The unprecedented downturn that has hit airlines worldwide due to the coronavirus pandemic is having ripple effects that may make weather forecasts less accurate. The reason? Fewer planes flying deprives weather models of prized data used to simulate future conditions.

Few people realize that when they’re on a commercial flight, be it an Airbus or a Boeing, the aircraft they’re flying in is constantly sending data on the air mass surrounding the plane back to the ground. This data then gets piped into computer models at weather centers in the United States, where it’s used as an input to the Global Forecast System (GFS) model, as well as in Europe, where the European model takes advantage of such data, as well.

Commercial flights are the equivalent of thousands of extra weather balloons launched each day, providing crucial data on air pressure, temperature, wind speed and direction, and, in some cases, humidity where such information is scarce, such as high above the open North Pacific Ocean. In terms of the density of measurements, aircraft soundings, as such data is known, dwarf the volume of the twice-a-day weather balloons launched from 900 weather stations around the world.

In the United States, more than 3,500 commercial aircraft provide more than 250 million observations per year, according to National Weather Service spokesperson Susan Buchanan.

Both the National Oceanic and Atmospheric Administration (NOAA), the Weather Service’s parent agency, and the European Center for Medium-Range Weather Forecasting (ECMWF) are closely watching the decline in flights to see whether it cuts into their models’ reliability.

The loss of data is particularly acute in Europe, where flight restrictions have gone into effect throughout the European Union, and U.S. airlines have announced steep capacity cuts in an attempt to stay in business.

According to an ECMWF statement, aircraft reports come in second, behind satellite data, in terms of their impact on forecast accuracy. The European center, which operates the world’s most accurate weather model, reports that between March 3 and 23, aircraft-generated weather reports across Europe declined by 65 percent. There was also a 42 percent global decline in such reports during the same period. Such data is provided via a program known as aircraft meteorological data relay, or AMDAR.

Officials at ECMWF are anticipating declines in U.S. AMDAR data, as well as similar data in Australia, as coronavirus-related flight cuts continue. Qantas, the Australian carrier, is cutting all international flights, for example, as well as 60 percent of its domestic capacity. The ECMWF has investigated the impacts that removing aircraft data has on its forecast model simulations and found that there is a particularly large difference visible at the jet stream level, where planes typically cruise during flight.

Removing all aircraft data from the European model would decrease short-range wind and temperature forecasts at about 30,000 feet by up to 15 percent, with a statistically significant degradation of 3 percent when it comes to surface pressure forecasts. Studies published in academic journals also show that weather data gathered using commercial aircraft is a significant contributor to forecast skill.

The ECWMF statement quotes a NOAA employee, Christopher Hill, as warning that the loss of such data is “likely to generate some measure of impact on the output of our numerical weather prediction systems.”

But Buchanan says this won’t necessarily translate into less-accurate forecasts, since National Weather Service meteorologists look at multiple models, as well as model blends and weather observations, when preparing their forecasts.

“It’s too soon to quantify the exact impact because the decrease is only occurring for certain flights and routes, and while there is a reduction of commercial passenger flights, we still receive valuable aircraft data from overnight cargo and package carriers,” Buchanan said in a statement.