The Radiosonde Network: The Twice-Daily Data Launch Behind Every Winds Aloft Forecast
Every winds aloft forecast pilots rely on traces back to a global network of 900 balloon-launched radiosondes that measure the atmosphere twice daily from surface to near-space.
Twice a day, at 0000 UTC and 1200 UTC, roughly 900 weather stations around the world simultaneously release helium-filled latex balloons carrying small instrument packages into the atmosphere. Those balloons climb to altitudes above 100,000 feet, transmitting continuous atmospheric measurements before bursting and drifting back to the surface. Every winds aloft forecast, icing outlook, and turbulence prediction in a preflight briefing is built on that data.
What Is a Radiosonde and How Does It Work
The instrument attached beneath each balloon is called a radiosonde - a name combining radio transmission with the French word for probe. The package weighs roughly 300 grams, about the weight of a large apple, and contains four sensors: temperature, humidity, pressure, and GPS.
During flight, the radiosonde transmits continuously on a frequency around 400 MHz. A ground station antenna tracks the signal and records the complete data stream as the balloon climbs. GPS provides precise altitude, position, and wind data at every level of the ascent.
The balloon itself starts at approximately one meter in diameter at launch. As the balloon rises and atmospheric pressure drops, the helium expands. By the time it reaches the upper stratosphere - between 30 and 35 kilometers, or roughly 100,000 to 115,000 feet - the balloon has grown to six or eight meters across. Then it bursts. A small parachute deploys, and the radiosonde drifts back to earth.
Most are never recovered. At roughly $200 per unit, with approximately 90 launches per day across the United States alone, that is significant hardware falling into cornfields, forests, and the ocean every 24 hours. The National Weather Service considers this an acceptable operating cost for the data produced.
Why the Launch Times Are Synchronized Globally
The simultaneous launch times - 0000 UTC and 1200 UTC - are not arbitrary. They represent a global agreement coordinated through the World Meteorological Organization. At those two moments, weather services on every continent launch simultaneously, creating a consistent atmospheric snapshot across the entire planet.
That synchronized measurement campaign has been running, in continuously updated form, since the 1930s. The first radiosondes were developed independently in France and the Soviet Union in the early part of that decade. Robert Bureau in France is credited with the term and the basic concept of a radio-transmitting meteorological balloon. Pavel Molchanov in the Soviet Union developed a parallel system around the same time. The United States began operational launches in 1937. The underlying concept has not changed fundamentally in nearly 90 years, though sensors, transmission systems, and GPS tracking are vastly more capable than the original designs.
Why Satellites Alone Cannot Replace Balloon Soundings
Satellites are extraordinary instruments for measuring cloud tops, sea surface temperatures, water vapor in broad columns, and outgoing radiation. But they measure the atmosphere from above - which makes them better at detecting horizontal variation than vertical structure.
A satellite cannot tell you precisely what the temperature is at 12,000 feet over Wichita versus at 14,000 feet over Wichita. The radiosonde can.
This vertical profile of the atmosphere - technically called an upper-air sounding - is the primary input for initializing numerical weather prediction models. Every time the National Weather Service runs its Global Forecast System, every time the European Centre for Medium-Range Weather Forecasts runs its model, the initial conditions are drawn substantially from radiosonde soundings taken hours earlier. The models apply the physics equations governing atmospheric motion to those observations and project the atmospheric state forward in time.
The forecast you check before a flight is the output of that process. The radiosonde launched hours earlier is the starting point.
How Sounding Data Feeds the Specific Forecast Products Pilots Use
Forecast Winds and Temperatures Aloft - the product your flight planning software shows when you check winds aloft - is generated by National Weather Service numerical models. The accuracy of that forecast is directly tied to how well the model’s initial conditions match the actual atmosphere. In regions with dense radiosonde coverage, the model initialization is tighter and the forecast tends to verify better. Over the open ocean, where launch sites are few or nonexistent, the model relies more heavily on satellite data and aircraft measurements, and forecast uncertainty grows.
The Forecast Icing Product, maintained by the National Center for Atmospheric Research in partnership with the FAA, uses model output calibrated against radiosonde measurements of temperature and moisture. Icing requires a specific combination of conditions - supercooled liquid water at temperatures below freezing. Identifying where those conditions exist at altitude requires knowing the actual vertical temperature and moisture profile. Surface observations alone cannot provide that. Sounding data is central to building that picture.
The Graphical Turbulence Guidance (GTG) product forecasts turbulence using model-derived atmospheric parameters including wind shear and temperature gradients. Radiosondes directly measure wind direction and speed at every altitude throughout the ascent, and those measurements reveal the shear layers and temperature inversions associated with clear-air turbulence. The models interpolate between sounding locations to build a continuous spatial forecast.
When your avionics or weather app displays a turbulence forecast and you are deciding whether to request a different altitude, the chain of evidence behind that recommendation starts with a balloon launched hours earlier from a station hundreds of miles away.
Understanding the Honest Limitations of the Sounding Network
Launch sites in the continental United States are spaced an average of 300 to 400 kilometers apart. In the western mountain states, where complex terrain creates significant weather variability over short distances, some areas are farther still from the nearest launch site. The model bridges those gaps through mathematical interpolation - which it does well in stable, slowly evolving weather regimes, but with more uncertainty in rapidly changing situations, particularly convective environments where conditions vary over tens of kilometers rather than hundreds.
The products are most accurate within a few hundred kilometers of a sounding station and within the first six hours after the observation cycle. That reliability degrades progressively with distance and time elapsed.
Practically: a winds aloft forecast for a morning departure, initialized from the 0000 UTC sounding taken overnight, is working with relatively fresh data. A forecast for a late afternoon departure still running off the same morning sounding - 12 or more hours after data collection - has had more time for the atmosphere to evolve away from the model’s assumptions. Worth factoring into any altitude choice on longer cross-countries over complex terrain.
How to Access the Raw Sounding Data Yourself
The University of Wyoming Department of Atmospheric Science maintains a publicly accessible archive of radiosonde soundings. Any launch site in the world, any date and time, returns the actual measured vertical profile plotted on a thermodynamic diagram called a Skew-T log-P diagram.
Reading a Skew-T is a skill beyond standard pilot training, but even basic familiarity - where the freezing level sits, where moisture layers exist, where wind shear concentrates - gives a direct view of the raw data behind the forecast products you rely on. When shooting approaches in complex winter weather and trying to understand why the icing forecast concentrates the heaviest accumulation at a specific altitude, the nearest sounding is one of the most direct answers available. It is source data, not a derived product.
How Aircraft Feed Data Back Into the Forecast System
Modern commercial aircraft equipped with data link communications - primarily through the Aircraft Communications Addressing and Reporting System (ACARS) - automatically transmit atmospheric measurements during flight. Air data computers sense temperature and pressure. GPS provides wind data. That information is downlinked and fed into forecast models in near real time.
The program managing this in the United States is called Aircraft Meteorological Data Relay (AMDAR). The World Meteorological Organization coordinates the global version. Over the continental United States, thousands of AMDAR observations are generated daily. Over the ocean, where radiosondes are rare, reports from transoceanic flights are sometimes the only source of in-situ upper-air data over vast stretches of water.
Researchers have studied what happens to forecast accuracy when AMDAR data is withheld from model initialization. The degradation is measurable, particularly over oceanic regions. When major winter storms reduce commercial flight operations across a region, the simultaneous reduction in AMDAR reports shows up in model verification scores for the affected area hours later.
Why Pilot Weather Reports Still Matter
PIREPs are the direct human contribution from every pilot in the air. A report of moderate turbulence at 7,500 feet over the mountains, or rime icing encountered on an ILS approach, goes to the Aviation Weather Center (operated by NOAA). Forecasters incorporate it into validation of current forecast products and into updates to turbulence and icing advisories.
The PIREP system has real limitations - coverage is sparse in many areas, pilot characterization of conditions is inconsistent, and voluntary participation creates gaps. But when forecast products are being evaluated and updated, a fresh PIREP from a pilot who was just in that airmass beats model interpolation between sounding stations every time.
Where Upper-Air Observation Technology Is Heading
Windborne Systems, a private company, has been developing long-duration stratospheric balloons designed to loiter over a target region for weeks rather than the roughly 45 minutes of a traditional radiosonde flight. The concept addresses a real limitation: persistent, targeted upper-air profiling in areas where the fixed-station network has gaps, particularly over oceans and regions where radiosonde infrastructure is thin.
Research programs have explored using small uncrewed aircraft for atmospheric profiling in the boundary layer - the lowest one to two kilometers of the atmosphere where pilots operate during climb, descent, and approach. Drones cannot reach stratospheric altitudes, but for detailed low-altitude wind and turbulence profiling around complex terrain and busy terminal environments, targeted deployment offers something a fixed launch site cannot.
Commercial weather intelligence companies have been building data assimilation platforms that ingest measurements from a wide variety of sources, including atmospheric moisture information extracted from cellular network signal characteristics. These are supplements to the radiosonde network, not replacements. Every alternative method gets validated against upper-air soundings as the observational standard - because that is what the word standard means.
The most sophisticated forecasting infrastructure ever built - supercomputers running models with billions of grid points, continuously updating satellite imagery from GOES-16 and GOES-17, global surface sensor networks, commercial data aggregators - still depends on that twice-daily balloon launch as the backbone of the vertical observation system.
Key Takeaways
- 900 stations worldwide launch radiosondes simultaneously at 0000 UTC and 1200 UTC daily, producing the vertical atmospheric profiles that initialize every major weather prediction model.
- Winds aloft, icing, and turbulence forecast products are only as accurate as the sounding data used to initialize the models - reliability degrades with distance from a launch site and time elapsed since the observation.
- Satellites measure horizontally well but cannot replace the vertical resolution of a radiosonde sounding; that distinction is why the balloon network remains the backbone of the system.
- Commercial aircraft (via AMDAR) and pilots (via PIREPs) actively contribute to the same forecast system they depend on - the aviation system and weather forecasting system are deeply intertwined.
- The University of Wyoming maintains a free, publicly accessible archive of sounding data that lets pilots examine the raw atmospheric profile behind any forecast product.
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