You may not row in dense fog. For the club's purposes, dense fog means fog thick enough that you cannot see the flagpole in Higginson Park clearly from the clubhouse.
In any event, you should use your own judgement, as fog can be patchy. On weekend mornings, the coaches and safety officer will take the fog into account when setting the safety light status.
For those with a longer memory, please not the the fog rules have changed. The old fog rules were based on being able to see the white house and steamer jetty.