What will the weather be like next weekend? Between the end of the curfew and the start of summer, everyone is seized with a furious desire to squat the terraces and go for the weekend. And under the sun so much to do! So, we frantically look at the weather apps to see if the gods of good weather are with us. Except that having a precise idea of the weather will be in a few days seems much more complicated since the start of the pandemic.
Why ? Because planes are real flying weather stations, which collect valuable data used by forecasters. By causing the drop in global air traffic, the coronavirus has seriously disrupted their work. Will the gradual resumption of commercial flights then mark the return to more reliable weather forecasts?
Planes, “a mine of information for forecasters”
Pressure, temperature, wind… Meteorologists therefore use measurements taken by aircraft instruments to establish their forecasts, Météo-France indicates. This is the AMDAR system, from theWorld Meteorological Organization (WMO), which collects this data and transmits it to ground stations. AMDAR transmits daily more than 800,000 observations on air temperature, as well as wind speed and direction. All this thanks to a network of “43 airlines and several thousand planes”.
On each aircraft in flight, “measurements are taken every ten seconds during take-off or landing,” Météo-France describes. This high frequency is necessary to properly describe the different layers of atmosphere crossed by the airplane. In cruising flight, around 10 kilometers altitude, the frequency drops to one measurement every 2 to 3 minutes ”. All of this data “feeds into numerical prediction models or is used for nowcasting.” In short, “planes are a mine of information for forecasters around the world, public or private. The data they collect are used to establish all of our forecasts ”, explains Paul Marquis, independent meteorologist and founder of the site. E-Meteo Service.
Up to 3 days of loss of reliability
But that was before global air traffic fell by 75 to 90%. “When the pandemic broke out, many discovered the determining role of information collected by civil aviation in the preparation of weather forecasts,” confides Paul Marquis. With the traffic collapse, we lost 2 to 3 days of reliability on our 10-day forecasts, which has serious consequences for us, who used to make very reliable forecasts for several days. For more than a year, it has been difficult to make forecasts that are more than 90% reliable beyond 5 days ”.
And sometimes even in the shorter term. “This spring has been particularly difficult, our forecasts could be corrected at 24-48 hours, or even the same day,” continues Paul Marquis. Not only because we are still missing aviation data, but also because this year the period from late March to early June was critical due to very unstable weather conditions. We lived the hottest second half of June ever recorded in France, points out the meteorologist, with anarchic and random depressions, translated by violent thunderstorms ”. In this case, “satellites and radars allow us to know in real time where the precipitation and thunderstorms are, and to calculate their immediate trajectory”.
Different tools to compensate
To establish the forecasts for several days, “we use the data collected by the stations distributed more on the mainland. However, the oceans represent more than 70% of the surface of the Earth, which means much less data ”, underlines Paul Marquis. But forecasters are not completely helpless by the fall in air traffic, and can count on already existing tools to compensate. Starting with the “space component of the Observation System”, reassures the WMO, which recalls that “30 meteorological satellites and 200 research satellites transmit observations continuously”. In addition, there are “more than 10,000 surface weather stations, 1,000 aerological stations, 7,000 ships, 100 moored and 1,000 drifting buoys, hundreds of weather radars as well as 3,000 specially equipped commercial aircraft (which) daily measure key relative parameters. to the atmosphere, to land and to the surface of the oceans ”.
In addition, Météo-France indicates that it has doubled the number of its radiosondes. Weather stations send weather balloons into the atmosphere to collect temperature, humidity and wind data at different altitude levels. Then the forecast models integrate these measurements into their calculations: “the nature and movement of air masses are thus better known, cloud layers and areas of turbulence or air instability and stability are better localized. “.
For his part, Paul Marquis “works 2 to 3 hours more per day, especially since human analysis of raw data is always essential to smooth forecasts according to local particularities. Normally, we update our forecasts once or twice a day, but since the pandemic, we are rather at 3 or 4 daily bulletins ”.
More reliability expected this summer
Forecasts watched by the meteorologist’s clients. Individuals, “farmers and winegrowers to protect their crops, town halls to anticipate crisis management in the event of storms or heavy rainfall, and event organizers who are resuming their activity”. Who all rely on the most reliable forecasts possible. And therefore on a resumption of air traffic. However, “it is estimated that the level of 2019 could be reached between 2024 and 2027”, predicted in May the CEO of Aéroports de Paris, Augustin de Romanet.
“Green at heart, the meteorologist that I am still impatiently awaits for air traffic to resume,” says Paul Marquis. Because already, since the beginning of June [marqué par l’assouplissement des restrictions sanitaires et une petite reprise du trafic aérien], we regained 24 hours of reliability over the two to three days that we had lost ”. A horizon accompanied by clearings: “In July-August, we generally have better visibility in the longer term, with the return of the Azores high, which allows us to know that it will be sunny and warm for about ten years. days. Between that and the increase in air traffic during the summer holidays, this should make us gain even more reliability ”.
However, if the WMO recognizes that data from airplanes is “one of the main elements” for forecasting the weather, it also recalls that they can “disappear” depending on the circumstances (here the pandemic). Hence his call to “have additional systems, (…), even when the Covid-19 crisis will be a thing of the past”.