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Weather: Difference between revisions
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[[File:Wiki PlaceTower.jpg|thumb|270px|[[3D clouds]] in [[OSG]] with shaders.]] | [[File:Wiki PlaceTower.jpg|thumb|270px|[[3D clouds]] in [[OSG]] with shaders.]] | ||
[[ | [[File:weather_scenario.jpg|thumb|270px|The Weather Scenario window in FlightGear 1.9.1.]] | ||
[[ | [[File:New_Weather_Scenario.jpg|thumb|270px|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. | FlightGear has an impressive weather system, including real weather fetch, predefined weather scenarios, [[3D clouds]] and lightning. | ||
==How FlightGear creates weather== | == 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 [http://en.wikipedia.org/wiki/International_Standard_Atmosphere International Standard Atmosphere]. | 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 [http://en.wikipedia.org/wiki/International_Standard_Atmosphere International Standard Atmosphere]. | ||
FlightGear's fundamental atmosphere parameters calculated are: | FlightGear's fundamental atmosphere parameters calculated are: | ||
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* visibility | * visibility | ||
===Atmosphere=== | === 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. | 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. | 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. | ||
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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. | 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 === | ||
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 | 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) | * coverage (clear, few, scattered, broken, overcast) | ||
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Once again, definition of the base cloud set is in preferences.xml and runtime adjustments can be done via the menu '''Environment->Clouds''' | 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 == | ||
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. | 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 | You can either pass a metar string as a command line option like | ||
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==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]] is [[METAR]] used. | 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]] is [[METAR]] used. | ||
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Such a message is established every hour. | Such a message is established every hour. | ||
===Explanation=== | === Explanation === | ||
'''Example:''' | '''Example:''' | ||
{| class="prettytable"; border="1" style="border-collapse:collapse; border:1px solid silver;" | {| class="prettytable"; border="1" style="border-collapse:collapse; border:1px solid silver;" | ||
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# [[ICAO]] Identifier (4-letter) | # [[ICAO]] Identifier (4-letter) | ||
# Issuance Time DDHHMMz (UTC) | # Issuance Time DDHHMMz (UTC) | ||
#*COR (CCD in Canada) if correction to observation | #* COR (CCD in Canada) if correction to observation | ||
#Wind | # Wind | ||
#*First 3 digits: True Wind direction or average if variable (VRB). | #* 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) | #** 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 | #* Next 2 digits: Mean speed and units | ||
#** KT=knots, KMH=kilometers/hour, MPS=meters/second | #** KT=knots, KMH=kilometers/hour, MPS=meters/second | ||
#*G (gust) as needed – 2 or 3 digit maximum speed | #* G (gust) as needed – 2 or 3 digit maximum speed | ||
#*Calm will be indicated by 00000KT | #* Calm will be indicated by 00000KT | ||
#*Example: 18012G22KT 150V240 | #* Example: 18012G22KT 150V240 | ||
#Horizontal Visibility | # Horizontal Visibility | ||
#*Prevailing Visibility (PV) | #* Prevailing Visibility (PV) | ||
#**Statue miles (SM) and fractions (US & Canada only) or, | #** Statue miles (SM) and fractions (US & Canada only) or, | ||
#**4 digit minimum visibility in meters, and, | #** 4 digit minimum visibility in meters, and, | ||
#**Lowest value and direction, as required (shown as a remark) | #** Lowest value and direction, as required (shown as a remark) | ||
#*Runway Visual Range (RVR) | #* Runway Visual Range (RVR) | ||
#**R: Runway Designator, L/R/C as needed, “/” | #** R: Runway Designator, L/R/C as needed, “/” | ||
#**P/M: Plus/Minus (US only) | #** P/M: Plus/Minus (US only) | ||
#**4 digit value (feet/meters) | #** 4 digit value (feet/meters) | ||
#**V (variability) with tendency U/D/N (up/down/no change) | #** V (variability) with tendency U/D/N (up/down/no change) | ||
#**Example: R18R/1200FTV/U | #** Example: R18R/1200FTV/U | ||
#Present Weather (Constructed sequentially): | # Present Weather (Constructed sequentially): | ||
#*Intensity | #* Intensity | ||
#*Descriptor | #* Descriptor | ||
#*Precipitation (Dominant type is listed first if more than one type reported) | #* Precipitation (Dominant type is listed first if more than one type reported) | ||
#*Obscuration | #* Obscuration | ||
#*Other | #* Other | ||
#Sky Cover | # Sky Cover | ||
#*Cloud Description | #* Cloud Description | ||
#**Amount in eights (octas) | #** Amount in eights (octas) | ||
#**SKC=Sky Clear (clear below 12,000 for ASOS/AWOS) | #** SKC=Sky Clear (clear below 12,000 for ASOS/AWOS) | ||
#**NSC=No significant clouds | #** NSC=No significant clouds | ||
#**FEW=Few (1/8 to 2/8 sky cover) | #** FEW=Few (1/8 to 2/8 sky cover) | ||
#**SCT=Scattered (3/8 to 4/8 sky cover) | #** SCT=Scattered (3/8 to 4/8 sky cover) | ||
#**BKN=Broken (5/8 to 7/8 sky cover) | #** BKN=Broken (5/8 to 7/8 sky cover) | ||
#**OVC=Overcast (8/8 sky cover) | #** OVC=Overcast (8/8 sky cover) | ||
#Temperature/Dewpoint (whole °C) (preceded by M=minus) | # Temperature/Dewpoint (whole °C) (preceded by M=minus) | ||
#*First 2 digits = temperature | #* First 2 digits = temperature | ||
#*Second 2 digits = dewpoint | #* Second 2 digits = dewpoint | ||
#Altimeter setting (QNH) and indicator (A=InHg, Q=hPa) | # Altimeter setting (QNH) and indicator (A=InHg, Q=hPa) | ||
#Supplementary Information | # Supplementary Information | ||
#*RE = Recent weather followed by weather codes | #* RE = Recent weather followed by weather codes | ||
#*WS = Windshear, followed by: | #* WS = Windshear, followed by: | ||
#**TKOF/LDG (takeoff/landing) | #** TKOF/LDG (takeoff/landing) | ||
#**RWY (2 digits runway identifier and designator L/R/C) | #** RWY (2 digits runway identifier and designator L/R/C) | ||
#*RMK = Remark | #* RMK = Remark | ||
#**SLP = Sea Level Pressure | #** SLP = Sea Level Pressure | ||
#**T00221083 (Expanded temp/dewpoint) | #** T00221083 (Expanded temp/dewpoint) | ||
#**1st, 5th digits: 0=plus, 1=minus | #** 1st, 5th digits: 0=plus, 1=minus | ||
#**2nd-4th digits: temp (decimal missing) (02.2) | #** 2nd-4th digits: temp (decimal missing) (02.2) | ||
#**6th-8th digits: dewpoint (decimal missing) (-8.3) | #** 6th-8th digits: dewpoint (decimal missing) (-8.3) | ||
#Trend Forecast (2 hours from time of observation) (Not used in US) | # Trend Forecast (2 hours from time of observation) (Not used in US) | ||
#*PROB and 2 digits (30 or 40) = probability 30% or 40% | #* 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 | #* Used to indicate the probability of occurance of alternate element(s) or temporary fluctuations | ||
#*Change Indicator | #* Change Indicator | ||
#**BECMG = Becoming (used where changes are expected to reach or pass through specified values | #** BECMG = Becoming (used where changes are expected to reach or pass through specified values | ||
#**TEMPO = Temporary (fluctuations of less than one hour duration | #** TEMPO = Temporary (fluctuations of less than one hour duration | ||
#**NOSIG = No significant change | #** NOSIG = No significant change | ||
#*Forecast Wind (same as item 4) | #* Forecast Wind (same as item 4) | ||
#*Forecast Visibility (as item 5) (9999 indicates 10Kilometers vis or greater) | #* Forecast Visibility (as item 5) (9999 indicates 10Kilometers vis or greater) | ||
#*Forecast Weather (as item 6) | #* Forecast Weather (as item 6) | ||
#*Forecast Cloud (as item 7) | #* Forecast Cloud (as item 7) | ||
== TAF == | == TAF == | ||
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===Explanation=== | === Explanation === | ||
This TAF example of a 30-hour TAF, released on November 5 2008 at 1730 UTC: | This TAF example of a 30-hour TAF, released on November 5 2008 at 1730 UTC: | ||
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The first line contains identification and validity times. | The first line contains identification and validity times. | ||
*'''TAF''' indicates that the following is a terminal area forecast. | * '''TAF''' indicates that the following is a terminal area forecast. | ||
*'''KXYZ''' indicates the airport to which the forecast applies (ICAO airport code). | * '''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. | * '''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. | * '''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. | 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. | * '''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. | * '''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. | 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. | * '''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 remainder of the line has similar formatting to the other forecast lines. | ||
The final line is for errata, comments, and remarks. | 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. | * '''RMK NXT FCST BY 00Z''' indicates a remark that the next forecast will be issued by 0000 UTC. | ||
===External Links=== | === External Links === | ||
* [http://aviationweather.gov/static/help/taf-decode.php Decoding a TAF] | * [http://aviationweather.gov/static/help/taf-decode.php Decoding a TAF] | ||
==Related content== | == Related content == | ||
* [[Howto: Fetch live aloft data]] | * [[Howto: Fetch live aloft data]] | ||
[[Category:FlightGear feature]] | [[Category:FlightGear feature]] | ||