Radiosonde:
A radiosonde is a small weather station coupled with a radio transmitter. The radiosonde is attached to helium- or hydrogen-filled balloon, called variously a weather balloon or a sounding balloon, and the balloon lifts the radiosonde to altitudes exceeding 115,000 feet. During the radiosonde’s ascent, it transmits data on temperature, pressure, and humidity to a sea-, air-, or land-based receiving station. Often, the position of the radiosonde is tracked through GPS, radar, or other means, to provide data on the strength and direction of winds aloft. Thus the radiosonde flight produces a vertical profile of weather parameters in the area above which it was launched. [Radiosonde Museum of North America]
Noted in this xkcd comic. This image is from Radiosonde Museum of North America.
