Weather balloons carry radiosondes—small instrument packages—through the atmosphere to measure vertical profiles of pressure, temperature and humidity. Tracking a radiosonde’s position also reveals wind speed and direction aloft. Unlike a surface station, one flight samples changing conditions from near the ground into the stratosphere, transmitting observations to a receiver as it rises. Forecast centres combine these profiles with satellites, radar, aircraft and other measurements to initialise numerical weather models and assess atmospheric stability. The balloon eventually expands as outside pressure falls and normally bursts, while the package descends. A sounding is therefore a moving sequence through one column of air, not a permanent tower or a complete picture of the atmosphere. Its value comes from consistent observations, quality control and networks of launches.
Key takeaways
- A radiosonde measures pressure, temperature and relative humidity during ascent.
- GPS tracking provides winds from the instrument’s movement.
- Pressure falls with height, while temperature changes through atmospheric layers.
- Each sounding samples a drifting path over roughly a few hours, not one fixed vertical line.
- Long records require adjustment for changing instruments and observing practices.
What are weather balloons and radiosondes?
The balloon supplies lift, usually using helium or hydrogen. The radiosonde is the electronic package suspended beneath it, with sensors, a radio transmitter, power and positioning equipment.
“Weather balloon” commonly describes the combined flight, but the balloon itself does not measure the atmosphere. The radiosonde produces the observations.
NOAA distinguishes a basic radiosonde observation from a rawinsonde observation in which tracked position supplies winds, although everyday meteorological usage often groups the measurements together.
Why surface observations are not enough
Weather develops in three dimensions. Surface warmth can sit beneath colder air, moisture can be concentrated in a layer and winds can change speed or direction with height.
A surface station cannot directly show these vertical structures. Balloons reveal where air becomes saturated, how stable layers are arranged and how winds steer systems aloft.
Earth Site’s introduction to meteorology and weather provides context for combining atmospheric observations into forecasts.
Measuring pressure
Atmospheric pressure generally decreases with altitude because less air lies above. Pressure is also a useful vertical coordinate in meteorology.
A sensor records pressure throughout ascent. Together with temperature, humidity and positioning, this helps establish the height and state of atmospheric levels.
The relation between pressure and height depends on the air column’s temperature and density, so one universal conversion is insufficient for precise work.
Measuring temperature
A small temperature sensor must respond rapidly while being exposed to moving air. Solar radiation can warm it, while ventilation and corrections influence the final value.
Temperature often falls through the troposphere but can rise in an inversion. Near the tropopause, the pattern changes as the balloon enters the stratosphere.
A vertical temperature profile helps determine stability. An air parcel rising into surroundings that remain warmer behaves differently from one that stays warmer than its environment.
Measuring humidity
Relative humidity describes water vapour relative to the amount needed for saturation at that temperature. Because saturation depends strongly on temperature, relative humidity can change even if water-vapour content does not.
Humidity sensors encounter low temperatures and rapidly changing conditions aloft. Response time and calibration matter, especially through sharp layer boundaries.
Forecasters use temperature and moisture profiles together to identify cloud layers, dry air and conditions relevant to convection.
How position becomes wind
The radiosonde drifts with the surrounding air while GPS records its position. Changes in horizontal position over time yield wind speed and direction.
This is an indirect measurement of wind, not a tiny spinning cup carried beneath the balloon. The calculated wind represents motion along the flight path.
Strong changes with height, called wind shear, can matter for storms and aviation. A single surface wind cannot represent them.
A fictional sounding
Imagine an illustrative balloon launched on a warm afternoon. It records 24°C and humid air near the surface, a rapid temperature decrease through the next kilometre and a dry layer above.
At higher levels, the wind turns and strengthens. These invented observations could support questions about whether rising clouds will continue developing and where they will travel.
They do not guarantee a storm. Meteorologists combine the profile with lifting mechanisms, radar, satellite imagery and model guidance.
Why the balloon bursts
As pressure around the balloon falls, the lifting gas expands. The balloon grows until its material can no longer stretch and it bursts.
A parachute slows the instrument package during descent. Flights can rise above 30 kilometres and drift far from release, depending on winds.
The exact path and maximum height vary. Treating a typical flight figure as a guarantee would ignore balloon fill, atmosphere and equipment.
A sounding is not perfectly vertical
Wind carries the radiosonde horizontally, sometimes many kilometres. The reported profile samples a sloping trajectory through evolving air.
For broad weather analysis this remains highly useful, but small local features may lie away from the path. Time also passes during ascent.
Comparing a balloon with a satellite at one nominal location therefore requires matching time, space and measurement meaning.
From observations to forecast models
Numerical weather prediction begins with an estimate of the atmosphere’s state. Data assimilation combines many observations with a previous model forecast while accounting for uncertainty.
Radiosondes are valuable because they measure vertical structure directly and consistently. They help constrain temperature, moisture and wind where surface stations cannot.
Models then calculate atmospheric evolution using physical equations. Observations initialise and evaluate them; they do not simply get converted into a weather icon.
Reading a sounding diagram
Meteorologists often plot temperature and dew-point-related information against pressure on a specialised chart such as a skew-T diagram. Wind symbols appear at corresponding levels.
The separation between temperature and dew point indicates moisture conditions. The slopes and shapes help assess stability, cloud layers and freezing levels.
Like any graph, the axes and transformations need attention. Earth Site’s guide to reading climate graphs concerns a different diagram but reinforces the habit of checking variables, units and time periods before interpreting a pattern.
Quality control and uncertainty
Sensors can lag, drift or suffer radiation effects. Telemetry can fail. Automated checks identify impossible jumps and inconsistencies, while metadata record instruments and procedures.
A suspicious value is investigated, not automatically erased because it looks unusual. Extreme atmospheric conditions are real possibilities.
Comparison with nearby observations and model backgrounds helps detect errors, but disagreement does not prove the instrument is wrong.
Weather records and climate records
One launch describes weather at a time and place. Many consistent launches can contribute to climate records of upper-air change.
Instrument types, launch times and processing methods have changed. NOAA’s radiosonde climate products address temporal inhomogeneities so that an equipment change is not mistaken for atmospheric change.
Earth Site’s explanation of weather and climate helps distinguish an individual profile from a long-term distribution.
Why balloons remain useful in the satellite era
Satellites provide broad and frequent coverage, especially over oceans. Many satellite instruments infer atmospheric properties from measured radiation rather than placing a sensor directly in a layer.
Radiosondes offer high-resolution in-situ profiles along their path and help calibrate or evaluate remote observations. Aircraft and ground-based systems add other strengths.
The observing system works through complementarity. Asking which instrument makes all others obsolete misunderstands the different quantities and coverage they provide.
Networks make one launch more useful
A profile gains context when compared with soundings upstream, downstream and from previous hours. Coordinated launch times allow analysts to map broad pressure surfaces and wind fields.
Extra launches may be made when severe weather or research needs justify denser observations. The value depends on where and when uncertainty is greatest, not simply on launching the largest possible number.
Coverage remains uneven, especially over oceans and sparsely populated regions. Satellites, ships, aircraft and drifting systems help fill those gaps.
What happens to the instrument?
After burst, a parachute reduces descent speed and the package may land far from its launch site. Services label instruments so finders can follow local return or disposal instructions.
Recovery rates vary, so expendable equipment creates material and environmental questions. Reusable concepts must still preserve measurement quality, safe flight and reliable communications.
Operational meteorology balances those practical costs against the public value of forecast observations. That decision belongs in the full account of the system.
Conclusion
Weather balloons transform the invisible vertical atmosphere into a measured profile. Pressure, temperature, humidity and tracked winds reveal layers that control clouds, storms and large-scale flow. A sounding remains a drifting, time-limited sample with measurement uncertainty, but networks and quality control turn those samples into powerful forecast and climate evidence. Their continued use demonstrates that simple flight technology can carry sophisticated observations into places satellites view differently.
Frequently asked questions
How high does a weather balloon go?
Many radiosonde flights exceed 30 kilometres, but burst height varies with balloon, fill and atmospheric conditions.
Does the balloon measure wind directly?
Usually wind is calculated from the radiosonde’s changing GPS position as it drifts with the air.
Why are balloons launched at regular times?
Coordinated observations give forecast systems comparable snapshots across a network and support consistent long-term records.
Can a radiosonde be reused?
Packages are generally designed as expendable instruments, though some are recovered. Data are transmitted during flight, so recovery is not required for the main observation.
Have satellites replaced weather balloons?
No. Satellites provide exceptional coverage, while radiosondes provide direct vertical profiles. The methods complement and check one another.
