Pilot has near midair collision with WX type balloon, apparently inadvertently released from ground.

Attributes
ACN	112810
Time
Date	198906
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : rst
State Reference	MN
Altitude	agl bound lower : 1500 agl bound upper : 1500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : ilg
Operator	general aviation : personal
Make Model Name	Small Transport
Flight Phase	cruise other
Flight Plan	None
Aircraft 2
Make Model Name	Experimental
Flight Plan	None
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : atp
Experience	flight time last 90 days : 40 flight time total : 3200 flight time type : 20
ASRS Report	112810
Events
Anomaly	conflict : nmac
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 100 vertical : 100
Supplementary
Primary Problem	Ambiguous

Narrative:

Sightseeing pleasure flight in an small transport X over north side of rochester about 1500 AGL. Suddenly large balloon (later determined to be a WX balloon spc with something dangling several hundred feet below it) appeared at 12 O'clock high. Took evasive maneuver down and to the left when second balloon appeared below and to the right. This one was moored and I assume the one we missed came loose from its mooring. Second pass by confirmed these were WX type balloons and not carrying passenger. Suggest these balloons be reported to tower by ground operators when they leave their mooring and become hazard to aviation.

Original NASA ASRS Text

Title: PLT HAS NMAC WITH WX TYPE BALLOON, APPARENTLY INADVERTENTLY RELEASED FROM GND.

Narrative: SIGHTSEEING PLEASURE FLT IN AN SMT X OVER N SIDE OF ROCHESTER ABOUT 1500 AGL. SUDDENLY LARGE BALLOON (LATER DETERMINED TO BE A WX BALLOON SPC WITH SOMETHING DANGLING SEVERAL HUNDRED FEET BELOW IT) APPEARED AT 12 O'CLOCK HIGH. TOOK EVASIVE MANEUVER DOWN AND TO THE LEFT WHEN SECOND BALLOON APPEARED BELOW AND TO THE RIGHT. THIS ONE WAS MOORED AND I ASSUME THE ONE WE MISSED CAME LOOSE FROM ITS MOORING. SECOND PASS BY CONFIRMED THESE WERE WX TYPE BALLOONS AND NOT CARRYING PAX. SUGGEST THESE BALLOONS BE REPORTED TO TWR BY GND OPERATORS WHEN THEY LEAVE THEIR MOORING AND BECOME HAZARD TO AVIATION.

Data retrieved from NASA's ASRS site as of August 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.