LACHAUX (63) 09.12.2023

On 21 October 2022, 2 witnesses in Lachaux (63) noticed powerful distant flashes, generally to the north, on a surveillance camera's recordings. Similar flashes recurred on 8 November 2022 from 18:15 to 20:15; 8 March 2023 from 21:45 to 22:30; 9 May 2023 from 21:30 to 22:00; and 9 December 2023 from 19:30 to 21:30. The appearance was consistent across the sightings: white according to T2 and more orange according to T1, to the northwest, north or northeast. On 9 May and 9 December, they also saw the lights directly and T2 recorded them by phone. Neither the witnesses nor the surveillance system detected sound. Checks of French lightning-strike archives found storms at each observation date and time, generally approximately 100 km north of the witnesses (see the investigation report). GEIPAN therefore considered distant lightning, with thunder too far away to hear or record. Its lightning-and-thunderstorm specialist advised that: - Thunder is generally audible up to 30–40 km, and occasionally 70 km for the strongest strikes, shorter than the distances involved. - An elevated observer with an unobstructed view, as these witnesses had northward, can see distant landscape features illuminated by lightning, including treetops on the high hills along the Puy-de-Dôme–Allier boundary. Lightning itself can also be visible low on the horizon from 150–200 km, producing the powerful horizon flashes seen in the videos. - Such distant lightning is sometimes called heat lightning, which the specialist considered a misleading name because it is simply too far away for thunder to be audible. - The strike-recording system logs cloud-to-ground lightning but not intercloud flashes. Uncertainty remained for 8 March 2023: one storm lay westward, in the wrong direction, and another was described as too distant, at about 200 km. Conversely, on 9 May storms were less than 10 km away and should normally have been recognisable. Despite these qualifications, GEIPAN classified the case A: distant lightning.