Radiosondes are small "weather stations" that are attached to weather balloons.
Radiosondes are equipped with a thermometer, humidity sensor, sometimes a pressure sensor, and nowadays, a GPS sensor for the position determination (and hence information about altitude, pressure, wind direction and wind speed). Consequently, radiosondes provide vertical profiles up to 30-35 km altitude of temperature, humidity, pressure, wind speed and wind direction. These data are transferred in real-time and high temporal resolution (every 1 second nowadays) to a ground receiving station at the launch site via radiowaves, hence the name "radio"sondes. A special frequency range of the radiowaves is reserved for these meteorological observations.
The radiosonde data from sites worldwide - currently about 1000 active sites - are transferred via the Global Telecommunications System (GTS) to the National Meteorological and Hydrological Services (like the RMI) and to consortia like the European Centre for Medium-Range Weather Forecasts (ECMWF) to provide the initial states of the Numerical Weather Prediction models. In other words, these data are assimilated in the weather forecast models. Because of the high vertical resolution of the radiosonde data, they are important for interpretation of the large scale meteorological and atmospheric dynamic conditions. They are also needed to locate temperature inversions and heights of low and high wind speed (low level jets, jet stream,...), and to characterise the conditions around cloud levels.
The Royal Meteorological Institute of Belgium carries out radio soundings by weather balloon launches at several locations. Regularly, three times a week in Uccle, including measurements of the vertical distribution of ozone (see ozonesondes), at the Belgian Princess Elisabeth station in Antarctica during the austral summer (see subpage here) and in the near future in Abuja, Nigeria.