On any given launch day somewhere in the tropics, a rubber weather balloon rises with an ozonesonde and radiosonde suspended beneath it, sending measurements of ozone, pressure, temperature and humidity back to the ground as it climbs. By October 2023, NASA’s accounting of the SHADOZ network had reached 10,000 archived ozone and pressure-temperature-humidity profile pairs after 25 years of operation.
It was supposed to last three years. Instead, the Southern Hemisphere ADditional OZonesondes network became one of the longest-running coordinated records of tropical ozone profiles.
NASA says SHADOZ was originally proposed as a three-year project to increase the number of ozone profiles available for satellite algorithms in an undersampled part of the atmosphere. By its 2023 milestone, the network coordinated 14 long-term stations and produced roughly 20 percent of the data from long-term ozonesonde stations worldwide.
What an ozonesonde actually is
An ozonesonde is a lightweight instrument carried beneath a weather balloon alongside a standard meteorological radiosonde. According to NOAA’s Global Monitoring Laboratory, its electrochemical sensor pumps ambient air through a potassium iodide solution, where ozone produces an electrical signal proportional to the amount present.
The radiosonde sends the ozone reading together with pressure, temperature and humidity back to a receiving station on the ground. A typical balloon climbs to roughly 35 kilometres in about two hours before bursting, producing a detailed vertical profile from the surface through much of the stratospheric ozone layer.
The instruments are generally treated as expendable because recovery is uncertain, but “launched and lost” does not mean every sonde disappears permanently. NOAA’s Hilo ozonesonde program, for example, reports recovering and reusing a minority of its instruments after descent.
Why the tropics were the blind spot
By the late 20th century, ozone observations were much denser at mid and high latitudes than across the tropics. NASA’s history of SHADOZ notes that Natal, Brazil, supplied the main regular tropical soundings during the 1970s and 1980s, supplemented by intermittent measurements from American Samoa and Hawaii.
That gap mattered because satellite instruments do not simply read a perfectly resolved ozone profile from orbit. Retrieval algorithms have to infer the vertical distribution from measured radiation, and a sparse tropical observing network left fewer direct profiles against which those satellite estimates could be developed and checked.
Major field campaigns helped build the infrastructure that later made SHADOZ possible. NASA’s TRACE-A campaign in 1992 used aircraft and ozonesondes across the tropical Atlantic, while PEM-Tropics-A in 1996 extended intensive atmospheric measurements across the Pacific.
Facilities, launch gas, trained operators and international collaborations already existed at several sites by the time Anne Thompson and colleagues at NASA Goddard organized SHADOZ in 1998. The plan was modest: coordinate launches, standardize the data, build a common archive and improve the tropical profiles available for satellite work.
How a three-year project became a 25-year network
SHADOZ did not survive because its instruments suddenly became exotic. It survived because a coordinated tropical record became more useful as it grew longer.
NASA Goddard and Wallops Flight Facility supplied coordination, archiving and some equipment, NOAA contributed measurements at several locations, and meteorological agencies, universities and research institutes operated stations around the world. By 2023, NASA described the partnership as involving organizations from 14 nations on five continents.
The record is long, but it is not uniformly continuous. NASA’s 2023 station inventory identified 14 stations with at least 10 years of operation, while several individual records contain interruptions; Tahiti, Malindi and Cotonou also contributed shorter historical records that remain preserved in the archive.
That stop-start history is part of the network rather than an exception to it. Watukosek in Java, for example, resumed balloon launches after a multiyear interruption, a station history also explored in this SpaceDaily background piece on the Indonesian ozonesonde program; despite interruptions across the network, the 2023 archive still accumulated about 400 profile pairs from its 14 long-term stations.
Why satellites still need balloons
The central reason SHADOZ lasted is the same reason it was created: satellite measurements need independent observations close to the atmosphere itself. Instruments aboard missions such as NASA’s Aura satellite can observe ozone over enormous areas, while sondes supply finely resolved profiles through the troposphere and stratosphere.
Over La Réunion, NASA’s 2023 review reported that long-running comparisons between ozonesondes and satellite instruments remained stable to within a few percent over roughly two decades. That kind of agreement is not automatic, because the chemistry, preparation procedure, sensor manufacturer and data corrections used by an ozonesonde can all introduce biases.
One major quality-control effort was the 2017 Jülich Ozonesonde Intercomparison Experiment involving SHADOZ operators. The resulting 2019 paper in the Bulletin of the American Meteorological Society reported that SHADOZ stations following recommended protocols measured total ozone within about 3 percent of the Jülich reference instrument.
Those laboratory comparisons helped feed into standardized operating procedures used across the wider global ozonesonde community. The result is a two-way check: sondes remain crucial for satellite validation, while mature satellite records can also expose changes or drifts in a station’s balloon record.
What 25 years of tropical profiles have shown
The value of a multidecade archive becomes clearest when researchers look for trends. A 2021 Journal of Geophysical Research: Atmospheres study led by Thompson examined SHADOZ profiles from 1998 through 2019 across five tropical regions, separating the free troposphere from the lowermost stratosphere.
The study found pronounced seasonal and regional variation. Free-tropospheric ozone generally showed its strongest positive trends from February through May, while apparent ozone declines in the lowermost stratosphere largely disappeared when the researchers expressed altitude relative to the changing tropopause rather than using fixed geometric heights.
That result mattered because an apparent chemical decline in ozone could instead reflect the boundary between the troposphere and stratosphere shifting upward. Adding three more years of observations through 2022 did not overturn that interpretation in NASA’s 2023 assessment.
A newer 2025 analysis in Atmospheric Chemistry and Physics extended the SHADOZ trend calculations through 2023. The authors reported that the additional four years changed the earlier picture little: total tropospheric ozone trends were generally small, around 0.5 to 1 Dobson Unit per decade, except over Southeast Asia, and sonde and OMI/MLS satellite trends agreed within their uncertainties at four of five analysed sites.
The newer study also found that adding thousands of commercial-aircraft profiles produced little change in the derived trends, evidence that SHADOZ’s relatively sparse balloon schedule can still resolve multidecade behaviour. The archive’s strength is not continuous observation above every station, but repeated measurements made in sufficiently consistent ways for changes separated by years to remain comparable.
What 10,000 profiles mean now
Ten thousand profiles does not mean 10,000 simultaneous views of the tropical atmosphere. Each profile is one narrow vertical slice taken during one balloon flight, separated from the next by days or weeks and from neighbouring stations by hundreds or thousands of kilometres.
What makes the number substantial is its accumulated span. NASA estimated in 2023 that SHADOZ supplied roughly 20 percent of the profiles from all long-term ozonesonde stations while sampling a region equivalent to about 35 to 40 percent of Earth’s surface; the archive was receiving more than 300,000 data-user hits in an average year, and more than 20 publications referenced SHADOZ data in 2023 alone.
The network has continued to change since that milestone. The current NASA Goddard SHADOZ archive lists 16 current sites, including Quito and Palau, and continues to recommend its Version 6 data product; NASA’s 2023 discussion of Version 7 described it as an eventual reprocessing effort rather than a finished release.
The hardware remains recognizably simple: a balloon, a chemical cell, a radio and a small package disappearing upward until the balloon finally bursts. What has accumulated beneath that routine is the unusual part, a measurement record that began as a three-year attempt to fill blank spaces in satellite algorithms and was still adding new profiles more than a quarter-century later.