Translation guide
A small balloon used to measure atmospheric conditions, especially wind speed and direction at high altitudes.
A balloon released to observe upper-air winds, typically tracked by theodolite or radar.
Standard technical term for a pilot balloon used in meteorological observations.
測風気球を放球して上層風を観測した。
We released a sounding balloon and observed upper-level winds.
Loanword from English 'pilot balloon', commonly used in meteorological contexts.
パイロットバルーンの上昇速度から風速を計算する。
Wind speed is calculated from the ascent rate of the pilot balloon.
More explicit term meaning 'balloon for wind measurement', less common than 測風気球.
測風用気球は通常、水素ガスで満たされる。
Sounding balloons are usually filled with hydrogen gas.
In everyday conversation, 'sounding balloon' is rarely used. The Japanese terms are technical and mainly appear in meteorological or scientific contexts. If you mean a toy balloon, use 風船 (ふうせん).