Luftwaffe 'Vapour trail marking'?

[Bombphoon]

Luftwaffe 'Vapour trail marking'?

On p.82 of 'Battle of the Flames', David Needham's new book on the Nottingham Blitz, he states that on May 8th 1941, after KG100 and KG26 had located Nottingham with navigational beams, they generally marked their target for the main bomber force with flares or 'by ringing it with a vapour trail if visibility was good'.

He continues: 'At 23.37 hours, a Heinkel of KG26 approached the city from the south east. It was flying at a height of around 4,000 ft and it began to mark out a rectangular box in the sky by means of a vapour trail. The box extended from Tollerton to just past Trent Bridge and then ran down river to Colwick, enclosing the south-east part of the city. Air Raid Warden Salter was at his post in West Bridgford and he watched the aircraft marking out the box in the sky and then he noticed flares burning down the Trent Valley.'

Have you ever heard of this 'vapour trail marking'? Did it happen? How was it created? Are the eye-witnesses just confusing aircraft trails passing through smoke clouds and forming lines? If it did exist, how could Luftwaffe crews see it at night or distinguish it from other aircraft contrails etc? I have spoken to eye-witnesses who say that often the bombers criss-crossed over cities on their bombing runs to specific targets and I wonder if this, as a by-product, is what left 'vapour trail marking'-type patterns?

Thanks