The pandemic that has idled scores of commercial flights is having a little-noticed consequence for meteorologists, whose forecasts rely in part on data collected from planes. That means a crucial eye in the sky has weakened just as spring flood waters rise across North American and Europe, and farmers are preparing to plant wheat, corn and soybeans.

“In terms of importance, aircraft data are usually in the top five,” said Chris Davis, a scientist at the National Center for Atmospheric Research in Boulder, Colorado.

With flights all but halted in many nations, meteorologists have seen a steep decline in more than 700,000 daily weather observations from aircraft. There could be a resulting dip in weather-forecasting accuracy, meteorologists warn, but fortunately there are possible workarounds to augment the remaining flights.

Forecasters have faced this challenge before. After the Sept. 11th terror attacks in the U.S., the grounding of global air traffic caused a deterioration of computer weather models. The European Centre for Medium-Range Weather Forecasts found between a 4% to 5% loss in its modeling abilities, Davis said.

The decrease in flights as a result of the Covid-19 pandemic hasn’t been that severe so far. About 17% of all trips worldwide have been cancelled, according to FlightAware, an airline tracking service, and 34% of U.S. flights have been grounded.

That has led the European Centre to see 45% drop-off in incoming reports from airplanes since March 1. One difference is that the pandemic and its economic fallout could last longer. The loss of airplane data could continue into the summer, the European Centre said.

Airplanes have been gathering weather data since WWI. The U.S. Weather Bureau first paid pilots in 1919 to carry instruments attached to wing struts to a height of 13,500 feet (4,100 meters). The flyers who got higher were paid a bonus.

The modern data-collection network took shape in 1998 when the World Meteorological Organization created the Aircraft Meteorology Data Relay Panel, which led to a fully automated system for gathering weather data from commercial, private and military aircraft. The flight information is radioed to ground stations and relayed to meteorologists. That data is combined in forecasting models with satellite observations and readings from weather balloons, ground stations and even buoys at sea.

The heaviest concentration of daily aircraft weather reporting comes from the U.S., Western Europe and Japan. In the U.S. alone there are about 250 million observations per year, said Susan Buchanan, a spokeswoman for the National Weather Service. Jets from American and Delta as well as cargo haulers FedEx and UPS contribute weather data, according to the National Oceanic and Atmospheric Administration.

Total loss of airplane data can have an impact. The European Centre, whose Euro weather models are touted as the gold-standard among many forecasters, ran a simulation last year that eliminated all airliner reports and found short-range forecast skill for temperature and wind fell by about 15%, with a smaller loss in prediction of high- and low-pressure patterns.

The U.S. government, which runs weather models of its own, doesn’t anticipate a great loss of accuracy. “Even though a decrease in critical data will likely negatively impact forecast model skill, it does not necessarily translate into a reduction in forecast accuracy,” said Buchanan. She cited “billions of Earth observations from other sources that feed into our models.”

There are reasons to be optimistic. The heavy cancellations haven’t reduced flights by military and cargo haulers or private aircraft. “Unlike 9/11, there still are a lot of planes flying, so the impact is muted right now,” said Todd Crawford, chief meteorologist at the Weather Company, an IBM business.

In addition, the forecast models themselves are better than in 2001. “There is some redundancy in observations, enough that some compensation can occur if one type is lost,” said Davis of the National Center for Atmospheric Research. “The system can be tuned such that other observations take up some of the slack. Not perfect, but fairly resilient.”

Some of the data could also be made up by additional weather balloon launches, an undertaking that is often used in anticipation of weather emergencies such as a hurricane bearing down on a coastline. Balloons are currently launched twice a day at 850 weather stations worldwide, climbing to heights up to 115,000 feet while radioing back information every second.

Above all is the fleet of satellites keeping an eye on weather down below. Crawford said these provide the most important observations.

There are also flights that almost never stop. The National Oceanic and Atmospheric Administration and the U.S. Air Force Reserve have a cadre of pilots dedicated to flying weather missions. The Hurricane Hunters, as they’re called, are currently flying into the Pacific gathering information on atmospheric rivers. In June, these pilots will transfer back to their summer jobs: searching for hurricanes in the Atlantic and eastern Pacific.

–With assistance from Alan Levin and Mary