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Aviation History
1953
1953 - 1384.PDF
538 RADAR SOUNDING ' —of the Upper Air: New Developments in the Design of Balloon-borne Reflectors THIS article traces progress in the collection of upper-air meteoro logical data by means of balloons, from early visual-tracking methods to the present-day radar technique. It includes descriptions of new types of radar targets used in such work, and with the development of which the author has been associated. IT is well known that operational requirements of jet air liners call for accurate meteorological forecasts of con ditions at various heights on the routes, and that in order to obtain such forecasts it is necessary to provide a three- dimensional synoptic picture of the atmosphere to com plement the available ground charts. A network of sounding stations has been set up covering all the major airline routes throughout the world, and information obtained by these stations is centralized and used to plot contour charts of definite pressure-values at given periods of the day. From the charts thus produced it is then possible to derive line forecasts giving the weather conditions likely to be experienced by aircraft following any particular route. For plotting these upper-air charts and for forecasting the drift of air masses, we must first measure the pressure, temperature, humidity and wind velocity at particular heights, at known times, and at places spaced as uniformly as possible throughout the area to be covered by the charts. This information could be obtained by meteorological-aircraft flights, but to do so on an extensive scale would be a costly procedure. Instead, therefore, use is made of suitable measuring equipment carried by free balloons which rise to heights of between 60,000 and 100,000ft, and which are carried along by the winds during the ascent. Until the advent of radio and radar techniques, meteorographs were attarhed to such balloons. These instruments automatically recorded the changes in the various meteorological elements as the balloon ascended. The method, however, has one serious dis advantage which makes it impracticable for modern usage: the information is not available until the balloon bursts and the equip ment, which descends on a parachute, has been recovered. The delay in obtaining the information is too great to allow it to be of any value for day-to-day forecasting. In this method, also, the upper winds were measured by visual theodolite-tracking of the balloons, but the need for clear skies free from cloud was a serious limitation. The introduction of radio and radar methods as a means of The ocean weather-ship "Weather Explorer." Aft is a balloon-filling hangar. In this picture the hull is partly hidden by a heavy swell. FLIGHT, 16 October 1953 For radar wind-finding the radar reflector is suspended below a sounding balloon in conjunction with a parachute. Radiosonde apparatus, seen at the bottom, is also included when data on pressure, temperature and humidity are required. The reflector shown is of the "Umbrawin" type. obtaining immediate upper-air information is an important step forward in meteorological instrumentation. To measure the pressure, temperature and humidity at the various heights, a small radio transmitter is now suspended below the sounding balloon. Suitable meteorological elements are incorporated in the trans mitter and their readings are immediately transmitted back to the ground station. The information received at the station is therefore available within a matter of minutes of the balloon reaching any particular height, and a complete record showing the variations in the various meteorological factors from ground level to bursting-height is now available less than an hour after the release of the balloon. An extensive network of upper-air sounding stations has been set up under an international agreement covering all the major airlines of the world. It includes the ocean weather-ships main tained by Norway, Holland, Great Britain and the U.S.A. In addition to their meteorological purpose, these vessels also serve a vital secondary need in providing bases from which life-saving facilities are available for any aircraft in distress on the ocean crossing. An intricate network of teleprinter and radio telephone lines from each sounding station to the central forecasting office allows complete upper-air charts over the whole network of stations to be compiled within a very short time of the sounding information being available. Until the development of radar techniques during the last war, the wind velocity at any particular height was measured by simple radio direction-finding methods, using the signals from the radio sonde equipment which provided the pressure, temperature and humidity information. While this method was a great advance on the previous visual-tracking method, in that it was independent of local weather conditions such as cloud and fog, the information obtained was subject to serious inaccuracies. These inaccuracies were due chiefly to the use of comparatively low-frequency radio transmission, the normal wavelengths employed being between five and ten metres. At these wavelengths, errors in direction- finding are introduced by changes in the local ground conditions adjacent to the sounding station and, although many improve ments were introduced, the final accuracy attainable was still far from that required by the forecasting services in plotting the upper air charts. Using the simple direction-finding technique, it is also impos sible to measure directly the height of the radio transmitter attached to the balloon, though this information can be obtained by calculations based on the pressure-measurement given by the radio-sonde. Although heights calculated from the pressure-
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