Date: 202007
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Fog; 0.062
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Situational Awareness
Second reporter function: Pilot Flying; Captain
Anomaly: Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: Taxi
Result: General Flight Cancelled / Delayed
Contributing factors: Procedure
Primary problem: Procedure

Narrative: ZZZ Tower was closed for early morning departure. Visibility was fluctuating between 1/4 and 1/16. A takeoff alternate was needed and obtained from Dispatch. We determined that 1/4 visibility was needed to takeoff because the Tower was closed and automated weather did not report RVR. We pushed and taxied with the understanding that we would not takeoff until the visibility was 1/4 or better. While waiting at the end of the runway; Dispatch sent an ACARS message with RVR values.  As we discussed the legality of that information; the reported visibility improved to better than 1/4. We departed without incident.   I don't know of any errors that were made. If we can legally take off with Dispatch reported RVR; then we were only delayed an additional three to five minutes unnecessarily. There are obvious references to Dispatch weather in the manual especially for arrival weather.  However; it was not so obvious for this exact scenario.


Second narrative: The Crew arrived at the aircraft and prepared for departure. During that time we noticed the visibility dropping rapidly due to fog. The ASOS was varying between 1/16 and 1/4 miles of visibility. ZZZ Tower was closed during that time of day; and the ASOS weather is the only visibility report available. During the Release review I noted no takeoff alternate was given. I called the Operations Center; and a new Dispatcher had just taken over. Once brought up to speed he gave ZZZ1 as the alternate. We discussed the weather; fuel onboard; and departure on Runway 6. We decided due to the varying visibility we would taxi out and depart provided we had the visibility. Which at the time I believed 1/4 was my limitation. (At the time of the call the visibility was 1/4).The Release was amended and the new weather provided and we pushed normally. During the taxi out the field dropped to 1/16 SM with no RVR reported on the field. We taxied out single engine with 4;000 pounds of extra fuel and our intention was to wait until 1/4 was again reported. The clearance was already given by NY Center. We only needed the Center release. While waiting; we received an ACARS message from Dispatch with an RVR report for Runway 6. The RVR given would have allowed us to depart; but I believed we were required to have 1/4. Shortly after that another RVR report was sent with even greater RVR values. The field was improving rapidly. Confused by the intent of the message I called Dispatch and asked if we were able to accept dispatch given RVR over ACARS and be legal to depart (ASOS still reporting 1/16)?The Dispatcher responded with 'I don't see why not; but let me ask someone.' This was not the informed answer I was hoping for. While I was on hold; the ASOS then reported 1/4 SM. After the Dispatcher returned; I told him we had the required visibility; we were departing; and I would call for an explanation in ZZZ2. We departed normally and arrived 30 minutes late. However; due to another flight out of ZZZ2 I wasn't able to receive clarification on the matter until a few days later. The explanation from Dispatch was 'yes' they can provide the RVR through SWIFT and/or WSI if RVR is being reported on the intended runway; even though we as Flight Crew can only hear ASOS weather not reporting RVR. This seems to be a reasonable and legal answer. However; after reviewing the FOM specifically Chapter 17 and the exemptions. I was not able to find any guidance relieving me of the 1/4 SM restriction while operating at an uncontrolled field with no RVR reported on the field. I specifically looked in NGRVR and the EWINS section. I may very well have missed or misunderstood the information available or how that information was presented in this situation; and I would be grateful to know where to find this information. However; if it is not defined; but legal that information should be disseminated to the Pilot Group as well.


Synopsis: Flight Crew reported departing airport late due to lack of RVR values and below minimum weather.