On the evening of the 20th March 1918 a thick mist began to settle across the ground occupied by the 3rd and 5th British Armies, their trenches stretching from Arras to the Oise River.
As the night settled in it became so thick that the routine patrols sent out to probe the German lines failed to reveal a great deal apart from two things: all their prisoners were very eager to be out of the way and they represented a lot of different regiments.
At 0440 hours Colonel Bruchmüller's terrifying bombardment began. To sow the seeds of doubt in the allies mind as to where the main point of the attack would be made almost the entire front line from Flanders down and into Champagne (French held territory) was pounded.
Shortly before 0700 hours the bombardment to the north on the British First Army's front slackened and it was remarked that on the old Cambrai battlefield around Flesquières the ground had been saturated in mustard gas. Whilst lethal, it is also a gas that lingers so its presence indicated that the Germans had little intention of trying to attack in the area.
The communications systems in the Fifth Army's area were particularly badly disrupted with shell patterns landing on cable junctions with formidable accuracy. It was remarked that the craters were evenly spread and touched each other.
The sun rose, but only as a pale glow behind the fog, which remained as dense as ever in front of General Gough's Fifth Army
It was by now obvious to the British General Staff where the attack was going to be delivered and both Gough and General Byng (Third Army) to his north ordered their men to stand to. The soldiers deployed to their positions in the Forward Zone but were wearing gas masks and staring out into an impenetrable white landscape.
In accordance with Colonel Bruchmüller's carefully considered plan the number of gas shells was slowly diminished in favour of high explosive and the Forward Zone increasingly became the subject of the bombardment. This eventually reached a crescendo of shell and trench mortars around 0930 hours.
Even whilst the shelling had been taking place German engineers had been out ensuring that the British wire was cut.
At 0940 hours and like phantoms in the mist the Stormtroopers commenced their attack. They found that much of the British front line was obliterated with none or few survivors, but the redoubts in front of St Quentin had weathered the storm and were in a better state to try and defend themselves.
The problem for the British was: defend themselves against whom and from which direction. They fired off distress flares but these were never seen - absorbed by the mist. Communications were all but non-existent and even the pigeons had been gassed.
In general it would be true to say that the fog was at its thickest near the Oise and slowly thinned as the front progressed towards Arras. Around St Quentin it would be well after midday before the RFC could see anything from above.
In places the visibility was down to 10 metres and the defenders were operating on fixed lines of fire - in other words the enemy are supposed to be coming from that direction so that is the field that we fire into.
Unfortunately the Stormtroopers were not operating to such a plan, making the most of the fog they flowed around any redoubt that put up any opposition.