Date: 201302
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: 1/2
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Landing
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Weather; Airport; Human Factors
Primary problem: Ambiguous

Narrative: Twenty five to thirty minutes out we picked up our first ASOS report: 300 scattered 5;000 overcast 1 1/2 miles visibility; mist. Shortly after that; Dispatch called to report that the visibility had dropped to 1/2 mile but that it had improved. We also verified that our alternate; was still good. We monitored the ASOS as we were being vectored onto the ILS approach. The weather was changing rapidly. It went to 300 broken 3/4 mile visibility then 300 overcast 3/4 mile visibility then 200 overcast 3/4 mile visibility. Then; as we were intercepting the localizer; the report was 200 FT vertical visibility 3/4 mile visibility freezing mist. From the outer marker I could see the approach lights and most of the high intensity runway lights. During descent; as the angle of sight decreased; fewer runway lights were visible. With 3/4 mile visibility still being reported and a runway 2 miles long I was expecting to see about 1/3 of the runway at touchdown. From 200 FT (DH) I could clearly see the green runway end identifier lights and at least 4-5 runway lights on each side. This seemed to agree with the 3/4 mile report. In any case it was more than 1/2 mile. I continued the approach and called 'landing'. By the time we got into the flare for landing it was clear to me that we did not have 1/2 mile visibility; but by that point I believed it was safer to continue the landing rather than initiate a missed approach. The landing was firm with some sideways movement; but not as bad as some landings performed by some new first officers. As we continued down the runway the visibility continued to drop. We taxied to the end of the runway and turned off. By that time the visibility was so low that we could not see the terminal. It was not safe to taxi unescorted. We stopped; asked for; and received an airport pick up truck to guide us to the terminal. Even as we were taxiing in; the ASOS was still reporting 3/4 mile visibility.Do not operate in low visibility environments without real time visibility updates. This ASOS was reacting to and reporting the rapidly changing ceiling conditions but was not reporting the rapidly changing visibility conditions. This is not acceptable when conducting approaches to minimums; at night; at an uncontrolled airport. If I had to make an educated guess at what the RVR readings would have been if the airport had been equipped with RVR it would be 2;800-1;800-800.




Synopsis: B757 Captain reports landing at an uncontrolled airport with an ASOS reported visibility of 3/4 mile and an ILS required visibility of 1/2 mile when actual visibility may have been less than 1/2 mile. A vehicle is required to lead the aircraft to the gate.