Article:Strange Black Line Photographed In Folkestone, Kent on May 10, 2023 at 8:05am.
The witness stated. "The building at the bottom of the black line is eurotunnel and when I saw it, it lasted about 4 mins."
This looks like a contrail shadow to me. An "edge shadow" where the viewer is in-line with the contrail and the sun, both of which are likely behind the camera. Discussed in more detail here:
Local Weathermen also thought a shadow, but seemed unfamiliar with this type:
Article:Mike Clay, chief meteorologist at the local station Bay News 9, was also left stumped by the unusual black line shooting down from a cloud.
The weatherman shared the pictures with some of his colleagues, one of whom came up with a possible explanation, suggesting that the dark line could be the shadow of a contrail.
Contrail is the condensation trail left behind jet aircraft. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapour pressure and low temperature.
For a contrail shadow to be seen, atmospheric conditions must be just right, with the sun hanging low on the horizon.
That not entirely true, as the sun can be quite high. You can get contrail shadows any time of the day.
We can geolocate the viewpoint:
Giving us a line of sight to Chunnel building:
Almost due west, consistent with the morning sun?
Date is May 10, 2023 at 8:05am (BST) 0705 UTC (0005 PDT, my timezone for Suncalc)
A perfect match for the line of sight.
Finding the plane needs only one that leaves a portion of its contrail aliged with the sun. The straight line of the shadow is not the exact shadow the contrail, but rather a "ray" of shadow viewed edge on. Given the time, it is likely a US to Europe plane, of which there are several. The track of the plane is not the same as the trace of the contrail, which. usua gets blown off by up to 10°, so a possible plane is Eurowings 69, which did go directly over Folkstone
Reported wind from 218, SSW, which is consistent with the difference in contrail and ground track