What Is a Weather Balloon? The Measurement Forecasts Are Built On

The package hanging below the balloon is called a radiosonde and is deliberately cheap, since it is not recovered:

• •A temperature sensor, typically a thermistor or fine wire, which must respond quickly and resist solar heating error
• •A humidity sensor, historically the least reliable component and much improved by thin-film capacitive designs
• •A pressure sensor, or pressure derived from satellite-determined altitude in modern units
• •A satellite navigation receiver, which gives position continuously, and from the drift of that position the wind speed and direction at each level are calculated, since the balloon moves with the air
• •A radio transmitter sending data continuously to the ground station
• •A balloon of latex or synthetic rubber filled with hydrogen or helium, which expands as pressure falls until it bursts, typically at around thirty kilometres, and a small parachute to slow the descent

The worldwide network launches at fixed universal times, principally midnight and midday, which means stations release at local times ranging from before dawn to mid-afternoon depending on longitude. The reason is that a weather model needs a snapshot of the atmosphere at one instant to start from, and measurements taken at different times would describe different states that cannot be assembled into a consistent picture. That coordination is maintained by international agreement and has run for decades, producing one of the longest consistent global datasets in any science, which is what makes it valuable for detecting climate trends as well as for forecasting. The resulting profile is called a sounding, and the standard way of displaying it, a thermodynamic diagram plotting temperature and dew point against pressure, allows a forecaster to read off stability, cloud base, the likelihood of thunderstorms and the depth of the layer in which air will rise, which are the quantities that determine what the weather will do.

Satellites observe the atmosphere continuously and globally and measure it differently. A satellite sounder infers temperature and humidity from radiation emitted in particular wavelength bands, which returns information averaged over deep layers rather than a sharp vertical profile, so fine structure including shallow inversions and sharp moisture boundaries is smoothed out, and those features frequently determine whether fog forms or a thunderstorm develops. Radiosondes measure directly at high vertical resolution, which is why they remain the reference against which satellite retrievals are calibrated and validated. The two are complementary rather than competing, and modern forecasting assimilates both alongside aircraft measurements, ground stations, radar, ships and buoys. The gaps are geographic: the network is dense over land in wealthy regions and sparse over oceans, Africa and polar regions, and reduced launches at under-resourced stations measurably degrade forecast quality downstream, which was demonstrated when pandemic disruption to aircraft reports and some launches was shown to have reduced accuracy.

Roughly a thousand radiosondes are launched daily worldwide and almost none are recovered, so the practice releases a substantial quantity of electronics and plastic into the landscape annually, which is a genuine if minor environmental criticism and has prompted work on biodegradable components and recovery incentives. Each carries a label asking finders to return it, and return rates are low. The balloons themselves burst into fragments that persist. Hydrogen is used for filling in many places because it is far cheaper than helium and it is flammable, which requires careful handling and has caused accidents. Beyond the operational network, balloons carry scientific payloads for ozone measurement, aerosol sampling and atmospheric chemistry, and high-altitude ballooning has become an accessible amateur and school activity, with cheap trackers and cameras reaching the stratosphere for modest cost, which requires aviation authority notification and produces the familiar photographs of a curved horizon against black sky.

Radiosondes carried by balloons measure temperature, humidity and pressure directly while rising, with wind derived from how the balloon drifts, and they burst around thirty kilometres up. Launches happen at the same universal times worldwide so that models can start from a consistent snapshot. Satellites cover the globe and infer conditions averaged over deep layers, so they cannot resolve the sharp features that decide fog and thunderstorms, which is why direct soundings remain the calibration reference.