Bob Ford describes this amazing sighting:
Impossible to count how many, but on one scan across a single field I counted 100 individuals in flight and settled. And there were two more fields like this! All were feeding on knapweed, not dispersing. All were fresh-looking. No helice females were seen at all.
If you see a Clouded Yellow on the wing, they are very distinctive – a mustard yellow, quite different from the bright lemon yellow of the Brimstone.