RADIOSONDES -- An Upper Air Probe


The radiosonde is a balloon-borne instrument platform with radio transmitting capabilities. Originally named a radio-meteorograph, the instrument is now referred to as a radiosonde, a name apparently derived by H. Hergesell from a combination of the words "radio" for the onboard radio transmitter and "sonde", which is messenger from old English.

The radiosonde contains instruments capable of making direct in-situ measurements of air temperature, humidity and pressure with height, typically to altitudes of approximately 30 km. These observed data are transmitted immediately to the ground station by a radio transmitter located within the instrument package. The ascent of a radiosonde provides an indirect measure of the wind speed and direction at various levels throughout the troposphere. Ground based radio direction finding antenna equipment track the motion of the radiosonde during its ascent through the air. The recorded elevation and azimuth information are converted to wind speed and direction at various levels by triangulation techniques.

A rawinsonde (or radio wind sonde) is a radiosonde package with an attached radar reflector that permits radio-direction finding equipment to determine the wind direction and wind speed at various altitudes during the ascent of the package.


While various efforts were attempted at remotely sensing the atmosphere with instruments onboard unmanned free balloons, the current type of radiosonde dates back to January 1930, when Pavel A. Molchanov, a Russian meteorologist, made a successful radio sounding into the stratosphere. He launched his radiosonde at Pavlovsk. His goal was a cheap, and expendable means of sounding the atmosphere for temperature, moisture and wind data.

Radiosondes were first used by the U.S. Weather Bureau in 1936. During that year a radiosonde network of several stations was inaugurated to obtain upper air soundings on a routine basis. This network replaced the kite and aircraft sounding programs. Currently, 70 radiosonde stations are distributed across the continental United States. Radiosondes are launched from these stations twice daily, just prior to 0000 and 1200 UTC.

Radiosondes can be launched in almost any type of weather. While the radiosonde is reasonably durable, severe thunderstorms and heavy precipitation may cause instrument failure or radio interference.


The complete radiosonde system, or rawinsonde, consists of a balloon-borne radiosonde instrument package , a radio receiver , a tracking unit and a recorder . Additional Radiosonde background information is provided.


The main component of the radiosonde is a sturdy, lightweight, white cardboard (or plastic) instrument package, approximately the size of a large shoe box. The package is attached to the train containing the balloon and parachute. The following weather sensing instruments are located within or attached to this package:


The resistance thermistor is a white ceramic covered metallic rod that serves as a temperature sensor on most American radiosondes. The diameter of the rod is approximately 0.7 mm and its length is no more than 2 cm. The electrical resistance of this rod changes with a change in the air temperature. To increase contact with the air, the thermistor is located on an outrigger, extended a distance from the outside of the instrument package. The thermistor is white to minimize the heating by sunlight. The temperature range for the thermistor lies between approximately +40° C to 90° C.


The hygristor is a humidity sensor consisting of a glass slide or plastic strip covered with a moisture sensitive film of lithium chloride (LiCl) and a binder; metal strips are located along the edges. The electrical resistance of the chemical changes with a change in the atmospheric humidity. The hygristor is located within the instrument package at a place where the outside air passes the hygristor. The hygristor on most radiosondes is designed to record the ambient relative humidity in the range from 15% to 100%.


The radiosonde measures pressure by means of an aneroid barometer, consisting of a small, partially evacuated metal canister. This temperature compensated instrument is central to the instrument package. The volume of the canister expands as the radiosonde ascends, in response to a reduction in the atmospheric pressure aloft. The aneroid is designed to register pressures from 1040 mb to 10 mb or less. The aneroid also serves another function as described below. A pen arm is attached to the aneroid.


A switching mechanism, called a baroswitch, was used on older radiosondes to switch between the two sensor and the reference elements. This baroswitch involves a contact arm connected to the barometer passes over the commutator bar. The commutator bar is a selector switch with 180 contact points; the switch occurs at approximately 10 mb intervals. The contact arm, responding to the pressure changes, moves across the contact points on the commutator bar.

Newer model radiosondes use a capacitive transducer with an aneroid capsule.


Alternating conducting and insulating strips are used to change the frequency. The conducting strip transmits humidity and reference information while the insulator strip transmits temperature information. Each fifth contact is a reference contact.


A miniature radio transmitter generates the FM radio frequency carrier, operating on a modulated carrier frequency of 1680 MHz. Variable modulation is used to transmit the collected information. That is, the radio frequency is changed by the position of the contact arm on the baroswitch. The radio transmitter is located in the pointed plastic cylinder attached to the base of the instrument package. In flight this conical antenna housing is pointed downward.


A small battery is contained in the radiosonde package to serve as the power supply for the weather sensing instruments and the radio transmitter. The battery is activated and tested prior to launch.


The radiosonde package is carried aloft by a spherically shaped balloon. The balloon is made of a film of natural or synthetic rubber (neoprene). Before launch, a neoprene balloon is inflated with lighter-than-air gas, typically helium, to approximately 6 foot (2 meter) diameter. This size provides sufficient lift to carry a radiosonde payload of several pounds. The thickness of the balloon skin ranges from 0.002 to 0.004 inch at the time of inflation, but becomes 0.0001 inch just before the balloon bursts. As the balloon ascends, it expands in size from approximately 6 feet to a diameter between 24 and 32 ft before it bursts. The balloon carries the instrument package to an altitude of approximately 25 mi (27-37 km) where the balloon bursts (at a pressure of approximately 10 mb).

An attached parachute returns the instrument package safely to the ground. Return mailing instructions are included in the instrument package. Those radiosondes that are found and returned can be refurbished for subsequent flights, saving a considerable amount of the cost of a new radiosonde.


The following equipment is located at the upper air observing station to track the radiosonde, receive the telemetry data and process these data into a useable form.



The antenna receives the telemetry signal transmitted from the radiosonde. Highly directional radio direction finding antenna is used also to obtain the wind speed and direction at various levels in the atmosphere by tracking the radiosonde and determining the azimuth and elevation angles. The ascent rate of the radiosonde is known and timed between intervals.


(Output to other devices)




Before launch, calibration of the baroswitch is made, with appropriate adjustment. The battery is activated. The balloon is carefully inflated to such a size that it will provide the proper lift. The balloon, parachute and instrument package are attached. The radio equipment is tuned.

Just before launch the surface weather conditions are measured.

The balloon and instrument train is launched. Care is taken so that the radiosonde does not become entangled with local obstacles.

Tracking of the radiosonde is begun immediately upon launch. Visual observations are continued until the radar tracking has locked on to the ascending instrument package. The data are recorded automatically during the flight and then processed for transmission.


The radiosonde transmits temperature and relative humidity data at each pressure level. Winds aloft are determined from the precision radar tracking of the instrument package. The altitudes of these levels are calculated using an equation (the hypsometric equation) that relates the vertical height of a layer from the mean layer temperature, the humidity of the layer and the air pressure at top and bottom of the layer. Significant levels where the vertical profiles of the temperature or the dewpoint undergo a change are determined from the sounding. The height of the troposphere and stability indices are calculated. These data are encoded into standardized RAOB messages and transmitted worldwide over standard communications network.

A plot of the vertical variations of observed weather elements made above a station is called a sounding. The plots of the air temperature, dewpoint and wind information as functions of pressure are generally made on a specially prepared thermodynamic diagram The altitude can be determined from the pressure by evaluating the hydrostatic equation. Mandatory and significant levels are determined. The data are encoded into the standard RAOB messages and transmitted by conventional communications networks to the National Meteorological Center.

Currently, 70 RAOB stations are distributed across the continental United States. Upper level observations are taken every 12 hours (0000 and 1200 UTC).


The dropsonde is an instrument package that represents a modification of a radiosonde. The dropsonde (or dropwindsonde) is used to collect low level weather data especially over ocean areas. These instruments have been used frequently by aircraft penetrating hurricanes. The dropsonde is literally dropped from an aircraft at flight level. The data collected by the dropsonde are radioed back to the aircraft. A parachute is deployed after the dropsonde leaves the aircraft to slow the flight of the dropsonde.


Meteorologists measure atmospheric conditions from the earth's surface to an altitude of approximately 30 km above sea level through twice daily radiosonde ascents. These balloon-borne instruments are sent aloft just prior to 0000 UTC and 1200 UTC on each day. During their ascent, they radio back to the ground-based receiving station a nearly continuous stream of information until the balloon bursts at approximately 10 mb. Radiosonde observations (also called RAOB) include the observed air temperature, pressure, moisture and wind information at various levels in the atmosphere.

Within two hours after the radiosonde has been launched, the RAOB data have been encoded and transmitted over a communications network to the National Meteorological Center. At this center, the data can be processed for analysis on upper air charts and for use in numerical weather prediction models. To accomplish this task, all upper air stations are to report RAOB data for certain mandatory pressure levels. To speed the transmission process, the RAOB operator encodes only the temperature and dewpoint data for significant pressure levelsalong with the mandatory pressure levels. The significant pressure levels are those points ascertained from the plotted sounding where a significant change in the temperature and or dewpoint profile is detected.

Current RAOB information for Great Lakes Region

Last revision 10 June 1996

© Copyright, 1996 Edward J. Hopkins, Ph.D.

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