Fog can develop and/or move very quickly . Starting an approach in deteriorating waether can end in landing in fog . The other day i tried to get out of my house as the fog was coming in ....as i started it was clear with fog around 500m away ...as i started to lift it came over me and i had to land again and abort ....all in 2 -3 minutes . If you were not in the heli you dont know the circumstances . There are a lot of armchair experts here who like nothing more than to slag off other pilots . Sad but true .