Weather

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3D clouds in OSG with shaders.
The Weather Scenario window in FlightGear 1.9.1.
The new Weather Scenario window in FlightGear CVS.

FlightGear has an impressive weather system, including real weather fetch, predefined weather scenarios, 3D clouds and lightning.

How FlightGear creates weather

Weather in reality is the state of the atmosphere at a given time for a given place. Calculating the complete atmosphere or even a small part of it will easily eat up all your computing power and result in very limited frame rates. FlightGear only calculates the state of the atmosphere for a vertical line beginning at earth's center straight through your aircraft up to approx. 100,000ft AGL. Most calculations are based on the International Standard Atmosphere. FlightGear's fundamental atmosphere parameters calculated are:

  • temperature
  • dew point
  • pressure
  • density
  • wind (three dimensional)
  • visibility

Atmosphere

The line for which FlightGear calculates the atmosphere is divided into two major segments and several sub segments. The bottom major segment (the one closest to the ground) is called the boundary layer. It reaches up to 500ft above ground and connects directly above it to the so called aloft layer. By definition, in the boundary layer, the atmosphere - mainly the wind - is affected by the structure of the earth's surface, while in the aloft layer there is a free flow of air. Within the boundary and the aloft layer, one up to many layers of atmosphere may be defined that have certain characteristics, like wind direction and speed. The basic weather definition in FlightGear is

  • Boundary layer, 0ft
    • wind 270@3KT
    • visibility 10SM
    • pressure 29,92INHG / 1013hPa
    • temperature 15, dewpoint 5
  • Boundary layer, 500ft
    • wind 280@6KT
  • Aloft layer, 3000ft
    • wind 300@10KT
  • Aloft layer, 6000ft
    • wind 310@20KT
  • Aloft layer, 9000ft
    • wind 320@30KT

All other values are derived from these parameters. When the aircraft changes altitude and passes through these layers, a linear interpolation between the parameters for the entry above and below your position is performed to compute the values for your exact altitude. The atmospheric parameters described here are defined in preferences.xml and can be changed at runtime by selecting Environment->Weather Conditions from the menu. The entry "Boundary transition depth" defines the thickness of the zone where the effects of the boundary layer and the aloft layer mix. When you fly within 500ft of the border between boundary and aloft layer, both tables are interpolated.

Clouds

Clouds are - other than in real life - not visible humidity, when dew point and temperature match. They are a separate system and so are defined separately. They are also stacked in layers and for each layer the defining parameters for clouds are

  • coverage (clear, few, scattered, broken, overcast)
  • the elevation of cloud base (height above ground, not altitude!)
  • the thickness (distance from cloud base to cloud top)
  • the span (how far away do the clouds reach)

Once again, definition of the base cloud set is in preferences.xml and runtime adjustments can be done via the menu Environment->Clouds

Defining Weather

Defining weather can be a tedious task. Setting all the parameters for each layer and defining clouds is not everybody's fun. To get a weather definition done quickly, FlightGear has a build in METAR interpreter. This can read the coded weather information from a METAR and apply a more or less reasonable weather, that matches the conditions described in the METAR. Since a METAR only describes the weather a a station on the ground, many parameters, esp. for the higher atmosphere are just plain guesses which just try to be reasonable. You can either pass a metar string as a command line option like

--metar="EDDH 280920Z 28020KT 9999 SCT030 14/07 Q1020 NOSIG"

or use the Weather Scenario dialog from the menu at Environment->Weather Scenario, where you can select a predefined weather, enter your own METAR or even enable live weather data. This options enables a task, that calculates your nearest airport and fetches the current METAR for that station from NOAA weather service. An internal interpolation routine applies weather changes smoothly when changing the METAR.


METAR

A METAR is a codified observation message indicating an airfield weather conditions observed at a given time. There are different ways of messaging weather reports, but in FlightGear METAR is used. The data is retrieved from NOAA.

The METAR-message can be found acros the menu (Weather > Weather Scenario). To have the actual weather (or playing with ATC) you need to enable the Real weather fetch in the FlightGear Wizard.

Such a message is established every hour.

Explanation

Example:

2008/03/15 12:24 KSFO 151224Z 05012KT 10SM SN BKN050 02/M08 A3016 RMK AO2 SLP228 T00221083
1 2 3 4 5 6 7 8 9 10 11
  1. Date
  2. ICAO Identifier (4-letter)
  3. Issuance Time DDHHMMz (UTC)
    • COR (CCD in Canada) if correction to observation
  4. Wind
    • First 3 digits: True Wind direction or average if variable (VRB).
      • Note: If the wind direction varies 60° or more, the direction will be indicated with a V (e.g. 180V250)
    • Next 2 digits: Mean speed and units
      • KT=knots, KMH=kilometers/hour, MPS=meters/second
    • G (gust) as needed – 2 or 3 digit maximum speed
    • Calm will be indicated by 00000KT
    • Example: 18012G22KT 150V240
  5. Horizontal Visibility
    • Prevailing Visibility (PV)
      • Statue miles (SM) and fractions (US & Canada only) or,
      • 4 digit minimum visibility in meters, and,
      • Lowest value and direction, as required (shown as a remark)
    • Runway Visual Range (RVR)
      • R: Runway Designator, L/R/C as needed, “/”
      • P/M: Plus/Minus (US only)
      • 4 digit value (feet/meters)
      • V (variability) with tendency U/D/N (up/down/no change)
      • Example: R18R/1200FTV/U
  6. Present Weather (Constructed sequentially):
    • Intensity
    • Descriptor
    • Precipitation (Dominant type is listed first if more than one type reported)
    • Obscuration
    • Other
  7. Sky Cover
    • Cloud Description
      • Amount in eights (octas)
      • SKC=Sky Clear (clear below 12,000 for ASOS/AWOS)
      • NSC=No significant clouds
      • FEW=Few (1/8 to 2/8 sky cover)
      • SCT=Scattered (3/8 to 4/8 sky cover)
      • BKN=Broken (5/8 to 7/8 sky cover)
      • OVC=Overcast (8/8 sky cover)
  8. Temperature/Dewpoint (whole °C) (preceded by M=minus)
    • First 2 digits = temperature
    • Second 2 digits = dewpoint
  9. Altimeter setting (QNH) and indicator (A=InHg, Q=hPa)
  10. Supplementary Information
    • RE = Recent weather followed by weather codes
    • WS = Windshear, followed by:
      • TKOF/LDG (takeoff/landing)
      • RWY (2 digits runway identifier and designator L/R/C)
    • RMK = Remark
      • SLP = Sea Level Pressure
      • T00221083 (Expanded temp/dewpoint)
      • 1st, 5th digits: 0=plus, 1=minus
      • 2nd-4th digits: temp (decimal missing) (02.2)
      • 6th-8th digits: dewpoint (decimal missing) (-8.3)
  11. Trend Forecast (2 hours from time of observation) (Not used in US)
    • PROB and 2 digits (30 or 40) = probability 30% or 40%
    • Used to indicate the probability of occurance of alternate element(s) or temporary fluctuations
    • Change Indicator
      • BECMG = Becoming (used where changes are expected to reach or pass through specified values
      • TEMPO = Temporary (fluctuations of less than one hour duration
      • NOSIG = No significant change
    • Forecast Wind (same as item 4)
    • Forecast Visibility (as item 5) (9999 indicates 10Kilometers vis or greater)
    • Forecast Weather (as item 6)
    • Forecast Cloud (as item 7)

TAF

In meteorology and aviation, TAF is a format for reporting weather forecast information, particularly as it relates to aviation. "TAF" is an acronym of Terminal Aerodrome Forecast or, in some countries, Terminal Area Forecast. Generally, TAF's can apply to a 9- or 12-hour forecast; some TAFs cover an 18- or 24-hour period; and as of November 5,2008, TAFs for some major airports cover 30 hours periods. The date/time group reflects the new 30 hour period in Coordinated Universal Time (UTC), as always.


Explanation

This TAF example of a 30-hour TAF, released on November 5 2008 at 1730 UTC:

TAF KXYZ 051730Z 0518/0624 31008KT 
     3SM -SHRA BKN020
     FM052300 30006KT 5SM -SHRA OVC030 
     PROB30 0604/0606 VRB20G35KT 1SM 
     TSRA BKN015CB 
     FM 060600 250010KT 4SM -SHRA OVC050
     TEMPO 0608/0611 2SM -SHRA OVC030=  
     RMK NXT FCST BY 00Z=

The first line contains identification and validity times.

  • TAF indicates that the following is a terminal area forecast.
  • KXYZ indicates the airport to which the forecast applies (ICAO airport code).
  • 051730Z indicates that the report was issued at 1730 UTC on the 5th of the month.
  • 0518/0624 indicates that the report is valid from 1800 UTC on the 5th until 2400 UTC on the following day.

The remainder of the first line and the second line contain the initial forecast conditions. Variations of the codes used for various weather conditions are many.

  • 31008KT indicates that the wind will be from 310 degrees at 8 knots.
  • 3SM -SHRA BKN020 indicates that visibility will be 3SM (statute miles) in light (-) showers (SH) of rain (RA), with a broken ceiling (at least 5/8 of the sky covered) at 2,000 feet.

Each line beginning with FM starts a new forecast period.

  • FM052300 indicates the next period lasts from (FM) 2300 UTC on the 5th to 0000 UTC on the 6th.

The remainder of the line has similar formatting to the other forecast lines.

The final line is for errata, comments, and remarks.

  • RMK NXT FCST BY 00Z indicates a remark that the next forecast will be issued by 0000 UTC.

External Links

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