A weather balloon, also known as a sounding balloon, is a balloon (specifically a type of high-altitude balloon) that carries instruments to the stratosphere to send back information on atmospheric pressure, temperature, humidity and wind speed by means of a small, expendable measuring device called a radiosonde. To obtain wind data, they can be tracked by radar, radio direction finding, or navigation systems (such as the satellite-based Global Positioning System, GPS). Balloons meant to stay at a constant altitude for long periods of time are known as transosondes. Weather balloons that do not carry an instrument pack are used to determine upper-level winds and the height of cloud layers. For such balloons, a theodolite or total station is used to track the balloon's azimuth and elevation, which are then converted to estimated wind speed and direction and/or cloud height, as applicable.

Weather balloons are launched around the world for observations used to diagnose current conditions as well as by human forecasters and computer models for weather forecasting. Between 900 and 1,300 locations around the globe do routine releases, two or four times daily.^[1][2][3][4]

One of the first people to use weather balloons was the French meteorologist Léon Teisserenc de Bort. Starting in 1896 he launched hundreds of weather balloons from his observatory in Trappes, France. These experiments led to his discovery of the tropopause and stratosphere.^[5] Transosondes, weather balloons with instrumentation meant to stay at a constant altitude for long periods of time to help diagnose radioactive debris from atomic fallout, were experimented with in 1958.^[6] The drone technology boom has led to the development of weather drones since the late 1990s.^[7] These may begin to replace balloons as a more specific means for carrying radiosondes.^[8]

The balloon itself produces the lift, and is usually made of a highly flexible latex material, though chloroprene may also be used. The unit that performs the actual measurements and radio transmissions hangs at the lower end of the string, and is called a radiosonde. Specialized radiosondes are used for measuring particular parameters, such as determining the ozone concentration.

The balloon is usually filled with hydrogen, though helium – a more expensive, but viable option nonetheless – is also frequently used. The ascent rate can be controlled by the amount of gas with which the balloon is filled. Weather balloons may reach altitudes of 40 km (25 mi) or more, limited by diminishing pressures causing the balloon to expand to such a degree (typically by a 100:1 factor) that it disintegrates. In this instance the instrument package is usually lost,^[9] although a parachute may be employed to help in allowing retrieval of the instrument. Above that altitude sounding rockets are used to carry instruments aloft, and for even higher altitudes satellites are used.

Weather balloons are launched around the world for observations used to diagnose current conditions as well as by human forecasters and computer models for weather forecasting. Between 900 and 1,300 locations around the globe do routine releases, two or four times daily, usually at 0000 UTC and 1200 UTC.^[1][2][3][4] Some facilities will also do occasional supplementary special releases when meteorologists determine there is a need for additional data between the 12-hour routine launches in which time much can change in the atmosphere. Military and civilian government meteorological agencies such as the National Weather Service in the US typically launch balloons, and by international agreements, almost all the data are shared with all nations.

Specialized uses also exist, such as for aviation interests, pollution monitoring, photography or videography, and research. Examples include pilot balloons (Pibal). Field research programs often use mobile launchers from land vehicles as well as ships and aircraft (usually dropsondes in this case). In recent years, weather balloons have also been used for scattering human ashes at high altitudes. The weather balloon was also used to create the fictional entity 'Rover' during the production of the 1960s TV series The Prisoner in Portmeirion, Gwynedd, North Wales, UK in September 1966. This was retained in further scenes shot at MGM Borehamwood UK during 1966–67.^[10]

While weather forecasting is increasingly reliant on satellites and radar technology, it still heavily involves the use of weather balloons. These devices, launched from thousands of stations worldwide, ascend into the atmosphere to collect meteorological data. The United States, for example, releases approximately 76,600 balloons annually, while Canada launches 22,000.^[11]

Weather balloons, after reaching an altitude of approximately 35 kilometers, burst, releasing their instruments and the latex material they are made of. While the instruments are often recovered, the latex remains in the environment, posing a significant threat to marine ecosystems. Studies have shown that a substantial portion of weather balloons eventually end up in the ocean. For instance, one Australian researcher collected over 2,460 weather balloon debris from the Great Barrier Reef, estimating that up to 300 balloons per week may be released into the marine environment. This environmental impact underscores the need for sustainable alternatives in weather data collection.^[11]

Scientists and environmentalists have raised concerns about weather balloons' environmental impact. The latex material, which can persist in the ocean for extended periods, can harm marine life, including sea turtles, birds, and fish. Efforts to minimize the environmental impact of weather balloons include developing biodegradable materials and improved recovery methods. However, the continued reliance on weather balloons for meteorological data challenges balancing the need for accurate weather forecasts with environmental sustainability.^[11]

• Atmospheric sounding
• Ceiling balloon
• High-altitude balloon
• SCR-658 radar
• Skyhook balloon
• Timeline of hydrogen technologies
• High-altitude platform
• UFOs

• Atmospheric Soundings for Canada and the United States – University of Wyoming
• Balloon Lift With Lighter Than Air Gases Archived 24 February 2014 at the Wayback Machine – University of Hawaii
• Examples of Launches of Instrumented Balloons in Storms – NSSL
• Federal Meteorological Handbook No. 3 – Rawinsonde and Pibal Observations
• Kites and Balloons – NOAA Photo Library
• NASA Balloon Program Office – Wallops Flight Facility, Virginia
• National Science Digital Library: Weather Balloons – Lesson plan for middle school
• Pilot Balloon Observation Theodolites – Martin Brenner, CSULB
• StratoCat – Historical recompilation project on the use of stratospheric balloons in the scientific research, the military field and the aerospace activity
• WMO spreadsheet of all Upper Air stations around the world (revised location September 2008)