B727 flight crew concern regarding WX information report of visibility prior to departure.

Attributes
ACN	392230
Time
Date	199801
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : dsm
State Reference	IA
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tower : cak
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	climbout : takeoff ground other : taxi
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 78 flight time total : 9513 flight time type : 4070
ASRS Report	392230
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	other anomaly other other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	other

Narrative:

The WX had been lowering since our arrival from oma. As we prepared our paperwork for departure, the ATIS reported 1/4 SM and fzfg. We anti-iced and started engines. As we taxied to runway 31R, a new ATIS called the visibility 1/2 SM and fzfg. As we approached the runway the tower reported tower visibility to be less than 1/4 mi. We asked the tower if this was official runway visibility, but were told it was tower visibility. We needed 1/4 mi runway visibility for takeoff, so we told the tower we would wait a few mins as they said a new ATIS was coming up soon. A min or so later, a B757 landed on runway 31R. We asked the tower when the new ATIS would be up and they said the visibility is still reported at 1/2 mi. We saw the B757 land and roll out in front of us and the visibility down the runway was good (the fog had formed a low ceiling above the surface). So with that, we accepted takeoff clearance and made an uneventful takeoff. My concern is with the gray area there seems to be between official reported visibility and 'tower visibility' which can vary greatly especially in foggy conditions and in sits where there is a tall tower far from the runway or WX reporting facility.

Original NASA ASRS Text

Title: B727 FLC CONCERN REGARDING WX INFO RPT OF VISIBILITY PRIOR TO DEP.

Narrative: THE WX HAD BEEN LOWERING SINCE OUR ARR FROM OMA. AS WE PREPARED OUR PAPERWORK FOR DEP, THE ATIS RPTED 1/4 SM AND FZFG. WE ANTI-ICED AND STARTED ENGS. AS WE TAXIED TO RWY 31R, A NEW ATIS CALLED THE VISIBILITY 1/2 SM AND FZFG. AS WE APCHED THE RWY THE TWR RPTED TWR VISIBILITY TO BE LESS THAN 1/4 MI. WE ASKED THE TWR IF THIS WAS OFFICIAL RWY VISIBILITY, BUT WERE TOLD IT WAS TWR VISIBILITY. WE NEEDED 1/4 MI RWY VISIBILITY FOR TKOF, SO WE TOLD THE TWR WE WOULD WAIT A FEW MINS AS THEY SAID A NEW ATIS WAS COMING UP SOON. A MIN OR SO LATER, A B757 LANDED ON RWY 31R. WE ASKED THE TWR WHEN THE NEW ATIS WOULD BE UP AND THEY SAID THE VISIBILITY IS STILL RPTED AT 1/2 MI. WE SAW THE B757 LAND AND ROLL OUT IN FRONT OF US AND THE VISIBILITY DOWN THE RWY WAS GOOD (THE FOG HAD FORMED A LOW CEILING ABOVE THE SURFACE). SO WITH THAT, WE ACCEPTED TKOF CLRNC AND MADE AN UNEVENTFUL TKOF. MY CONCERN IS WITH THE GRAY AREA THERE SEEMS TO BE BTWN OFFICIAL RPTED VISIBILITY AND 'TWR VISIBILITY' WHICH CAN VARY GREATLY ESPECIALLY IN FOGGY CONDITIONS AND IN SITS WHERE THERE IS A TALL TWR FAR FROM THE RWY OR WX RPTING FACILITY.

Data retrieved from NASA's ASRS site as of July 2007 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.