radiosonde

A radiosonde is an electronic instrument package that hangs beneath a hydrogen- or helium-filled weather balloon. The instruments in a radiosonde are used to measure temperature, air pressure, and relative humidity as they float upward to a maximum height of 19 miles (30.6 kilometers). (Relative humidity is the amount of moisture in the air, expressed as a percentage of how much moisture are can hold at a given temperature.) Attached to radiosondes are radio transmitters, which continuously send upper-air readings back to weather stations on the ground.

Some radiosondes, called rawinsondes, emit a signal so their location can be tracked by radar on the ground. The path of a rawinsonde gives an indication of how wind speed and wind direction change with height.

Radiosondes in the United States contain parachutes, so they can gently drift back to the ground.