishing fleets, they began to solve their forecasting problems by a more detailed analysis of the air masses of the upper air.
In their study of the life history of cyclones, the Norwegian school observed that the tropical and polar regions are covered by masses of air in which the temperature and humidity remain fairly constant at any given level. Whenever a body of air is stagnated or moves slowly for a sufficient length of time over any large region having uniform surface characteristics and where the sun’s heat is rather evenly distributed throughout the area, it will acquire definite properties of temperature and humidity. No matter how long the air mass is stagnated over such a region, these characteristics do not change appreciably, and thereafter such air may be identified by these characteristics acquired in its source region. For example, air remaining over the Gulf of Mexico for any length of time will soon become a homogeneous, evenly heated mass of high temperature and humidity. Once it moves northward out of its Gulf source region, these properties will be modified by the latitude temperature gradient, but the change will take place essentially in the lower layers of such an air mass, leaving the upper air still sufficiently Unaffected so that it may be identified readily as a Gulf air mass by an aërograph sounding. Similar reasoning holds for air moving down from a polar source region. Under the influence of pressure differences, two such masses of air, even when drawn into close proximity, will endeavor to maintain their separate identities, which results in a line of demarcation between them called a “front.” It is along these fronts, or surfaces of discontinuity, that most of our cyclones develop, and where they provide the medium for mixing air masses of different characteristics this process usually results in a change of weather. The system of identifying air masses and locating and tracking the boundaries or fronts between them is now known as the air mass, or frontal method of weather analysis. The only practical way at present to identify these different air masses is to obtain a cross-sectional record of the temperature and humidity of the mass by use of an aërograph attached to the wing of an airplane.
The upper air data essential to this system of forecasting are obtained by a self-recording instrument called an aërograph or meteorograph which gives a continuous record of temperature, pressure, and humidity aloft during the flight; The aërograph, which is a combined barograph, thermograph, and hygrograph, is secured to the struts of the plane by bungee cord so that the shocks during take-off and landing and the vibration during flight may be minimized. It is suspended in an outer bay of the plane to provide for good ventilation and to be free from any thermal influence by the engine or exhaust gases. The climb to 17,000 feet usually takes about 50 minutes, for if the rate of ascent exceeds 300-350 feet per minute, lag in the instrument, particularly in the hygrograph element, will give inaccurate readings. During flight the aërological observer notes cloud bases and heights, turbulence, visibility and haze both horizontally and vertically, precipitation, ice formations on plane, etc., all of which is of later assistance in the selection of significant points on the traces. In working up the record, the computer determines as significant values those points on the trace where the temperature shows marked changes. This divides the air mass into levels where the temperature changes uniformly throughout any select- strata. Additional significant values, if found, are selected from the humidity trace. After these significant points have decided upon and their values determined from the record sheet, the computer plots these data on an adiabatic chart in order to convert the pressure reading of the aneroid element into height determine graphically the correct altitude for each level. By means of the diabatic chart he also computes the potential temperature, or that temperature mass of air at each significant point would have if it were brought adiabatically to standard atmospheric pressure. The vapor pressure and dew point of each significant point are also computed.
While the adiabatic chart shows the forecaster a cross-sectional picture of the conditions of stability of the air masses aloft, it does not give an estimate of the amount of energy there is available in the upper air to do work, that is, to produce precipitation, thunderstorms, etc. Since, however, entropy is proportional to the potential temperature obtained from the adiabatic chart, it is possible to obtain an “indicator card” of the atmosphere by use of the tephigram, which is an energy diagram on which ordinates are potential temperature and abscissae are absolute temperature.
The available energy found on a tephigram may indicate the approach or passage of a “front,” or it may give evidence of convention and thunderstorms—distances within a homogeneous air mass, that is, one in which the temperature and moisture content at any given level remain in fairly constant. The tephigram is lotted daily at the various aërological tons in order to analyze local weather conditions. It gives the forecaster an indication of the thermodynamical structure the upper air masses in the vicinity of the place of observation, thus enabling him to make short-range, local predictions without having to depend solely upon an elaborate network of synoptic reporting stations. The tephigram is of greatest value to the isolated observer; and to an aërological officer, on an aircraft carrier engaged in a mission which required a minimum of communication or radio silence, it would be an invaluable source of meteorological information in preparing aircraft weather advices.
As soon as the data are worked up, the significant temperature, pressure, and humidity points are encoded and transmitted daily by teletype and radio to the forecast centers of the U. S. Weather Bureau since all government activities pool weather information whenever possible.
The Navy, having recognized that this air mass system was particularly applicable to aviation weather problems, was one of the pioneers in the work of obtaining upper-air soundings by aërograph and airplane. The records of the Naval Air Station, San Diego, show that the first aërological flight was made as early as February IS, 1922. In 1933, the Science Advisory Board, appointed by the President, recognized the importance of this method of analysis of upper-air conditions for more accurate forecasting by recommending the adoption of the system of air-mass analysis by the U. S. Weather Bureau. It further recommended the establishment of a network of stations throughout the United States where, in cooperation with the Army and Navy, daily altitude flights could be made, in order to reconstruct as nearly as possible vertical cross sections of upper air over the country from the Atlantic to the Pacific. Regular flights are now being made by the Navy at Lakehurst, N. J., Anacostia, D.C., Norfolk, Va., Pensacola, Fla., Seattle, Wash., Sunnyvale and San Diego, Calif., and Pearl Harbor, T. H., and also from units of the fleet. To complete the network throughout the country, seven Weather Bureau stations, six Army posts, one National Guard and the Massachusetts Institute of Technology also take these daily upper-air soundings.
The aërograph, of course, has certain limitations in weather forecasting. Often it is impossible to make an aërograph flight due to unfavorable weather conditions. Again, changes may be taking place so rapidly aloft that data obtained only a few hours before are almost valueless to the forecaster by the time they are worked up. The data obtained from the record sheet are not always accurate, for the hygroscopic element, which consists of a spread human hair assembly, has considerable lag, and must be frequently calibrated and kept free from dirt. A small error in relative humidity will show up as a proportionally greater error in the amount of available energy pictured on the tephigram.
Some over-zealous advocates of the air mass system would lead the casual reader to believe that once an air mass has been identified and the fronts located, the problem of weather forecasting then becomes very simple. Unfortunately, these air masses do not maintain their source characteristics throughout their life cycle. It is impossible to label a mass of air and then trace it across the surface of the earth indefinitely, for purely polar or tropical air in its migration soon comes under other influences, such as topography, latitude temperature gradients, etc., and thus they become transitional masses, with “fronts” which are sometimes difficult to identify. It must be borne in mind that the isobars are still the most important lines on the weather map.
However, most meteorologists will agree that the air mass and frontal system of analysis is perhaps the greatest advance that has been made in the forecasting of meteorological elements since the adoption of the synoptic weather chart, and thus by making these daily upper-air soundings by aërograph, the service is not only seeking to solve its own particular weather problems, but it is also making another peace-time contribution to scientific research.