noun
weather balloon; sounding balloon
A balloon used for meteorological observation, carrying instruments to measure atmospheric conditions.
気象庁は毎日観測気球を飛ばしている。
The meteorological agency launches weather balloons every day.
noun
A balloon used for surveillance or reconnaissance, often in military or espionage contexts.
敵軍は観測気球を使ってこちらの陣地を偵察した。
The enemy forces used an observation balloon to scout our position.
noun
trial balloon
Figurative use: a tentative measure or statement to test public opinion or reaction, similar to the English idiom.
政府は増税の観測気球を上げたが、世論の反発は強かった。
The government floated a trial balloon about a tax increase, but public backlash was strong.