The instrument packages the National Weather Service attaches to their balloons are called radiosondes. The term radiosonde is a contraction for radio-sounding device. These instruments measure the pressure, temperature, and moisture of the air through which they pass. The instruments also indicate wind direction by their changing position during the flight.
Each radiosonde transmits its data to a ground-based telemetry system (antenna and receiver). The telemetry system forwards the signals to another module (signal processing system) that decodes them into meteorological units. Data are then passed to a computer that generates a table of the results.
Air pressure decreases with altitude, so weather balloons grow larger and larger as they rise. When a balloon reaches its elastic limit and bursts, the radiosonde falls to the ground, slowed slightly by a small parachute. If you ever find a radiosonde, follow the instructions printed on the outside of the package to mail it back to the National Weather Service so it can be reconditioned and reused.
for more information on radiosondes.