Two years ago I wrote an article on how a high altitude weather balloon flight can go wrong. I like to periodically update this article when I come across more ways that balloon flights can go wrong. Here I would like to include the ‘slow puncture’. During balloon inflation or manufacture, weather balloons can develop small holes that cannot be spotted during balloon inflation. If a weather balloon has a small hole then the lighter than air gas will obviously escape after launch. This may not become apparent until you have let go and notice that your balloon ascent rates are unusually slow. The general consequence is that as the balloon expands, gas will escape. This will lower ascent rates to the extent that it will probably not reach a point where is fails by bursting. Thus is will slowly and steadily ascend, then slowly and steadily descend.
I have had personal experience of this. I did a flight for a cosmetic company, launching a balloon close to Milton Keynes. The balloon ascended for 25 minutes then slowly descended for 25 minutes. We recovered it from a recently harvested field, bobbling about with the balloon partly inflated. This can be a real hazard, greatly increasing the chance of entanglement in power or telegraph lines.
I also have the experience of watching a flight from Warrington. Here the leak was much slower. The balloon ascended for well over an hour. It then slowly descended. By the time it reached ground level, it had drifted out into the North Sea; much further east than the forecast showed. This payload was lost.
There are few ways to overcome this problem. You can listen out for gas leaks after balloon inflation but may not be able to hear micro leaks. You can routinely use a balloon cut off such as Doongara; set to remove the balloon after a set period of time. This would have certainly helped avoid the total disaster that the flight from Warrington experienced.