Once had an HS125 envelop the whole of the runway and short finals in front of me into fog. I was following about 3 miles behind in a Puma. Conditions were clear starry night, 100% RH but no wind so therefore no mixing = no fog. The HS 125 provided the mixing and instantly the fog began to form and although I had the required visual referneces at decision height, when we got down to the runway we could only see one runway light ahead of us. I had to use the GPS to locate the turn off!
Also started up a Wessex at Valley early one morning in very similar conditions. The effect of the rotor disturbing the crisp, clear air was to fog out the whole of the dispersal area so that we could no longer see where to taxi. It took several hours to clear.
Beater, to quote VeeAny - you don't know what you don't know.
JJ