National Weather Service United States Department of Commerce

Upper Air Observations & Maps

NWS Upper Air Program

Weather balloon being prepared for flight


The weather experienced on the ground is generated within the lowest 15 miles of the atmosphere.  The National Weather Service (NWS) uses multiple platforms to observe the weather: Doppler radar, satellite, aircraft observations, automated surface observation stations, etc.  However, the weather balloon remains the best platform for observing temperature, wind, relative humidity, and pressure above the ground. Weather balloons carry an instrument called a radiosonde which is tracked by specialized ground equipment. The NWS at Charleston, SC conducts rawinsonde observations at least twice a day. Balloons are launched at our office at 7 AM and 7 PM EDT and 6 AM and 6 PM EST. Weather balloons are simultaneously launched at 91 other NWS offices across the U.S. Special weather balloon flights are occasionally launched to observe the latest state of the atmosphere ahead of significant weather or to conduct research. The special upper air observations are usually launched at 2 PM/AM EDT and 1 PM/AM EST. The NWS has been flying weather balloons since the 1930s. The data collected by these weather balloon flights are critical for weather model performance and forecast accuracy.

About an hour before the scheduled release time, the observer will inflate the balloon within our Upper Air building. A latex balloon is filled with helium and will reach around 5 feet in diameter when filled with enough gas to lift 1,100-1,600 grams. Below the balloon, the observer will attach a bright orange parachute and over 75 feet of string that the radiosonde will be tied to. Before the radiosonde is attached to the weather balloon, it will undergo a “baseline” process to ensure that the sensors are operating properly. This process takes about 10-15 minutes and observers check to ensure data is being received by the radiosonde and the battery is in good health. 

After the weather balloon assembly is complete and the radiosonde baseline process is successful, the observer will consult the local FAA air traffic control for clearance to release the balloon. Once released, the radiosonde will send meteorological data in one-second intervals. This data will be received by a computer system in our office, and monitored and quality controlled by the observer.

The National Weather Service recently switched to a new system called MROS (Manual Radiosonde Observing System). Not only is the software different but so are the radiosondes. The GRAW radiosonde now only has a temperature and humidity component, which is sent back to the office via GPS and antenna tracking. The software then computes pressure, wind and wind gusts.


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The flight typically takes around two hours. During that that time the weather balloon can reach over 100,000 feet in altitude and may drift over 100 miles from our office.  As the balloon ascends through the atmosphere, the gas pressure inside the balloon remains the same as the pressure outside the balloon decreases. The difference in pressures will result in the expansion of the latex balloon during its flight. The expansion will continue until it reaches the elastic limit of the latex, then the balloon will burst and the radiosonde will drift back to the ground while being slowed by the parachute.

Radiosonde observations are used for several purposes, including:

  • Input for computer-based weather prediction models,
  • Local severe storm, aviation, and marine forecasts,
  • Weather and climate change research,
  • Input for air pollution research, and
  • Ground truth for satellite data.

According to the NWS Office of Public Affairs: “The technology has come a long way in the last 80+ years, and we've made significant progress to reduce environmental impacts. We've reduced the amount of material used in the balloon construction by about 20 percent in the last 10 years, and radiosondes weigh four ounces or less today, compared to 2 pounds in the 1990s. Today, balloons are made with a natural, biodegradable latex, and the parachute and string are also biodegradable.‚Äč”