If it's a standard radio sonde type balloon then it's made from latex rubber, like a giant condom. On a length of line they carry a small pack of disposable instruments with a note on it asking the finder to dispose of thoughtly, and that The Met Office doesn't want it back!
I cannot imagine the 'boys in blue' chasing one.
Now, if it's a barrage type balloon that's broken free, step forward the first person who can quote verbatum from MATS Pt 1 'Action When a Balloon Breaks Free of its Moorings'. . .
[This message has been edited by Lew Ton (edited 18 November 2000).]
[This message has been edited by Lew Ton (edited 18 November 2000).]