Assuming your valley was in the area you cite in your signature, try
14CFR 91.175
c) Operation below DH or MDA. Where a DH or MDA is applicable, no pilot may operate an aircraft, ..., at any airport below the authorized MDA or continue an approach below the authorized DH unless --
(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers,...;
(2) The flight visibility is not less than the visibility prescribed in the standard instrument approach being used; and
(3) ..., at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:
(i) The approach light system,... [other lights follow in (ii) to (x)]
Unlike the situation on this side of the Pond, I believe the assessment of flight vis is entirely up to the pilot for Part 91 ops.