B737 flight crew reported taking off with a mid-field RVR of 400 feet.

Narrative:

Preparing for takeoff from ZZZ rwy xxr with everything going just fine. It was a nice morning with great visibility and I had no reason to believe anything different. I watched other takeoffs as I taxied to the runway and saw no problems. As we finished the checklist and neared the hold short line we were cleared for takeoff with the following call; 'aircraft X; cleared for takeoff; midpoint RVR [runway visual range] 400.' as we rolled onto the runway I was saying; wait; what did he say? I looked down the full length of the runway seeing everything clearly all the way to the far end. There was no restriction to visibility at all. With that I assumed I either heard something wrong or it was a bogus call. The first officer and I both thought we were good for takeoff and continued. That was a mistake. The runway was perfectly clear; however; when we got to midfield our lights illuminated the fairly thick fog bank just beside the runway; obviously where the RVR equipment is placed. The call was midfield RVR 400 and I should have stopped; regardless of how unbelievable the call was; or how surprised I was by it. I was surprised enough by any RVR call at all that I didn't even think of it being below minimums. There had been absolutely zero visibility restrictions on the drive from the hotel or taxi out. I let the very last second nature of the information and the surprise of it distract me enough that we were airborne before I realized my error.

Original NASA ASRS Text

Title: B737 flight crew reported taking off with a mid-field RVR of 400 feet.

Narrative: Preparing for takeoff from ZZZ Rwy XXR with everything going just fine. It was a nice morning with great visibility and I had no reason to believe anything different. I watched other takeoffs as I taxied to the runway and saw no problems. As we finished the checklist and neared the hold short line we were cleared for takeoff with the following call; 'Aircraft X; cleared for takeoff; midpoint RVR [Runway Visual Range] 400.' As we rolled onto the runway I was saying; wait; what did he say? I looked down the full length of the runway seeing everything clearly all the way to the far end. There was no restriction to visibility at all. With that I assumed I either heard something wrong or it was a bogus call. The First Officer and I both thought we were good for takeoff and continued. That was a mistake. The runway was perfectly clear; however; when we got to midfield our lights illuminated the fairly thick fog bank just beside the runway; obviously where the RVR equipment is placed. The call was midfield RVR 400 and I should have stopped; regardless of how unbelievable the call was; or how surprised I was by it. I was surprised enough by any RVR call at all that I didn't even think of it being below minimums. There had been absolutely zero visibility restrictions on the drive from the hotel or taxi out. I let the very last second nature of the information and the surprise of it distract me enough that we were airborne before I realized my error.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1590047
Time
Date	201810
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Night
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Takeoff
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 338
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 587 Flight Crew Type 1000
Events
Anomaly	Deviation - Procedural FAR Deviation - Procedural Published Material / Policy