A rawinsonde balloon (i.e. a radiosonde that is designed to measure various meteorological parameters) is released into the air. Radiosondes are tracked by radar or radio theodolite and measure atmospheric conditions (especially wind speed and direction) during ascent. The information is transmitted back to surface observers. In the background are surface weather instruments and a mobile radar unit.
[Image credits: NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL),