Paris (AFP)

The coronavirus also causes unforeseen victims: meteorologists, deprived of the precious information usually transmitted in real time by airliners today nailed to the ground by the epidemic.

Very data-intensive, the digital models used every day for weather forecasts are constantly fed with millions of observations on the state of the atmosphere.

The vast majority of these surveys are carried out by dedicated, increasingly efficient satellites.

But to be more precise, especially in the lowest layers of the atmosphere, "about 10% of the data comes from on-board systems on airliners", explains to AFP Emmanuel Bocrie, director of the Media division at Météo France.

Commercial airliners normally transmit more than 700,000 observations worldwide daily on air temperature, wind speed and direction, as well as humidity and turbulence measurements during their phase. altitude rise then during the flight: this is the Amdar system, managed by the World Meteorological Organization (WMO), a UN agency.

With the epidemic, which has led to an 80% to 90% drop in world air traffic, "we have lost around two thirds" of these observations, underlines Mr. Bocrie.

A situation that worries meteorologists. "The continuous and amplified decrease in weather observations from aircraft could cause a gradual decline in the reliability of forecasts," warned one of the directors of WMO, Lars Peter Riishojgaard, in a press release in early April.

- Lack of surveys over the Atlantic -

The impact on forecast accuracy could be around 10%, calculated Météo France, which decided to compensate, with other national institutes, "to double the number of radiosondes" with sounding balloons.

This technique, invented in Trappes in the Paris region in 1929, consists of sending a large balloon, inflated with hydrogen and equipped with a probe, up to an altitude of more than 30 kilometers, where it explodes and falls back down. The measurements are more complete and more reliable than those of the probes on board aircraft, but the equipment is rarely recovered and cannot be reused, which induces a very significant cost.

While these operations had progressively decreased, Météo France returned to 4 daily launches from each of its sites in mainland France and overseas.

The device is automatic, but a team must come and rearm it every two weeks: "you have to come twice as often," explains Mr. Bocrie.

Too expensive, the technique is not applicable everywhere.

"In many developing countries, the transition to automated observations is not complete," and they are done manually, noted the WMO.

And the organization "noted a significant decrease" in these surveys from mid-March without attributing it exclusively to the pandemic.

Finally, radiosondes cannot replace planes over the oceans.

"Today there is a lack of observation above the Atlantic, which nevertheless conditions the weather on the European continent", explains for example Sébastien Brana, vice-president of the Infoclimat association which combines data from Météo France with those of its own network of observers.

"The risk is that in the event of a major meteorological event such as a storm, there is a little more uncertainty about the strength of the winds or the expected time of arrival on the French coast," says Emmanuel. Bocrie. Fortunately, in Western Europe, "the weather situation since the start of the health crisis is relatively easy, we are experiencing a high pressure situation which reduces the risk of storms".

© 2020 AFP