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Frequently Asked Questions (FAQ)

Questions About the Data

Questions about soundings

Questions about data availability

Other Questions Asked


Questions About the Data
Where do these data come from?
These data are from approximately 135 Southwest and United Parcel Service Airlines 737 and 757 aircraft. The NWS has purchased the sensors (called the Water Vapor Sensing System- WVSS-II) and installed them on some of the airlines aircraft. They measure temperature and water vapor, while the aircraft avionics produce ground relative wind speeds.
What is the Water Vapor Sensing System?
The Water Vapor Sensing System is produced by Spectrasensors. Details about the WVSS-II system can be found here.
Do the airlines get paid for producing these data?
No, the airlines allow their aircraft to collect the data so that airline and NWS meteorologists will be able to produce better weather forecasts and warnings.
How long have the data been available?
The NWS has been receiving an increasing amount of data from these aircraft since the late 1990s, but contractual agreements have prevented their public release until recently.
How do they compare to radiosondes and other upper air systems?
The WVSS-II soundings have been compared to NWS radiosondes and have been found to be of equal or greater quality. Here are some research papers detailing some of the comparisons:
What are these data used for?
The data are used in NWS, military and university numerical weather prediction models, as well as by airline and NWS meteorologists. Forecast applications can be found in these papers:
What can I do with these data?
Because these data are in the public domain, they can be freely used by anyone for any purpose. Please remember, though, that they are not subject to strict quality control and are not considered official data.

Questions about soundings
Why did the sounding I requested not go very high?
There are at least a couple of reasons. The first is that the sounding was from a short flight with a relatively low cruising altitude (say a flight from Chicago to Milwaukee). Another possibility is that the aircraft that produced the sounding was still climbing at the end of the time period you selected. If you selected a time period from 03z until 06z, and an aircraft took off at 0545Z, it may not have achieved much height before the data period ended. You could wait a few minutes until the plane climbs more and reload the latest data, or adjust the data period to extend a little further out.
Why is the sounding I requested missing data near the surface?
It is possible that the aircraft that produced the sounding was descending at the end of the time period you selected. If you selected a time period from 03z until 06z, and an aircraft was still descending at 0600Z, it will be missing the data near the ground. You should see the rest of the sounding if you adjust the ending time of the data period you selected.
Why does there seem to be a superadiabatic layer near the ground?
The air inlet for the temperature and moisture sensors is mounted on the aircraft fuselage, and is heated to prevent icing. Sometimes the heater is on while the aircraft is parked on the ramp, and will produce an erroneously warm temperature until the aircraft is just above the ground.

Questions about data availability
Why are there more soundings at certain cities?
These soundings come from Southwest and United Parcel Service Airlines aircraft only. Therefore, the soundings are most abundant near their hub airports (Louisville, Rockford, Miami and Ontario for UPS, and Baltimore, Fort Worth, Chicago, Denver and several West Coast Cities for Southwest) .
Why are there fewer soundings on Sunday?
While Southwest Airlines flies most of their aircraft on Sunday, most UPS Airlines aircraft pick up and deliver packages Monday through Saturday, with much less activity Saturday.
When will there be more soundings?
While there are no immediate plans to provide more soundings, the NWS plans to expand the program as money is available.

Other Questions Asked
What variables are available?
Most aircraft provide
  • latitude,longitude,altitude,time
  • temperature
  • wind direction and speed
  • Eddy dissipation rate (EDR), a measure of turbulence, is reported by about 100 UAL aircraft that fly mostly over the United States. (These data are experimental and should not yet be relied upon for turbulence verification. We expect quality-controlled EDR data to be available in 2005.)
  • Derived equivalent vertical gust (DEVG), another measure of turbulence, is reported by about 80 international (AMDAR) aircraft that fly over Europe and Asia. The translation between DEVG values and turbulence severity is as follows

    < 2 m/sNil
    2 - 4.5 m/sLight
    4.5 - 9 m/sHeavy
    > 9 m/sSevere

  • Water vapor, reported as dewpoint, is provided by a few aircraft. These data are experimental, have not been validated, and should be used with caution.
  • Icing information (yes/no) is reported by a few aircraft. These data are experimental, have not been validated, and should be used with caution.
How many data are there?
Currently, we are getting just about 140,000 wind and temperature observations per day, 100,000 of which are over the continental United States. These data come from more than 4000 aircraft. There are more data during the daytime than at night, but thanks to participation by some parcel-carrying airlines, nighttime coverage is substantial.

What is the storage/data transmission load?
  • about 1.8 megabytes per hour in netCDF format
  • 150 kilobytes per hour in the format downloaded to the java display
  • The non-java display downloads gif images which have sizes from 2 to 75 kilobytes, depending on the amount of data shown and whether a satellite background image is requested.
What quality control is applied to the data?
The ACARS data stream is very high quality, with less than 2% of the data showing problems. Nevertheless, all data shown on the web site or available for download have been quality-controlled at GSD. This processing includes interpolating location and time information to those data, such as high-resolution ascent data, that lack it. Data that fail quality control are indicated by a special color on the web plots, and several quality control and interpolation flags are available in the netCDF data that indicate exactly what test(s) and interpolation each datum was subjected to. Details of the quality control are available here.

What kind of altitude is reported?
The altitude-determining physical variable is pressure. This is converted to altitude by using a standard atmosphere. The standard atmosphere is used at all altitudes and under all pressure conditions. Thus, for instance, it is possible to have ACARS altitudes below ground level on days with high atmospheric pressure. (This is in contrast to other aviation reports such as voice PIREPS that use a standard atmosphere to compute altitude above 18,000 ft (MSL), but use the current altimeter setting for lower altitudes.)

Are the data averaged?
No. All ACARS data are point samples, not averaged in time or space. (Specifically, this means that the averaging time is approximately 1 sec or less.)

What is the resolution?
At flight altitudes (above about 23,000 ft), data are generally taken every 5-6 minutes. Near airports the data spacing is decreased by some airlines in several different ways:
  • 2000 and 1000 ft. vertical resolution below 18,000 ft. is quite common
  • On ascent, more than 150 aircraft provide data with a vertical resolution of 300 ft. for the first minute after take-off (up to about 2500 ft. AGL).
What is the accuracy?
  • Benjamin, Schwartz, and Cole (1999, Weather and Forecasting, in review) -- collocation study with ACARS reports. Estimated wind vector accuracy - 1.8 m/s, estimated temperature accuracy - 0.5 deg C. These numbers are much lower than the previous ACARS/RAOB differences, which by definition included RAOB error.
  • Decker, R., R.D. Mamrosh, and C.E. Weiss, 1999: ACARS operational assessment: description and interim results. Preprints, 3rd Conference on Integrated Observing Systems, Dallas, TX, Amer. Meteor. Soc., 24-27.
  • From Richard D. Mamrosh (1998): "Several studies have compared ACARS with other data sources. Among these, Lord, et. al. (1984) compared ACARS flight-level winds with radiosonde, cloud-motion and VAS thermally-derived winds. When ACARS was compared to radiosondes, root mean square (RMS) deviations were 7.4 degrees in direction and 5.3 m/s in speed. More recently, Schwartz and Benjamin (1995) compared ACARS ascent/descent winds and temperatures with radiosondes and found temperature differences were less than 2C on 94 percent of all occasions, and less than 1C better than 68 percent of the time. Wind speed RMS deviations were 4.1 m/s while direction RMS differences were 35 degrees (mostly due to light and variable wind situations)."
How timely are the data?
Data are processed at GSD every 10 minutes, starting on the hour. If you access the data shortly after they are processed, the most recent data may be only a couple of minutes old. However, some airlines buffer the data on board the aircraft. Reports on ascent and descent are generally buffered for 0 to 2 minutes (depending on airline and aircraft type), however some over-ocean reports may be buffered for several hours.
Gruesome additional details: Every 10 minutes, starting on the hour, data arriving since the end of the last complete hourly file are processed. Processing takes about 10 seconds. At 18 min past the hour, the last complete hourly file is generated. I.e., at 0118 a file containing data for 0000-0059:59 is generated. In order to catch late-arriving reports, this file is regenerated approximately 1,2,3,6, and 12 hours later. (Times are shifted by a minute or so that jobs don't overlap.) So, if you want to get the most complete data set, wait at least 12h 22m.

How is a "sounding" determined?
A "sounding" is a portion of an ascent or descent flight track from a single aircraft that can provide a vertical profile of wind, temperature, and sometimes dewpoint and other variables.
On both the java and non-java displays, a sounding plot will always be generated when you click on an ascending or descending portion of a flight track. However, some flight track portions are considered by the software to have sufficient resolution and vertical extent to be a meteorologically useful sounding. These are indicated on the java display by solid lines between the data points comprising the sounding. To be considered a (useful) sounding, the flight track portion must meet all of these criteria:
  • The bottom of the sounding must be <= 2000 ft AGL (and < 10000 ft MSL)
  • The bottom of the sounding must be within 50 nm of a known airport
  • The maximum data gap must be <= 5000 ft in the vertical.
  • (The top of the sounding is taken as 500 ft below the maximum altitude of a flight, or 50 temperature data points above the lowest point--whichever is less.)
Are retrospective data available?
On the web, 30 days of data are generally kept on line. For the java display, data back to 1 July 2001 may be downloaded on demand (from the MADIS archive) although data more than 30 days old take longer to load, initially.
Data since 1 July 2001 in netCDF format are available through GSD's MADIS program. Interested parties may fill out this data application.

How have the data been used?
  • NOAA forecast offices have been using GSD's ACARS data for several years. One on-line publication that describes the use of the data at one forecast office is THE USE OF HIGH- FREQUENCY ACARS SOUNDINGS IN FORECASTING CONVECTIVE STORMS by Richard D. Mamrosh of the National Weather Service, Chicago, Illinois. We also have begin compiling NWS Forecast Discussions that mention ACARS.
  • ACARS are critical asynoptic data for the Rapid Update Cycle (RUC), LAPS, and the Eta models, and are also assimilated into global models.
What airlines provide GSD with data?
Currently-participating U.S. airlines are: Alaska, American, Delta, Federal Express, Southwest, United, and United Parcel Service. In addition, the following airlines participate in the AMDAR program: AeroMexico, Air Canada Jazz, Air France, Air New Zealand, Air Nippon, Air Vanuatu, Asiana Airlines, Blue1, British Airways, Cathay Pacific, China Southern Airlines, EasyJet, Finnair, Japan Airlines, JetConnect (Qantas), Jetstar Airways, Jetstar Asia, KLM, Korean Air, Lufthansa, NAV Canada, Novair Scandinavia, Qantas, Scandinavian Airlines, Shandong Airlines, SkyTraders, South African Airways, and Thomas Cook Airlines.


AMDAR data sources explained, by Dave Helms, NWS.

  • AMDAR: Aircraft Meteorological Data Reports. The name used worldwide for automated meteorological reports from aircraft.
  • TAMDAR: Tropospheric AMDAR. A commercial measurement and communications developed by AirDat, LLC, initially under NASA sponsorship. TAMDAR is designed to be deployed on regional aircraft, and has been deployed on several regional fleets. TAMDAR data from the Mesaba Airlines fleet is available to the U. S. government; data from other fleets is generally restricted to GSD, for evaluation.
  • MDCRS: Meteorological Data Collection and Reporting System. MDCRS is funded jointly by the U.S. government and the seven participating airlines (American, Delta, Federal Express, Northwest, Southwest, United, and United Parcel Service), and operated by Aeronautical Radio, Inc.
  • ACARS: Aircraft Communications Addressing and Reporting System, a proprietary system run by Aeronautical Radio, Inc. (ARINC). ACARS is used by airlines to transmit a variety of proprietary air-to-ground communications. Seven airlines use ACARS to transmit meteorological data for the MDCRS system. These airlines also use ACARS to route their meteorological data directly to GSD where we also decode the data from the wide variety of formats used by the airlines. Thus, on the GSD AMDAR display, we refer to "ACARS" data as those data we decode here, and to MDCRS data as those data we reveive from the NOAA telecommunications gateway.
  • WVSS and WVSS-II: The Water Vapor Sensing System II (WVSS) has been developed by SpectraSensors through contracts with NOAA. The WVSS has undergone a series of engineering changes since its initial deployment in 2005, culminating with the most design changes in 2008. The latest design has corrected most problems with the system, yielding consistent quality mixing ratio observations for a range of warm/moist and cold/dry airmasses, probably as good or better than other operational in situ observing systems. As of August 2010, there are 30 WVSS in operations, 25 flying on United Parcel Service B-757 aircraft and another 5 flying on Southwest Airlines B-737 aircraft generating hundreds of soundings per day. The WVSS fleet will expand to 100 aircraft in the next year, with on-going assessments planned by multiple researchers; the results of these assessments will be published in scientific journal articles. (August, 2010)

Last updated 19 April 2017