weather balloon; sounding balloon
A balloon used for meteorological observation, carrying instruments to measure atmospheric conditions.
The meteorological agency launches weather balloons every day.
observation balloon; spy balloon
A balloon used for surveillance or reconnaissance, often in military or espionage contexts.
The enemy forces used an observation balloon to scout our position.
trial balloon
Figurative use: a tentative measure or statement to test public opinion or reaction, similar to the English idiom.
The government floated a trial balloon about a tax increase, but public backlash was strong.