AN ACR B727 HAS AN NMAC WITH A WX BALLOON AT FL310. THERE WAS NO ADVISORY AND NO TIME FOR AVOIDANCE ACTION.

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VERY SIMPLY; A WX BALLOON PASSED EXTREMELY CLOSE BY OUR COCKPIT WINDOW. IT APPEARED SO FAST; THERE WAS NO TIME TO REACT. WE NOTIFIED CTR ABOUT THE INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT ATC DID NOT KNOW OF THE WX BALLOON IN THEIR AREA AND THUS COULD NOT GIVE ANY ADVISORIES. THE AREA WAS NEAR THE WX FRONT ASSOCIATED WITH THE STORM SYS GOING THROUGH THE S. THE BALLOON WAS APPARENTLY SENT UP TO RETRIEVE SOME OF THAT INFO. HAD THE WX NOT BEEN VMC AT THEIR CRUISE ALT THEY NEVER WOULD HAVE SEEN THE BALLOON. ALL THE RPTR SAID WAS THAT IT WAS VERY CLOSE AND FRIGHTENING TO SEE WHEN THE CONSEQUENCES OF A MIDAIR WERE CONSIDERED. THEY ALL FELT 'VERY LUCKY' TO HAVE MISSED IT. RPTR WONDERS WHY THE BALLOONS CAN'T BE FITTED WITH AND GIVEN A XPONDER CODE ASSIGNMENT TO COMPLY WITH AN ALT RPTING REQUIREMENT/MODE C. RPTR SAID; 'THE SKIES ARE GETTING MORE CROWDED; IT'S ONLY A MATTER OF TIME WITH THESE THINGS.' RPTR STATED THAT THE PLT'S UNION WILL BE CONTACTED WITH REF TO THESE EVENTS.