weather balloon 
noun
A balloon equipped with meteorological apparatus which is sent into the atmosphere to provide information about the weather.The launch team had noted what NASA describes as ‘marginal’ wind conditions during the countdown, and wanted to double-check the data against information from a final weather balloon....
- On arriving there Peter and Chris got out of the vans and sent up a weather balloon to watch the wind direction.
- The Port Elizabeth weather office said there wasn't a weather balloon in that area and no calls reporting UFOs had been received.