How Radiosondes Measure Air Pressure, Temperature and Humidity
Since the 1930s meteorologists have taken upper air observations with small expendable instrument packages known as radiosondes. These instruments report pressure, temperature and humidity via a radio transmitter to ground tracking antennas as they ascend.
A radiosonde is carried aloft in a spherically shaped balloon, typically helium. At the point where the balloon reaches its elastic limit, a parachute slows its descent to Earth.
The temperature sensor in a radiosonde is a resistance thermistor, a white ceramic-covered metallic rod whose electrical resistance changes with air temperature. Most radiosondes also carry a hygristor, which is a plastic strip covered with a moisture-sensitive film of lithium chloride and binder. A change in humidity causes the resistance of the hygristor to increase.
The thermistor and hygristor in a radiosonde are connected to each other by a small circuit. The resistance of the hygristor is used to set the frequency of a miniature radio transmitter that transmits the temperature, humidity and pressure measurements to the ground.
The resulting radiosonde profiles are very useful to meteorologists and scientists studying the atmosphere. However, the main disadvantage of the radiosonde observation system is that it only provides profiles up to a maximum of 30 km height above sea level (heights are in metres above mean sea level, abbreviated as MASL). This limits their use mainly to the areas where balloon flights can take place.
Almost every radiosonde includes a humidity sensor (white, ceramic-covered metallic rod), which measures the water vapor content of the air. As the air temperature rises, the moisture content of the air rises and the humidity sensor records this information.
The data is interpreted at the launching station and transmitted to a worldwide communications network. The observations are relayed to weather forecast centers around the globe. Radiosonde observations are generally made twice daily at 0000 and 1200 UTC.
Special helium-filled meteorological balloons are used to elevate the radiosonde to very high altitudes, up to 100,000 feet or more. A small parachute slows the descent of the sonde, minimizing potential safety hazards. Several specialized versions have been developed, including dropsondes that are dropped from aircraft over hurricanes and other storms of interest; and ozonesondes that measure the amount of ozone in the atmosphere.
Radiosonde pressure measurements are taken with a small, temperature compensated aneroid barometer, usually housed in a partially evacuated metal canister. The canister volume expands as the sonde ascends, and the resulting pressure decrease is registered by the aneroid.
The instrument package also includes a GPS receiver, which sends GPS time difference data to a ground station. This information is used to locate the sonde with respect to known fixed beacons (LORAN). The position of the LORAN beacons, along with the model forecast winds, are used to calculate temperature, humidity, and wind speed and direction profiles as a function of height, pressure, and location.
PHOCS stores raw heterodyne power data, meta data, and background spectra for each measurement. Statistical calculations are performed for each windowed set of data, which are then used to fit spectral features such as the oxygen line shape. The resulting profiles are then compared to experimental data and used to inform spectral fitting algorithms for other absorption lines.
The radiosonde measures atmospheric conditions and transmits the data via a transmitter to tracking equipment on the ground. This equipment, known as a radio theodolite, determines where the balloon and sonde are in the atmosphere and provides information about wind speed and direction.
A radiosonde consists of a small, expendable package of instruments attached to a helium-filled free-flying weather balloon. The instrument package contains sensors for temperature (using a thermometer), pressure (using an aneroid barometer) and humidity. It also has a GPS-based transmitter that sends sensor and position data to tracking equipment on the ground.
At the launching station, the data are interpreted and entered into a worldwide communications network that relays it to weather forecast centers around the world. As the sonde ascends and descends, the temperature and humidity data provide valuable insight into boundary layer meteorology and can help identify locations of a tropopause. Generally, the air temperature decreases with altitude, but occasionally this is “inverted” and the air temperature increases with height. This is called a temperature inversion and it is often a feature of the underlying troposphere.