April 2008

Fifty years ago

WMO Bulletin Vol. VII, No. 2
April 1958

  cover

The picture on the cover

Although the tasks of meteorologists do not vary quite so rapidly as the weather they have to forecast, there can be no doubt that they are subject to a very marked secular variation. In the early days of organized meteorological services during the latter half of the 19th century, major attention was devoted to meeting the needs of the mariner and the farmer but since then the emphasis has been on the vital forecasts required for the aviator. The latest development in this field has been the development of jet airliners, firstly the ill-starred British Comet I, then the Russian 104 and now the American Boeing 707, Convair 880 and the Douglas DC/8, the French Caravelle and the improved versions of the British Comet. The picture on the cover shows one such aircraft. The large-scale introduction of these aircraft by the main airlines of the world in the near future poses many new problems for the meteorologist and these are now being studied as a matter of urgency by WMO.

In parallel with these developments in aeronautical meteorology, equally important advances are being made in other branches, among which might be mentioned agricultural meteorology and hydrometeorology.

Contents

The contents of the April 1958 Bulletin covered meteorological transmissions in Europe, reducing evaporation from reservoirs, aviation aspects of mountain waves, the 50th anniversary of the Australian Bureau of Meteorology (see MeteoWorld/February 2008/Anniversaries) and Part II of “The conquest of the third dimension”, as well as reports of the second session of Regional Association III, the second session of the Commission for Synoptic Meteorology and activities of regional associations and technical commissions.

The conquest of the third dimension

Presidential address at the second session of the Commission for Aerology,
Paris, June-July 1957

Part II

Part I of this address was published in the last issue of the Bulletin (Vol. VII, No. I (January 1958).

The development of aviation after the first world war placed a new vehicle for the exploration of the atmosphere at the disposal of aerologists. To obtain upper-air information, all that was necessary was to equip an aeroplane with a meteorograph and ask the pilot to make the visual meteorological observations which are a necessary complement of the information obtained from a meteorograph. For this purpose, however, the former Marvin and Bosch meteorographs for sounding balloons had to be somewhat modified for use in the new vehicle. It was necessary both to increase the accuracy of the meteorological instruments and to reduce their time lag. This was achieved in 1923 by J. Jaumotte (1887-1940) with the construction of an aircraft meteorograph capable of showing the frontal surfaces of cyclones thanks to the reliability and the extreme fineness of the trace.

Development of synoptic aerology

… In the early years of the century the frequency of the launchings seemed to be as important an element of success as their simultaneity. But in order to increase the frequency of soundings, it is necessary to have a meteorograph of low price and light weight, which can be handled easily even in rough weather and which is strong and easily constructed and calibrated. To design and construct an instrument which complied with such exacting and apparently contradictory conditions seemed to be almost impossible.  W.H. Dines (1855-1927) succeeded in solving the problem however, in 1907. He had the idea of doing away with the clockwork mechanism and of replacing the recording of changes as a function of time by the recording of temperature and relative humidity of air in terms of atmospheric pressure, which most fortunately varies continuously and regularly with altitude. Dines’s light meteorograph, which weighed hardly more than 50 g, gave a new impulse to meteorology. It became possible to launch balloons hourly, which contributed greatly to the success of the international aerological days.

The Norwegian theory of cyclones, deduced in 1918 simply from synoptic surface observations, had to be checked in the free atmosphere. The synoptic exploration of frontal cyclones required that soundings be carried out at very frequent intervals, the average being one launching per hour for two or three days with a temporary increase in the rate of launchings up to four per hour at certain critical moments. In 1924, Jaumotte followed up Dines’s idea with the construction of a very small meteorograph (hardly more than 9 cm in length) of very light weight (it only weighed 30g complete with casing), which retained all the qualities of sensitiveness and accuracy of his aircraft meteorograph while at the same time having the convenience and robustness essential for an instrument to be calibrated and reproduced on a large scale and to be launched at very frequent intervals. J. Bjerknes realized that Jaumotte’s instrument was suitable for the systematic exploration of disturbances of the polar front and the tropopause. In 1927, Jaumotte and Bjerknes carried out the first experiment in synoptic aerology at the aerological station at Uccle.

Radio-sounding techniques

While aircraft soundings have the double advantage over sounding balloons that they can be supplemented by visual observations and that they give data capable of immediate transmission to the national meteorological services, they are, nevertheless, incapable of giving regular information above 6 or 7 km. Such information is necessary for the diagnosis of atmospheric conditions and this has led to the need for radio-soundings, which are the application of radio-telegraphy to the technique of sounding balloons. On 3 March 1927, P. Idrac (1885-1935) and R. Bureau, at the Teisserenc de Bort Aerological Observatory at Trappes, carried out the successful experiment which marked the introduction of radio-soundings into meteorology and the beginning of a new phase in the technique of exploring the atmosphere. They achieved for the first time, a radio link between the stratosphere and the Earth by attaching a low-powered short-wave wireless transmitter to a free balloon. After the untimely death of Idrac, Bureau continued the study of the complex problem of radio-soundings alone and, in 1927, was the first to construct a radiosonde designed for the purpose of exploring the atmosphere systematically. The radiosonde transmits the meteorological data to the Earth either by mechanical action, as in the system invented by Bureau, or by the electromagnetic action of the sensitive elements on the transmitter. The positive results of Bureau’s first experiment an the great importance of radiosondes as a means of investigating the free atmosphere, led many instrument designers to follow his example; the number of radiosonde designers is still growing at the present day.

Radio-soundings have not only resulted in the inclusion of upper-air data in the daily programme of work of the national meteorological services, but they have also made it possible to extend the network of sounding stations to deserts, the polar ice-caps and the oceans.

The prodigious developments which have recently taken place, especially since the war, in the field of electronics and short-wave wireless telegraphy, have placed at the disposal of aerologists methods as numerous as they are diverse for dealing the problems of observing at a distance involved in the extension and improvement of the network of sounding stations. Thus it has been possible to replace the optical theodolite by the radio-theodolite and radar, which have enabled wind soundings to be made in all cloud conditions. But radar has many other possibilities. By means of the reflection of centimetric and millimetric waves from large drops and ice crystals it is possible to explore hydrometeors and shower clouds. In addition, the optical echo method makes it possible to measure the altitude of both base and summit of superimposed layers of cloud. Finally, the sensitiveness of radio waves of less than 10 m in wave-length to the slightest modifications in the structure of the troposphere make ultra-short waves an excellent medium for exploring the lower layers.

Transoceanic soundings

The immense possibilities of modern technology have made a powerful contribution to the present expansion of the network of sounding stations, particularly in the northern hemisphere where they now number some five hundred. The situation is less satisfactory in the southern hemisphere because of the vast oceanic areas. The southern IGY aerological network has only about one hundred stations in all of which 52 are in the tropical zone, 36 in the temperate zone and 20 in the Antarctic zone. Even if all the islands in the southern oceans had been used, the network would still be inadequate. Fortunately, however, modern technique affords a new method of overcoming this obvious insufficiency of the southern network. …Between 1947 and 1950, at the initiative of F.A. Spilhaus, successful tests were carried out in the USA with experimental launchings of polyethylene balloons of constant volume, of the Skyhook type. These showed that the technique of the floating balloon was both practical and economical.

It is to E.T. Orville, however, that we owe the idea of applying the technique of the horizontal sounding balloon to the horizontal exploration of the atmosphere over the oceans. He propounded his idea of trans-oceanic sounding in his presidential address delivered before the American Meteorological Society in January 1950. He proposed to explore the atmosphere by means of a balloon maintained by a suitable eletromechanical device, at a given pressure level over otherwise inaccessible ocean regions and equipped with radiosondes capable of being launched automatically at regular intervals for the purpose of sounding underlying layers of the atmosphere. A receiver attached to the balloon would collect the data from the radiosondes which would then be automatically retransmitted to coastal receiving stations. These radio-soundings would be located by a radio system. In addition, a network of radio direction-finding stations on the ground would record the precise position of the balloon every two hours or so, so as to make it possible to reconstruct its trajectory for the purpose of obtaining valuable information on the flow pattern at the isobaric level being explored.

The trans-oceanic soundings carried out between 1953 and 1956 by the Naval Research Laboratory, under the sponsorship of the US Bureau of Aeronautics, showed that the floating balloon made it possible not only to explore vast regions where no sounding station can be set up, but also to procure information such as the synoptic acceleration of the air, which the aerological network is unable to supply.

Modern techniques of measurement at a distance, in addition to enabling the fields of pressure temperature, humidity, hydrometeors and air flow to be explored, also supply us with information about the average vertical distribution of the electric potential gradient, the conductivity of the air, atmospheric ozone, the resultant flux of long-wave radiation, etc. Several types of sondes have already been constructed which enable all these quantities to be measured. Research work is now being carried on nearly everywhere and the results of the first tests justify our being optimistic.

Recent experiments with guided missiles

In short, we are now able to measure meteorological quantities in vivo in the free atmosphere. Unfortunately, sounding balloons cannot penetrate the layers where the air is so rarefied that the buoyancy force on the balloon becomes insignificant. Theoretically, a sounding balloon cannot exceed a height of 40 km but in actual practice, the ceiling limit is much lower, somewhere about 30 km. Up to about 10 years ago, only indirect methods were therefore available for determining the structure of the atmosphere above the ceiling of sounding balloons. Among these methods, we would mention those based on the abnormal propagation of sound, the propagation of radio waves, the observing of shooting stars, ozone, aurora, crepuscular rays and the light of the night sky.

Immediately after the close of the last world war, self-propelled missiles enabled the atmosphere to be directly explored up to a height which, nowadays, exceeds 350 km. … By 1 July 1954, 266 rockets, whose maximum altitude varied between 71 and 389 km, had been fired by various American institutions. The very promising results encouraged other countries to construct self-propelling missiles for exploring the highest layers of the Earth’s atmosphere.

For measuring pressure at great heights, the resources of vacuum technique were called upon. Depending on the pressure levels that require to be explored, gauges based on the variability with pressure, either of thermal conductivity or the intensity of an ionization current, must be used. The specific mass of the air can be deduced from a law relating to the ultrasonic displacement of a cone. The temperature of the air is then calculated from the atmospheric pressure, the specific mass of the air and its molar mass, which varies with the height above 100 km.

The wind at levels exceeding 30 km can be deduced from the trajectory of a teleguided missile, reconstructed by means of optical and direction-finding methods, from the position angles of the axis of the missile, ascertained with gyroscopic systems, and from the angle between this axis and the direction of the relative speed of the air, measured by an instrument placed in the head of the rocket. As the missile reaches a height of 60 km in 150 seconds, the wind speeds are practically instantaneously obtained and not mean speeds as in the case of a radio-wind or radar-wind ascent.

A technique for taking air samples has been developed in order to ascertain the chemical composition of the upper atmosphere. Instruments have been constructed which enable the ionic composition of upper layers, the intensity in these layers of cosmic rays, the ultra-violet spectrum of the sun and other quantities to be measured by self-propelled missiles.

But rockets, in their turn, also have their limitations. They only spend an exceedingly short time in the atmosphere and moreover they merely provide data along a single vertical. However it is frequently valuable to possess data over a long time interval and covering great areas of the Earth’s surface. In these cases, the artificial satellite is capable of rendering very great service. Tests are now being carried out. …

The means thus exist for measuring, everywhere in the space surrounding our planet, all the parameters—dynamic, physical and chemical—which enter into atmospheric processes. It will only be by assembling the results of all these measurements into a harmonious whole that we shall finally attain to a truly complete representation of all the phenomena which occur in the atmosphere.

J. Van Mieghem
4 June 1957

Australian Bureau of Meteorology
50-year jubilee

See Anniversaries in this issue and the February 2008 issue of MeteoWorld.

Meteorological transmissions in Europe

A joint ICAO/WMO Meteorological Telecommunications Meeting, Europe, was held from 24 February to 8 March in the Palais des Nations, Geneva, Switzerland, and was attended by representatives of 29 countries and observers from three other international organizations. The purpose of this meeting was to discuss regional telecommunications problems affecting both ICAO and WMO.

It has long been recognized that the present meteorological telecommunications system for aviation requires more uniformity and coordination, in order to achieve a more efficient use of equipment, funds, frequencies and manpower, The matter is becoming more urgent with the introduction of new types of aircraft, as this means that the transmission of meteorological information must be faster and more frequent over far greater distances. For instance, jet airplanes will need more frequent and more detailed information on weather conditions at the terminal airports.

After carrying out a complete review of the various telecommunications systems used for the exchange of meteorological information both for operational requirements and for general forecasting purposes, the meeting adopted a number of recommendations, of which the following are of special interest.

For the transmission of information to aircraft in flight, a new system of radio-telephony ground-air broadcasts on very high frequencies should be set up progressively … As a consequence of this increased use of radio-telephony, rado-telegraphy facilities will gradually disappear.

For the exchange of operational data (actual weather in detail at aerodromes and forecasts of weather conditions at theses aerodromes) between ground stations, it was recommended to set up, in addition to the existing landline teleprinter system, a new landline teleprinter system with 10 circuit areas in the European Region. …

For the exchange of information needed for the establishment of weather charts, it was recommended that the present landline teleprinter system should be improved and that the existing Morse broadcast should be replaced by the more rapid radio-teletypewriter broadcasts.

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