MET.TR.210 Observing meteorological elements

The following meteorological elements shall be observed and/or measured with specified accuracy and disseminated by automatic or semi-automatic meteorological observing system.

(a)     Surface wind direction and speed

The mean direction and the mean speed of the surface wind shall be measured, as well as significant variations of the wind direction and speed (gusts), and reported in degrees true and knots, respectively.

(1)     Siting

The meteorological instrument used to measure surface wind direction and speed shall be situated in such a way as to provide data which is representative of the area for which the measurements are required.

(2)     Display

Surface wind displays relating to each sensor shall be located in the aeronautical meteorological station. The displays in the aeronautical meteorological station and in the air traffic services units shall relate to the same sensors, and where separate sensors are required, the displays shall be clearly marked to identify the runway and section of runway monitored by each sensor.

(3)     Averaging

The averaging period for surface wind observations shall be:

(i)      2 minutes for local routine report and local special report and for wind displays in ATS units;

(ii)     10 minutes for METAR and SPECI, except that when the 10-minute period includes a marked discontinuity in the wind direction and/or speed; only data occurring after the discontinuity shall be used for obtaining mean values; hence, the time interval in these circumstances shall be correspondingly reduced.

(b)     Visibility

(1)     The visibility shall be measured or observed, and reported in metres or kilometres.

(2)     Siting

The meteorological instrument used to measure visibility shall be situated in such a way as to supply data which is representative of the area for which the measurements are required.

(3)     Displays

When instrumented systems are used for the measurement of visibility, visibility displays relating to each sensor shall be located in the aeronautical meteorological station. The displays in the aeronautical meteorological station and in the air traffic services units shall relate to the same sensors, and where separate sensors are required, the displays shall be clearly marked to identify the area monitored by each sensor.

(4)     Averaging

The averaging period shall be 10 minutes for METAR, except that when the 10-minute period immediately preceding the observation includes a marked discontinuity in the visibility, only those values occurring after the discontinuity shall be used for obtaining mean values.

(c)      Runway visual range (RVR)

(1)     The RVR shall be reported in metres.

(2)     Siting

The meteorological instrument used to assess the RVR shall be situated in such a way as to provide data which is representative of the area for which the observations are required.

(3)     Instrumented systems

Instrumented systems based on transmissometers or forward-scatter meters shall be used to assess RVR on runways intended for Category II and III instrument approach and landing operations, and for Category I instrument approach and landing operations as determined by the competent authority.

(4)     Display

Where the RVR is determined by instrumented systems, one display or more, if required, shall be located in the aeronautical meteorological station. The displays in the aeronautical meteorological station and in the ATS units shall relate to the same sensors, and where separate sensors are required, the displays shall be clearly marked to identify the runway and section of the runway monitored by each sensor.

(5)     Averaging

(i)      Where instrumented systems are used for the assessment of the RVR, their output shall be updated at least every 60 seconds to permit the provision of current, representative values.

(ii)     The averaging period for RVR values shall be:

(A)     1 minute for local routine report and local special report and for RVR displays in ATS units;

(B)     10 minutes for METAR and SPECI, except that when the 10-minute period immediately preceding the observation includes a marked discontinuity in RVR values; then only those values occurring after the discontinuity shall be used for obtaining mean values.

(d)     Present weather phenomena

(1)     The following present weather phenomena shall be reported, as a minimum: rain, drizzle, snow and freezing precipitation, including intensity thereof, haze, mist, fog, freezing fog and thunderstorms, including thunderstorms in the vicinity.

(2)     Siting

The meteorological instrument used to measure present weather at the aerodrome and its vicinity shall be situated in such a way as to provide data which is representative of the area for which the measurements are required.

(e)     Clouds

(1)     Cloud amount, cloud type and height of cloud base shall be observed and reported as necessary to describe the clouds of operational significance. When the sky is obscured, vertical visibility shall be observed and reported, where measured, instead of cloud amount, cloud type and height of cloud base. The height of cloud base and vertical visibility shall be reported in feet.

(2)     Siting

The meteorological instrument used to measure clouds amount and height shall be situated in such a way as to provide data which is representative of the area for which the measurements are required.

(3)     Display

When automated equipment is used for the measurement of the height of cloud base, at least one display shall be located in the aeronautical meteorological station. The displays in the aeronautical meteorological station and in the air traffic services units shall relate to the same sensors, and where separate sensors are required, the displays shall be clearly marked to identify the area monitored by each sensor.

(4)     Reference level

(i)      The height of cloud base shall be reported above aerodrome elevation.

(ii)     When a precision approach runway in use has a threshold elevation of 50 ft (15 m) or more below the aerodrome elevation, local arrangements shall be made in order that the height of cloud bases reported to arriving aircraft shall refer to the threshold elevation.

(iii)     In the case of reports from offshore structures, the height of cloud base shall be given above mean sea level.

(f)      Air temperature and dew-point temperature

(1)     The air temperature and dew-point temperature shall be measured, displayed and reported in degrees Celsius.

(2)     When automated equipment is used for the measurement of air temperature and dew-point temperature, the displays shall be located in the aeronautical meteorological station. The displays in the aeronautical meteorological station and in the air traffic services units shall relate to the same sensors.

(g)     Atmospheric pressure

(1)     The atmospheric pressure shall be measured, and QNH and QFE values shall be computed and reported in hectopascals.

(2)     Display

(i)      When automated equipment is used for the measurement of atmospheric pressure, QNH and, if required in accordance with point MET.TR.205(g)(3)(ii), QFE displays relating to the barometer shall be located in the aeronautical meteorological station with corresponding displays in the appropriate air traffic services units.

(ii)     When QFE values are displayed for more than one runway, the displays shall be clearly marked to identify the runway to which the QFE value displayed refers.

(3)     Reference level

A reference level for the computation of QFE shall be used.