Love that episode BTW.

Anyway, they are Barrage Balloons. Barrage Balloons were designed to clutter up airspace, and to force bombers and fighters to fly higher, or to discourage low altitude attacks. Not only were the balloons themselves an obstacle, but so were the steel cables supporting them. By raising and lowering them to different altitudes, and placing them around important targets, the ability of enemy aircraft to engage at optimal altitudes or locations was reduced.