NWS Radiosonde Observations - Factsheet

What is a Radiosonde?

Since the late 1930s, 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 2 meter (6 feet) wide balloon filled with hydrogen or helium.  As the radiosonde rises at about 300 meters/minute (about 1,000 feet/minute), sensors on the radiosonde measure profiles of pressure, temperature, and relative humidity. These sensors are linked to a battery powered, 300 milliwatt or less radio transmitter that sends the sensor measurements to a sensitive ground receiver on a radio frequency typically ranging from 1675 to 1685 MHz. 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 35 km (about 115,000 feet) and drift more than 300 km (about 180 miles) from the release point. During the flight, the radiosonde is exposed to temperatures as cold as -90^oC (-130^oF) 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 and bursts (about 6 m or 20 feet in diameter), a small parachute slows the descent of the radiosonde, minimizing the danger to lives and property.

Less than 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.  The instrument is harmless and safe to handle.  

Although all the data from the flight are used, data from the surface to the 400 hPa pressure level (about 7 km or 23,000 feet) are considered minimally acceptable for NWS operations.  Thus, a flight may be deemed a failure and a second radiosonde released if the balloon bursts before reaching the 400 hPa pressure level or if more than 6 minutes of pressure and/or temperature data between the surface and 400 hPa are missing.

Worldwide, there are over 800 upper-air observation stations and through international agreements data are exchanged between countries.   Most upper air stations are located in the Northern Hemisphere and all observations are usually taken at the same time each day (00:00 and/or 12:00 UTC), 365 days a year. When severe weather is expected additional soundings may be taken during the day at a select number of  stations.  Observations are made by the NWS at 92 stations - 69 in the conterminous United States, 13 in Alaska, 9 in the Pacific, and 1 in Puerto Rico.  NWS also supports the operation of 10 other stations in the Caribbean.  

How Are Radiosonde Data Used?

Understanding and accurately predicting changes in the atmosphere requires adequate observations of the upper atmosphere. Radiosonde observations are a primary source of upper-air data and will remain so into the foreseeable future.

Radiosonde observations are applied to a broad spectrum of efforts. Data applications include:

-    Input for computer-based weather prediction models;

-    Local severe storm, aviation, and marine forecasts;

-    Weather and climate change research;

-    Input for air pollution models;

-    Ground truth for satellite data

Radiosonde Replacement Program

The current radiosonde tracking systems are 1950's vintage and the data processing computer is a 1980's IBM PC/XT. These systems are obsolete and are increasingly difficult to maintain. NWS has begun a program to replace the ground systems at all NWS stations with a new system. The program has four objectives: 

-    Maintain system availability and

-    Reduce maintenance activity

-    Use less radio frequency to avoid radio interference with other users

-    Provide improved data sets with higher vertical resolution.