Since the late 1930s, upper air observations have been made by the NOAA National Weather Service (NWS) with radiosondes. The radiosonde is a small, expendable instrument package that is suspended 25 meters (about 80 feet) or more below a large balloon inflated with hydrogen or helium gas. 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 tracking antenna on a radio frequency typically ranging from 1675 to 1685 MHz. By tracking the position of the radiosonde in flight using GPS or a radio direction finding antenna, data on wind speed and direction aloft are also obtained (observations where winds aloft are also obtained from radiosondes are called "rawinsonde" observations). The radio signals received by the tracking antenna are converted to meteorological values and from these data significant levels are selected by a computer, put into a special code form, and then transmitted to data users. High vertical resolution flight data, among other data, are also archived and sent to the NOAA National Climatic Data Center.
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 -90oC (-130oF) and an air pressure only few thousandths of what is found on the Earth's surface. If the radiosonde enters a strong jet stream it can travel at speeds exceeding 400 km/hr (250 mph).
When released, the balloon is about 2 meters (about 6 feet) in diameter and gradually expands as it rises. When the balloon reaches a diameter of 6 to 8 meters (20 to 25 feet) in diameter, it bursts. A small, orange colored parachute slows the descent of the radiosonde, minimizing the danger to lives and property. At the present time, data are not collected while the radiosonde descends.
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.
I found a radiosonde. What should I do?
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.