This system had several weaknesses:
A three-dimensional picture of temperature, pressure, relative humidity, and wind speed and direction in the atmosphere is essential for weather forecasting and meteorological research.
During the latter part of the 19th century and the first quarter of the 20th
century, this information was obtained mainly by meteorographs sent aloft on tethered
kites, which automatically recorded, on a single sheet, the measurements of two or
more meteorological parameters such as air pressure, temperature, and humidity. This system had several weaknesses:
From 1925 to 1937, upper-air data were obtained by attaching meteorographs to airplanes, observations called APOBS. While planes continued to be used for this purpose until 1943, their use was greatly curtailed after radiosonde observations were inaugurated in 1937. Airplane observations also had several weaknesses:
The radiosonde is a small, expendable instrument package that is suspended below a large balloon filled with hydrogen or helium. The radiosonde consists of sensors used to measure several meteorological parameters coupled to a radio transmitter and assembled in a lightweight box. The meteorological sensors sample the ambient temperature, relative humidity, and pressure of the air through which it rises.
As the radiosonde is carried aloft, sensors on the radiosonde measure profiles of
pressure, temperature, and humidity. These sensors are linked to a battery powered,
300 milliwatt radio transmitter that sends the sensor measurements to a sensitive
ground receiver. The transmitter operates on a frequency ranging from 1668.4 to
1700.0 MHz, while the ground receiver can detect and process signals ranging from
1655 to 1705 MHz. By tracking the position of the radiosonde, information on wind
speed and direction aloft is also obtained.
The instrument package is attached to a biodegradeable plastic parachute, and together, they are suspended from a balloon. The balloon is a spherically-shaped film of natural or synthetic rubber (neoprene) and is inflated with a lighter-than-air gas such as hydrogen or helium. Even though various sizes are available, 600-gram and 1200-gram balloons are most widely used. The balloon, parachute and radiosonde, known collectively as a flight train, ascend at an approximate rate of 1000 feet per minute depending on weather conditions, amount of gas, and balloon size.
During the flight train's ascent, the radiosonde continuously transmits temperature, relative humidity, and pressure readings to the ground-based Radiosonde Tracking System which is housed in a fiberglass dome above the inflation shelter. Wind speed and direction are determined for each minute of the flight, generally 90 minutes. They are determined from changes in the position and direction of the flight train as detected by the Radiosonde Tracking System. When winds are incorporated into the observation, it is termed a rawinsonde observation, and all National Weather Service
upper air stations take rawinsonde observations.
The altitude reached by rawinsonde observations varies for several reasons:
Those observations that reach the bursting altitude of the regular 600-gram balloon attain an average height slightly in excess of 90,000 feet. The average bursting altitude for stations using the larger 1,200-gram balloon exceeds 100,000 feet. The average altitude reached for all observations is approximately 94,000, while a number of observations have risen to 125,000 feet or more. When the balloon reaches its elastic limit and bursts, the parachute slows the descent of the radiosonde, minimizing the danger to lives and property.
Significant changes in the meteorological parameters are entered into a minicomputer,
which converts them into coded messages. These messages are relayed through
various communications systems to all parts of the world. The information in these
messages is extremely important in weather forecasting, climatology, and research.
The National Center for Environmental Prediction (NCEP) prepares daily upper air
charts and forecasts based on the rawinsonde observations. After the data has been
used, it is archived at the National Climatic Data Center (NCDC), where it is available to
anyone upon request.
Although all the data from the entire flight is used, data from the surface to 400 millibars (mb) or about 24,000 feet is considered minimally acceptable for NWS operations. Thus, a flight is considered a failure and a second radiosonde is released if the balloon bursts before reaching the 400 mb level or if more than 6 minutes of data between surface and 400 mb are missing.
Worldwide, there are more than 900 upper-air observation stations using 15 major types of radiosondes. Most stations are located in the Northern Hemisphere and all observations are taken at the same times each day at 00:00 and 12:00 UTC (Greenwich Mean Time), 365 days per year. Observations are made by the NWS at 93 stations - 72 in the conterminous United States, 13 in Alaska, 10 in the Pacific, and 1 in Puerto Rico.
Through international agreements, radiosonde data are exchanged between countries
and are applied to a broad spectrum of efforts. Applications include: data initialization for global and regional numerical prediction models; input for air pollution/dispersion models; severe storm, general, aviation, and marine forecasts; ground truth for satellite data; weather research; and climate change studies. 
The network of National Weather Service rawinsonde stations in the United States are shown in the figure.
Approximately one-third of the radiosondes released by the National Weather Service are found and returned to the Instrument Reconditioning Branch in Kansas City, Missouri, where they are repaired and reissued for further use, some as many as seven times. Instructions printed on the radiosonde explain the use of the instrument, state the approximate height reached, and request the finder to mail the radiosonde, postage free, at any Post Office or hand it to a U.S. rural carrier. The finders receive the satisfaction in knowing that they are helping reduce the overall cost of operating the Government.
At present, the rawinsonde observation is the most accurate and cost-effective method for sampling the atmosphere. When combined with meteorological satellites and aircraft reports, a complete profile of the atmosphere to very high altitudes is possible.
Accurately predicting changes in the atmosphere ranging from severe thunderstorms to global climate change requires adequate observations of the upper atmosphere. The NWS radiosonde network is the primary source of upper-air data and will remain so into the foreseeable future.
The current operational numerical weather prediction (NWP) models for weather and hydrologic prediction are run on computers at the National Center for Environmental Prediction in Camp Springs, Maryland. Radiosonde data are fed into the models and, with other observations, assimilated to provide initial conditions for the model predictions. The highest resolution operational weather prediction model computes weather parameters on a mathematical grid 48 by 48 km horizontally, 38 levels vertically, and in 200 second time steps out to 48 hours. Experimentally, a 29 by 29 km, 50 vertical level model is being run regularly in addition to the 48 km resolution model.
Radiosonde sites in the conterminous United States are about 375 km (235 miles) apart. Since the NWP model grid resolution is much smaller than the spacing of radiosonde sites, the loss of data from only one or two radiosonde sites can have significant impacts on model forecasts. Studies have shown that when missing radiosonde observations occur, small scale weather features above the surface can be lost or inaccurately positioned in the data analysis, causing significant errors in the model predictions. This especially holds true during severe weather episodes.
NWS meteorologists also analyze individual radiosonde soundings to help them prepare short-term, local weather forecasts. Individual soundings help forecasters determine many local weather parameters including, atmospheric instability, freezing levels, wind shear, precipitabie water, and icing potential. The following are examples of local weather phenomena that are predicted with the aid of sounding data:
Unfortunately, the present spatial and temporal resolution of the radiosonde network
hinders the NWS from optimizing the accuracy of local weather forecasts because they
occur on much smaller time and space scales. Just one missing radiosonde sounding
can degrade the forecast accuracy of severe weather events and thus it is essential that
all radiosonde sites report observations on a routine basis.
The National Weather Service Office at Alabaster is part of the network of rawinsonde stations in the United States. The flight trains are prepared in the upper air inflation building located just to the north of the NWS building at the Shelby County airport.
Click here to see real-time upper air charts and soundings.
Last updated 02/16/97