The weather we experience on the ground is due to dynamic processes that take place throughout the atmosphere. Much like a surface weather observation to record the weather, we need to know what is taking place through the entire atmosphere. For over 60 years, upper air observations have been made by the National Weather Service (NWS) with radiosondes.
The radiosonde is a small, expendable instrument package that is suspended below a six feet wide balloon filled with hydrogen or helium. As the radiosonde rises at about 1,000 feet/minute (300 meters/minute), sensors on the radiosonde measure profiles of pressure, temperature, and relative humidity.
These sensors are linked to a battery powered radio transmitter that sends the sensor measurements to a ground receiver. By tracking the position of the radiosonde in flight, information on wind speed and direction aloft is also obtained. Observations where winds aloft are also obtained are called "rawinsonde" observations.
The radiosonde flight can last in excess of two hours, and during this time the radiosonde can ascend to over 115,000 feet (35,000 m) and drift more than 125 miles (200 km) from the release point. During the flight, the radiosonde is exposed to temperatures as cold as -130°F (-92°C) and an air pressure only few thousandths of what is found on the Earth's surface.
When the balloon has expanded beyond its elastic limit (about 20 feet in diameter) and bursts, a small parachute slows the descent of the radiosonde, minimizing the danger to lives and property.
Only about 20 percent of the approximately 75,000 radiosondes released by the NWS each year are found and returned to the NWS for reconditioning. These rebuilt radiosondes are used again, saving the NWS the cost of a new instrument. If you find a radiosonde, follow the mailing instructions printed on the side of the instrument.
Although all the data from the flight are used, data from the surface to the 400 mb pressure level (about 23,000 feet / 7000 m) are considered minimally acceptable for NWS operations. Therefore, a flight may be deemed a failure and a second radiosonde is released if the balloon bursts before reaching the 400 mb pressure level or if more than 6 minutes of pressure and/or temperature data between the surface and 400 mb are missing.
Worldwide, there are nearly 900 upper-air observation stations. Most are located in the Northern Hemisphere and all observations are usually taken at the same time each day (00:00 UTC and/or 12:00 UTC), 365 days a year. Observations are made by the NWS at 92 stations - 69 in the conterminous United States, 13 in Alaska, nine in the Pacific, and one in Puerto Rico. NWS supports the operation of 10 other stations in the Caribbean. Through international agreements data are exchanged between countries.
How Are Radiosonde Data Used?
Understanding and accurately predicting changes in the atmosphere requires adequate observations of the upper atmosphere. Radiosonde observations are the primary source of upper-air data and will remain so into the foreseeable future. Radiosonde observations are used over a broad spectrum of efforts 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.
Improvements in the radiosondes are coming. The National Weather Service is in the process of replacing the current radiosondes with new versions that use a Global Positioning System to track the flight.
Improvements in these observations will include:
- Requiring less operator interaction and maintenance,
- Providing complete high-resolution information to users,
- Providing consistent and accurate measurement of surface weather parameters at the point of balloon release, and
- Increase amount of data provided to users (at one second resolution or about every 20 feet through the atmosphere).