A BE400 pilot reports the laf ASOS reports visibility lower than actually exists.

Attributes
ACN	690500
Time
Date	200603
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : laf.airport
State Reference	IN
Altitude	msl single value : 2300
Environment
Flight Conditions	IMC
Weather Elements	Rain Fog
Light	Night
Aircraft 1
Controlling Facilities	artcc : zau.artcc tower : lax.tower
Operator	general aviation : corporate
Make Model Name	Beechjet 400
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 100 flight time total : 19000 flight time type : 2700
ASRS Report	690500
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	inflight encounter : weather other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted
Supplementary
Problem Areas	Weather Airport
Primary Problem	Airport
Situations
Airport	other physical facility

Narrative:

We were returning to laf. We were flying in rain and considerable lightning. The XA00 WX from the ASOS reported 700 ft broken and 1 1/4 mi visibility in moderate rain and thunderstorms. Wind was less than 10 KTS from the wnw. I was planning on a possible approach to minimums; given the ASOS. However; we could clearly see the runway from 2300 ft MSL 7-8 mi west of the airport. We landed with no problems and were able to see the runway all the way down the GS; even though the ASOS was still reporting 700 ft broken and 1 1/4 mi visibility in moderate rain and fog/mist. This seems to be a very large error in reporting the existing conditions. It seems this ASOS consistently reports visibility lower than actually exists.

Original NASA ASRS Text

Title: A BE400 PLT RPTS THE LAF ASOS RPTS VISIBILITY LOWER THAN ACTUALLY EXISTS.

Narrative: WE WERE RETURNING TO LAF. WE WERE FLYING IN RAIN AND CONSIDERABLE LIGHTNING. THE XA00 WX FROM THE ASOS RPTED 700 FT BROKEN AND 1 1/4 MI VISIBILITY IN MODERATE RAIN AND TSTMS. WIND WAS LESS THAN 10 KTS FROM THE WNW. I WAS PLANNING ON A POSSIBLE APCH TO MINIMUMS; GIVEN THE ASOS. HOWEVER; WE COULD CLRLY SEE THE RWY FROM 2300 FT MSL 7-8 MI W OF THE ARPT. WE LANDED WITH NO PROBS AND WERE ABLE TO SEE THE RWY ALL THE WAY DOWN THE GS; EVEN THOUGH THE ASOS WAS STILL RPTING 700 FT BROKEN AND 1 1/4 MI VISIBILITY IN MODERATE RAIN AND FOG/MIST. THIS SEEMS TO BE A VERY LARGE ERROR IN RPTING THE EXISTING CONDITIONS. IT SEEMS THIS ASOS CONSISTENTLY RPTS VISIBILITY LOWER THAN ACTUALLY EXISTS.

Data retrieved from NASA's ASRS site as of January 2009 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.