When severe thunderstorms, tornadoes, flash floods, or winter storms threaten the Washington region automated observations from airliners taking off and landing at airports as far away as Philadelphia give NWS forecasters a more detailed picture of what the atmosphere is up to.
In effect, the airliners are supplying sounding data up through the atmosphere like that from radiosondes attached to the weather balloons that Sterling and some other NWS offices normally launch twice a day.
Zubrick says forecasters can display reports from an airliner side by side with the most recent balloon sounding. “This gives you a idea of what the trends are; it’s kind of nice.”
Takeoff and landing data are only a small part of the information that airliners feed into the world’s weather forecasting models each day. Globally, airlines including those from the U.S. supply “well over 450,000 observations per day,” the World Meteorological Organization reported in May.
In the U.S., reports come from some jets flown by United, American, Delta, Northwest, Southwest, UPS, and FedEx. The UPS and FedEx data fill a gap since these airplanes fly mostly overnight and into the early morning when few passenger airliners are in the air.
Several studies over the years have shown that airliner observations, which supplement data from weather balloons and satellites, improve the output of the models that produce all forecasts, not just those for aviators.
Since jet airline flights spend most of their time cruising at altitudes above 25,000 feet most of the reports are from these high altitudes. That’s fine, since as broadcast meteorologists regularly remind us, the “jet streams” are major players in surface weather.
More upper-air observations improve predictions not only of upper air changes, but also of the resulting ground-level effects.
NWS offices also receive airliner take off and landing soundings because all participating airliners transmit reports of the temperature, wind speed and direction, atmospheric pressure, altitude, and latitude and longitude from the time the wheels leave the ground until they touch down on landing.
Weather reports from aircraft aren’t new; they date back to the 1930s when airliners began using radios and pilots would sometimes radio in information about weather conditions. Pilots today are encouraged to radio in reports of weather conditions, and these “pilot reports” are often helpful to other pilots or aviation forecasters. But they don’t begin to supply enough of the reliable, detailed data models or local forecasters need.
Today’s automated aircraft weather reports, which don’t require pilots to divert attention from flying, began in 1979 when a few airlines, other aviation-related companies, and national weather services established the system for automated reports.
From the beginning, the automated weather reports have flowed through the Aircraft Communications Addressing and Reporting System (ACARS), operated by Rockwell Collins/ARINC, with headquarters in Annapolis, Md. Aviation weather data make up only a small part of this communications channel to and from airliners. Other transmissions include automated aircraft performance information to airline maintenance departments, and updated weather observations and forecasts for the pilots (from non-aviation sources). When a flight attendant announces shortly before landing which gates passengers should go to for connecting flights the data probably arrived via ACARS.
In addition to temperature, wind, and location information, a few airlines began adding water vapor sensors in 2007. These sensors made by SpectraSensors, Inc. are on some Southwest Airlines, UPS, and foreign airplanes.
Measuring water vapor in the air an airplane is moving through at a few hundred miles per hour is more complicated than measuring temperatures and winds. Almost all airplanes, including single-engine trainers have outside air thermometers. Jet airliner flight management systems use navigational and airspeed data to calculate the speeds and directions of winds airplanes are flying through.
The water vapor sensors send outside air through a tube inside the airplane where a diode laser measures the air’s water vapor before it flows back outside. Humidity can be calculated from the water vapor levels.
Zubrick says the humidity data are especially useful because airplanes sometimes fly into areas with more or less humidity than recorded by the latest radiosondes sounding. Among other things, this information helps forecasters decide where clouds are likely to form or dissipate. When thunderstorms are possible water vapor measurements help “us keep up with changes in CAPE,” he says. (CAPE is Convective Available Potential Energy, which determines how powerful thunderstorms can become.)
Zubrick says forecasters also use water vapor data to calculate precipitable water in the air. This is the amount of precipitation that would fall if all of the water vapor condensed and fell. Forecasters know that when it’s two standard deviations “above climatology and you have thunderstorms they will be flash-flood producing.”
If you regularly look at the Sterling office’s “area forecast discussion” on the Web you’ll sometimes see references to “ACARS” data. “If you don’t see this it doesn’t mean we aren’t using it,” Zubrick says.
Although Weather services around the world, including the NWS, officially use “AMDAR” for “Aircraft Meteorological Data Reports,” meteorologists regularly refer to it as ACARS.
If you like to look at the sources of forecasts, such as the models, don’t waste your time looking for AMDAR data. The agreements that the NWS and other national weather services have with airlines say that real-time data is made available only to national weather services and airlines that provide the data at their expense.