Introduction to meteorology
Who sets the standard
The organisation ICAO works with, and the annex where the rules are found.
Works with ICAO to provide the framework. Also in LAW 001, among the international organisations.
Meteorological Service for International Air Navigation. Its objective is the safety, regularity and efficiency of international air navigation.
The world centres
A handful of centres serve the whole planet, each with one job.
Prepares gridded global forecasts.
- The two
- London, run by the UK Met Office, and Washington, run by NOAA.
Monitors satellite, ground and air data, and runs dispersion models to forecast where an ash cloud will move.
- Updates
- At least every 6 hours, while there is volcanic ash to report on.
Monitors tropical cyclones, using satellite, radar and other meteorological data.
- Updates
- At least every 6 hours.
Monitors space weather, and gives advisories on anything that might affect:
- The global centres
- PECASUS, ACFJ, NOAA and CRC.
- A regional centre
- SANSA.
How the WAFCs’ forecasts are provided and distributed, through WAFS and SADIS, is under Forecasting, below.
National and local
Below the world centres, each state has its own meteorological provider, and the offices that work for ATS.
A meteorological office shall be associated with each ATS unit. Which kind depends on the unit.
In Denmark one DMI unit, Civil Weather Forecasts and Warnings, does both jobs: it is the MWO for København FIR, and the aerodrome meteorological office at civil aerodromes such as Kastrup and Odense.
Where the information comes from
Weather information has many sources, and the airport is only one of them.
Surface observations
What an airport measures, the instrument that measures it, and the unit it is reported in. Every weather report uses the same units, so a number means the same thing wherever it is read.
Measured as direction and speed, by an anemometer representative of the conditions at the runway and touchdown areas. A windsock gives a visual indication. Wind is caused by differences in pressure.
Traditional mercury thermometers and hygrometers have been replaced by temperature and humidity probes.
Measured by visibility sensors: a transmissometer, or a forward scatter meter.
A present weather sensor measures the type and intensity of precipitation. It is often combined with the visibility and RVR sensors.
Measured with dual sensors.
Clouds, and the ceilometer that measures them, have the next section to themselves.
Clouds
How much of the sky is covered, and how high the cloud starts.
The lowest height of the visible portion of a cloud. A ceilometer is the equipment that measures it.
VVVertical visibilityNSCNil significant cloudCAVOKVisibility, cloud and present weather better than prescribed values or conditionsVisibility 10 km or more, no cloud of operational significance, no significant weather. A cloud of operational significance is one with its base below 5 000 ft, or below the highest minimum sector altitude if that is higher, or any cumulonimbus or towering cumulus. “Ceiling and visibility OK” is only an informal reading.
CAVOK needs no significant weather, not the absence of a SIGMET. All three conditions describe the aerodrome itself: its visibility, its cloud and its weather. A SIGMET comes from the meteorological watch office and plays no part in it.
Upper air observations
The air above the airport, measured on the way up.
- ClimbsFor about two hours, up to 30 km, until the balloon bursts.
- Comes downOn a parachute.
- MeasuresPressure, temperature and humidity. The wind is worked out by tracking the radiosonde’s position, today by GNSS.
- LaunchedTo the same schedule around the world: most stations send one up for 00 and 12 UTC.
What they give is a profile of the atmosphere from the ground up: one of the inputs to numerical weather prediction, in the next section.
Forecasting
How a weather model turns the laws of physics into a forecast, and how the world’s en-route forecasts reach their users.