A radiosonde is a small, expendable instrument package carried aloft by a weather balloon to measure vertical profiles of temperature, humidity, pressure, and wind from the surface to the stratosphere (typically 25–35 km altitude). It transmits data to a ground receiver via radio signal in real time. Radiosondes are the backbone of upper-air observation, providing the three-dimensional data that numerical weather models need to initialise their forecasts.
A typical radiosonde weighs about 250–300 grams and contains sensors for temperature (thermistor or thin wire), humidity (capacitive sensor), pressure (silicon-based), and a GPS chip for wind measurement (wind speed and direction are derived from the balloon's drift). The latex balloon inflates to about 1.5 m at launch and expands as pressure decreases with altitude until it bursts — typically at 30–35 km, where the pressure is less than 10 hPa. A small parachute slows the descent of the instrument.
The global radiosonde network consists of about 900 stations that launch simultaneously at 00:00 and 12:00 UTC every day — approximately 1,800 launches daily worldwide. This synchronised timing is essential because weather models assimilate all observations at the same analysis time. The data are plotted on skew-T/log-P diagrams (also called tephigrams) that meteorologists use to assess stability, identify inversions, calculate convective potential (CAPE), and predict thunderstorm severity. Despite advances in satellite remote sensing, radiosondes remain irreplaceable for their vertical resolution and accuracy.