US Team report indicates the concern was based on visibility during final glide...

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Things seemed to settle down at a “grim but acceptable” level and pilots proceeded with their tasks, finding good lift to around 9000’ in a few areas, and occasionally struggling to reach 6000’. Our local expert pilot and Task Setter – Bruce Taylor – launched as a scout. His initial report of 20+ km visibility was encouraging. But around 3:30 he encountered troublesome smoke and haze – bad enough to make final glides and finishes potentially dangerous. His advice, various forecasts and other input led (shortly before 4pm) to the decision to cancel the task. It was a serious disappointment to many pilots, most of whom had been racing hard in challenging conditions for 3 hours and more. They now had to fly home – up to 140 km, through the thickening smoke – with their efforts rendered meaningless.