Help understanding IFR WX mins

[eastern]

Other than the MDA, I've never understood IFR mins very well and observed a perfect example this morning. Our airport is located in a shallow valley and this morning was covered by fog generated by the small lakes that are also in the valley.

Before and after the airport, visibility was unlimited as the terrain is high enough that it was not affected by fog. However after entering the valley and driving by the airport, visibility was less than 1/2 mile and at times as low as 1/4 mile- However, flights continued to land the entire time I was in the valley-

Considering that visibility up to about 200 feet AGL was minimal, I they must have landed IFR- Can someone point me to the regs that cover such conditions?

Thanks!

[bookworm]

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.

[eastern]

Yes this is RDU- Sorry to leave out key info- Much thanks!