Date: 201201
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Fog; 0.5
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer
Human factors: Communication Breakdown; Situational Awareness; Workload
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Weather; Airport; Human Factors
Primary problem: Human Factors

Narrative: I was the First Officer on this flight. Weather was forecast 4 SM BR SKC; with a TEMPO of 1 SM BR SCT 003. We were also advised not to arrive before Tower opening and adjusted our pushback and timing enroute to ensure this. We received updates enroute from Dispatch; initially indicating deteriorating conditions and suggesting a new alternate; but later forecasting improving conditions at our arrival. We checked current weather conditions a couple of times after this ourselves using ACARS and found that the visibility was fluctuating; but remained greater than or equal to the 1/2 SM needed for the ILS approach. Once we were within radio range; I listened to the ASOS (ATIS was not available yet; as the Tower was still closed). The observation indicated 1/2 SM FG BKN 003 and I wrote this on the landing data card. The Captain built and briefed the approach for the ILS. Shortly after this; Center gave us the latest weather and asked us which approach we wanted to set up for. I don't remember the exact visibility given at this point; but we briefly discussed whether it was good enough for the RNAV since the Controller had asked us which approach we wanted. We decided it was not and both agreed that we still had the visibility for the ILS and this is what we requested. At this point Center cleared us direct to the IF for the ILS; cross at 3;000 FT; cleared for the ILS approach.Shortly before turning to intercept the localizer; a different Center Controller informed us that ATIS information 'Yankee' was now available. I replied that we understood this to mean that the Tower was now open; which Center confirmed. I told the Controller that we would get information 'Yankee'; and then we were then cleared to contact the Tower. At this point we were established on the ILS approach and I checked in with the Tower Controller and informed him of our position on the ILS. Tower then cleared us to land; but did not report the visibility. I then flipped over to the ATIS frequency on the other radio to get information 'Y'. The weather I recorded was: 300/5; 1-1/4 SM; FG OVC 001 2999. I relayed this to the Captain and we continued the approach. During this time I was also complying with normal aircraft configuration changes requested by the Captain for the approach. We completed the appropriate checklists and were configured and stable per the FOM. The flight visibility was less than the 1-1/4 SM we were expecting at the DA; but was not less than the 1/2 SM required for the approach. The Captain was able to see the required visual references and safely landed the aircraft. As we completed the rollout the visibility seemed to worsen on the departure end of the runway. During taxi we heard an aircraft query Tower as to whether the visibility was still 'a quarter mile.' The Controller responded that the visibility was still 'one quarter mile.' This was the first point at which we realized what we thought was the reported visibility of 1-1/4 SM may have been reported at 1/4 SM. When I checked ATIS 'Y' on final approach; I heard the visibility as 'one and a quarter mile' and relayed this to the Captain. However; it may have actually been 'one quarter mile' - which is how the Tower Controller phrased it to the requesting aircraft after we were on the ground.We taxied to our ramp without incident. We received no communication from Tower at this point that he had any concerns with our flight. I did not listen to the ATIS again; as I was focused on monitoring the taxi and then became busy with post flight duties. I was confident at the time that I heard the visibility correctly and relayed what I thought I heard to the Captain. At no point during this approach did we willfully try and disregard any regulations. By the time I was able to retrieve and relay the ATIS we were established on the approach and may have intercepted the glide path as well. In hindsight; it would have been easier for me to simply ask Tower or Center to read information 'Y' tous. Given the high workload at this phase of the flight; this would have ensured that both pilots heard and agreed to the reported visibility. Digital ATIS would also have useful in this situation; but was not available at this airport. I could have requested the ASOS over ACARS to confirm; but would still have needed to listen to the ATIS to obtain 'Y'.




Synopsis: Air Carrier First Officer discovers after landing that the prevailing visibility is 1/4 mile instead of 1 and 1/4 miles as was thought to be what was broadcast on the ATIS.