My recollection of Harvards at Ternhill in 1951 was that unrated students could fly solo provided the cloudbase was 800 ft minimum and visibility 500 yds but white rated could fly with 500 ft but needed a greater visibility, possibly 1000 or 1200 yds which even to my inexperienced eyes seemed all wrong. Of course the SBA needed to be serviceable and one would have been trained to use TBA. I can't remember anyone taking off to fly in those conditions though.