June 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 April 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.

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.

Cetyl alcohol reduces evaporation from reservoirs

Loss of water by evaporation from reservoirs has always been a serious handicap to development, particularly in arid and semi-arid areas, where such losses may amount to some 1 135 cubic metres per month per 0.5 ha of water surface. In the design of reservoirs, the necessity to allow for these losses adds greatly to their cost, especially where, due to uncertain rainfall, allowance has to e made for two or more consecutive drought years. Even where reservoirs can be expected to fill with water every year, it is often necessary to limit the amount of water beneficially used to a fraction of the amount lost by evaporation in order to avoid exhaustion of the supply before the end of the dry season. Nevertheless, such exhaustion does frequently occur and water has then to be hauled at great cost by road, rail or sea from ore favoured sources of supply. When this is not possible populations are obliged to migrate, sometimes abandoning unharvested crops and generally losing valuable cattle and other stocks, and sometimes human lives, for lack of water

An assessment of the financial value of water lost by evaporation is not a simple matter. IN some humid areas this value may be comparatively small but in most arid areas water is a priceless asset and its availability is the dominating factor in the lives of the people.

There are therefore good reasons, in fact urgent needs, for investigating any method of reducing evaporation and thereby making more water available to meet ever increasing requirements due to increasing populations and the rising standards of living of peoples, particularly in backward areas.

The fact that a film of cetyl alcohol can reduce evaporation from water surfaces was discovered in a laboratory over 30 years ago but the application of this knowledge to practical purposes only began early in 1953, when the Commonwealth Scientific and Industrial Research Organization of Australia turned its attention to using cetyl alcohol on reservoirs. In the following year, D.A. Davies, who was then director of the East African Meteorological Department, realizing the tremendous benefit to East Africa of reducing the excessive losses of water due to high evaporation rates, obtained some preliminary information of the work being started in Australia and decided that the subject should be studied by his department.

(There followed a description of trials in East Africa and a comparison of water balances.)

Assessment of results

Experiments on very small reservoirs in East Africa have been successful in reducing evaporation by varying amounts, the maximum being approximately 30 per cent.

Experience in the use of cetyl alcohol on reservoirs has proved that a film can be established and maintained on water surfaces and that reduction in evaporation can be achieved. However, before the process can be recommended for general use, further systematic research is necessary in the laboratory and the field to resolve problems connected with the method of applying cetyl alcohol to the water surface, with the effects on the film of wind and waves, the wastage of material due to biological oxidation. It is known that cetyl alcohol is non-toxic to humans and animals, but the effect of the film over long periods of time n fish and other underwater life has not yet been determined; this consideration, however, may not be of great importance where the major problem is to maintain water supplies for domestic use.

The cost of using cetyl alcohol cannot yet be predicted with any degree of accuracy but it is unlikely to be excessive. As in many cases of the application of scientific knowledge to useful purposes, research and improved techniques will undoubtedly lead to decreasing costs. The process has the exceptional advantage that it is most efficient where evaporation rates are high. It is usually in such areas that water has its greatest value.

F. Grundy

Aviation aspects of mountain waves

In the past, numerous aircraft accidents have occurred over mountains, for which there was at the time no satisfactory explanation. To make mountain flying safer, a considerable amount of research has been conducted in recent years to gain better understanding of airflow over mountains. Probably the most important information, from the aeronautical viewpoint, which emerged from this research concerns the standing waves which not uncommonly form to the lee of mountain barriers. Many past air accidents can now be explained in terms of these so-called mountain waves which, because of the vertical currents they set up, the severe turbulence they sometimes generate and their effect on the accuracy of pressure-altimeter readings and air navigation can constitute a real danger to the unwary pilot.

Mountain clouds

A visible evidence of mountain waves is provided by the stationary or quasi-stationary lens-shaped clouds which form downwind from mountain ridges and are arranged in bands parallel to these ridges. These clouds are appropriate to an airstream undergoing vertical oscillations associated with standing waves. As many as ten or more of these cloud bands have been observed at one time, whilst two or three bands are commonly seen.

Orographic lenticular clouds occur at different heights, sometimes simultaneously. They may be Stratocumulus, Altocumulus or Cirrus. The highest and most brilliantly iridescent of orographic clouds are the nacreous (mother-of-pearl) clouds which have been observed in polar latitudes at altitudes between 20 and 30 km. Owing to their great height they remain brightly illuminated long after the sun has set with respect to the terrestrial observer.

Other cloud types associated with mountain waves are the cap clouds and the rotor clouds. The former are low-hanging clouds with their base near or below mountain-top level and their relatively smooth top only a few thousand feet above the ridge. Their major portion is found over the windward slope. To an observer on the downwind slope, the clouds resemble a bank or a wall with fibrous fingers reaching partway down the lee slope. Rotor clouds appear in the form of stationary rolls at varying distances downwind from the ridge. They develop in standing eddies under the crests of the waves.

Flying aspects of mountain waves

Among the most important aspects of mountain waves from the viewpoint of aviation are the vertical currents associated with these waves. Downward currents of 5-10 m/s are not at all uncommon. In one extreme case in the Sierra Nevada (USA), a pilot estimated the vertical currents to be of the order of 40 m/s. Of great significance to aircraft is the fact that, when mountain waves develop, downdraught areas generally occur immediately downwind from the mountain crest, so that an aircraft flying upwind towards the ridge, if caught in one of these downdraughts, might not be able to regain enough altitude in time to clear the mountain.

Another aspect of mountain waves which affects aviation is the turbulence which is often associated with them. By far the most important and most common seat of severe turbulence in mountain waves is the area of the rotor clouds. Violent sharp-edged gusts exceeding 12 m/s have been measured in some Sierra waves and experienced pilots have reported complete loss of control of their aircraft for short periods while flying in the rotor area. Rotor turbulence generated by smaller ridges is generally not quite so severe; nevertheless it often constitutes a source of discomfort and possible hazard to aircraft.

Other aspects of mountain waves which adversely affect the performance and safety of aircraft are the air navigation errors caused by the fluctuations in the horizontal wind speed between crest and trough, the errors in pressure-altimeter readings and the increased probability and intensity of aircraft icing.

Detecting the presence of mountain waves

The easiest waves to detect are those accompanied by the typical wave clouds. Mountain waves, however, may occur with cloudless skies or with completely overcast skies. Under these circumstances other means of detecting the presence of waves must be employed. The possibility of using routine radiosonde ascents for this purpose has recently been demonstrated.

Requirements for mountain wave formation

The possibility of detecting the actual presence of mountain waves does not eliminate the necessity for determining the meteorological conditions which are favourable for wave formation and for advising pilots when these conditions obtain and waves are likely to occur.

Application of hydrodynamic theory

The above requirements are in substantial agreement with theoretical results obtained by applying the method of perturbations to the flow of a stably stratified current crossing a mountain. The most successful results to date have been obtained by R.S. Scorer who considered a frictionless, steady, laminar and isentropic flow, but allowed for the variation of lapse rate and wind with height. By applying the criterion obtained by Scorer to a representative radiosonde ascent made in the unperturbed current, it is possible to determine without much difficulty whether waves are likely to develop in a given air mass.

Theory also gives an insight into the factors affecting the wavelength and the amplitude of the standing waves. A resonance effect is shown to exist such that large amplitude waves occur only if the natural wavelength of the air stream corresponds to the size of the mountain ridge. Since the former increases with the wind speed, larger mountains would require stronger winds for large amplitude waves. This fact is confirmed by observations.

Application to aviation forecasting

Much useful information can be given to a pilot with regard to the effect of mountain waves on his air route by applying the theoretical and observational results briefly discussed above. …

M.A.A.

News and notes

Excellence award to Malayan selected ship

On 27 November 1957 the first Excellence Award to be made by the Singapore Government under the selected ship scheme was presented to Captain H.W. Richardson, master of the m.v. Kimanis.

The Malayan Meteorological Service has already recruited 17 selected ships based in Singapore and it is planned to recruit more in 1958. The m.v. Kimanis has made regular reports since January 1954.

All ships calling at Singapore are visited by a liaison officer from the Meteorological Service. The average number of ships’ reports now being received from such ships amounts to 1 200 per month.

Brussels International Exhibition

The first Universal and International Exhibition held since 1939 opened in Brussels on 17 April 1958.

… the contribution of WMO to the exhibition will be in the pavilion devoted to the Untied Nations and the specialized agencies The part of the pavilion illustrating the Organization’s activities includes photographs showing aspects of synoptic, aeronautical and agricultural meteorology and the International Geophysical Year activities. There is a facsimile link between the Belgian Royal Meteorological Institute and the WMO section and two weather charts are transmitted daily. A working model of a wind-driven generator serves to illustrate the use of wind energy and several different models of radiosonde are exhibited. The name of the Organization appears in relief on a cloud photograph seven metres in length.

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