A radiosonde is a small, lightweight package of meteorological instruments that measure temperature, pressure, humidity, and winds in the upper atmosphere. The package of instruments is attached to a helium filled free-flying balloon comprising an aneroid barometer and sensors for temperature (using the so-called "Thermocap" temperature sensor), humidity and a radio transmitter.
The balloon climbs to a height of 20-30 km above mean sea level at about 5 m/s sending information and data at intervals from these sensors to ground stations by transmitter, whereupon it bursts, returning the equipment to the ground by means of a small parachute. Other versions may report pressure alone (windsonde) or ozone concentration (ozonesonde).
The position of the balloon is tracked by radar and from its changes in position the wind velocities can be calculated. Then the sonde must carry a radar reflection shield in order to reflect the beam emitted by the radar. Sometimes a radiosonde is equipped with a GPS (global positioning system) or a radio receiver to detect LORAN-C signals and is then also known as rowinsonde.
Upper air data is reported up to four times per day at the synoptic hours of 00, 06, 12 and 18 GMT. The UK upper air network consists of six operational stations which release four radiosondes a day and four range stations which release sondes at irregular intervals according to the daily requirements of the army ranges. Such ascents occur predominantly during the morning. The six main radiosonde stations in the UK are Camborne, Lerwick, Albemarle, Watnall, Castor Bay and Herstmonceux. Currently, the global radiosonde network includes about 900 upper-air stations, and about two-thirds make observations twice daily (at 0000 and 1200 UTC). The network is predominantly land-based and favours the Northern Hemisphere.
