CHAPTER CONTENTS Page CHAPTER 2
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CHAPTER CONTENTS Page CHAPTER 2. MEASUREMENTS AND OBSERVATIONS AT AERONAUTICAL METEOROLOGICAL STATIONS .................................................... 553 2.1 General ................................................................... 553 2.1.1 Definitions ......................................................... 553 2.1.2 Units .............................................................. 553 2.1.3 Requirements ....................................................... 553 2.1.4 Methods ........................................................... 555 2.2 Surface wind .............................................................. 555 2.2.1 General ............................................................ 555 2.2.2 Instruments and exposure ............................................ 556 2.3 Visibility .................................................................. 556 2.3.1 Visibility for aeronautical purposes ..................................... 557 2.3.2 Prevailing visibility ................................................... 558 2.4 Runway visual range ........................................................ 558 2.4.1 General ............................................................ 558 2.4.2 Methods of observation .............................................. 558 2.4.2.1 Measurement by observers ................................... 559 2.4.2.2 Measurement by video. 559 2.4.2.3 Measurement by transmissometer ............................. 559 2.4.2.4 Measurement by forward-scatter or backscatter meters ........... 560 2.4.3 Instruments and exposure ............................................ 560 2.4.3.1 Transmissometers ........................................... 561 2.4.3.2 Forward-scatter meters. 562 2.4.3.3 Background luminance sensor ................................ 562 2.4.4 Instrument checks ................................................... 562 2.4.5 Data display ........................................................ 563 2.4.6 Accuracy and reliability of runway visual range measurements ............. 563 2.5 Present weather ............................................................ 563 2.6 Cloud .................................................................... 564 2.6.1 General ............................................................ 564 2.6.2 Observation methods. 565 2.6.3 Accuracy of cloud-base height measurements ........................... 565 2.7 Air temperature ............................................................ 566 2.8 Dewpoint ................................................................. 566 2.9 Atmospheric pressure ....................................................... 567 2.9.1 General ............................................................ 567 2.9.2 Instruments and exposure ............................................ 568 2.9.3 Accuracy of and corrections to pressure measurements ................... 568 2.10 Other significant information at aerodromes ................................... 569 2.10.1 General ............................................................ 569 2.10.2 Slant visual range .................................................... 569 2.10.3 Wind shear. 569 2.10.4 Marked temperature inversions. 570 2.11 Automated meteorological observing systems .................................. 570 2.12 Radar ..................................................................... 570 2.13 Ice sensor ................................................................. 571 2.14 Lightning detection ........................................................ 571 2.15 Other relevant observations ................................................. 571 REFERENCES AND FURTHER READING .............................................. 573 CHAPTER 2. MEASUREMENTS AND OBSERVATIONS AT AERONAUTICAL METEOROLOGICAL STATIONS 2.1 GENERAL 2.1.1 Definitions This chapter deals with the requirements for observations at aeronautical meteorological stations and the instruments and methods that are used. Synoptic observations measure at one location a representative value for a rather large area, but meteorological observations for aeronautical purposes are often made at several locations at the aerodrome and in the surrounding area, at more frequent intervals, to be representative of rather limited areas, such as the approach, touchdown and take-off areas. The meteorological measurements to be taken are for the most part essentially the same as those taken for other applications, and described in other chapters in this Guide. The exceptions are runway visual range (RVR), slant visual range and low level wind shear which are unique to this application. 2.1.2 Units The units for measuring and reporting meteorological quantities for aeronautical purposes are the same as for other applications, except that: (a) Surface wind speed may be measured and reported in metres per second or knots;1 and wind direction2 reported in degrees measured clockwise from geographic north3 (see section 2.2.1); (b) Cloud-base height may be measured in metres or feet. The choice of units is a matter for national practice, depending on the requirements of the aviation regulatory bodies. 2.1.3 Requirements The formal requirements for aeronautical observations are stated in the Technical Regulations, Volume II (WMO, 2013). Detailed guidance on procedures and practices is found in WMO (2014). Useful guidance on observing and monitoring meteorological conditions is contained in WMO (2003). Special attention should be given to aeronautical meteorological stations established on offshore structures in support of helicopter operations (ICAO, 1996). The requirements for uncertainty, resolution and range, and for currently achievable performance in meteorological measurements are given in Part I, Chapter 1, and the operationally desirable accuracies of some measurements are provided in the Technical Regulations, Volume II, Part II, Attachment A. Despite the excellent performance of modern aircraft, weather factors still have a marked effect on their operation. The reliability and representativeness of aerodrome observations are very 1 The unit of wind speed used is determined by national decision. However, the primary unit prescribed by the Technical Regulations, Volume II (WMO, 2013) for wind speed is the metre per second, with the knot permitted for use as a non-SI alternative unit (further information is given in ICAO, 2010). 2 Direction from which surface wind is blowing. 3 Because wind direction reported to aircraft for landing or take-off purposes may be converted into degrees magnetic, the display at the air traffic service unit usually presents direction with respect to the magnetic north. 554 PART II. OBSERVING SYSTEMS important in ensuring that landings and take-offs are made safely. The wind observation will determine the runway to be used, and the maximum take-off and landing weights. Temperature is also important and affects engine performance. Consequently, the load carried might have to be reduced, or the take-off would require a longer runway, particularly at airports in hot countries. Routine observations are to be made at aeronautical meteorological stations, at times and frequencies determined by the Member country to meet the needs of national and international air navigation, giving due regard to regional air-navigation arrangements. Special and other non-routine observations are to be made on the same basis. Routine observations at aerodromes should be made at hourly or half-hourly intervals, during all or part of each day, or as necessitated by aircraft operations. Special observations must be made when specified changes occur between routine observations in respect of surface wind, visibility, RVR, present weather and/or cloud. These specified changes are set out in the Technical Regulations, Volume II, Part II, Appendix 3, 2.3.2. These observations, in the form of coded reports of the METAR or SPECI types, are exchanged internationally between aeronautical meteorological stations. Other types of reports are intended only for aeronautical operations, and should be prepared in a form defined jointly by the meteorological and airport authorities. In view of the importance of meteorological observations for aircraft safety, it is essential that observers be correctly trained and have good eyesight. Observer training should include basic courses and regular refresher courses (for further information see the Technical Regulations, Volume I, Part II, 4, and WMO, 2012). Siting, installation and the nature of meteorological systems are specified in the Technical Regulations, Volume II, Part I, 4, with technical specifications and detailed criteria in the Technical Regulations, Volume II, Part II, Appendix 3. These specifications are summarized below. Special care is necessary in selecting appropriate sites for making observations, or for the installation of instruments at aeronautical meteorological stations, to ensure that the values are representative of the conditions at or near the aerodrome. In some instances, where information over a large area is required, it may be necessary to provide multiple installations for some instruments to ensure that values reported are representative of the entire area. For example, for long runways or for large aerodromes with several runways, where approach, touchdown and take-off areas may be as much as 2 to 5 km apart, the values of various parameters such as wind, cloud height, RVR, and so forth, measured at one end of a runway may be quite different from the conditions