DESCRIPTION, TYPES & COMPONENTS OF RADIOSONDES

Radiosondes are battery-powered telemetry devices that measure at high altitudes, pressure, temperature, relative humidity, wind and cosmic rays. It can also be seen as a small weather station that is attached to a water balloon with radio transmitting capabilities. Radiosondes play a vital role in most operational assimilation of atmospheric data.

There are mainly two classes of radiosondes; the Rawinsonde and the Dropsonde. 

A class of radiosonde whose position is tracked as it ascends in the atmosphere to give wind speed and direction and measures other atmospheric data is referred to as Rawinsonde. A radiosonde is called a rawinsonde if it’s tracked so that wind in the atmosphere is provided in addition to the pressure, temperature, and relative humidity data.

The other class of radiosondes are the ones that are usually released from airplanes and fall rather than being carried by weather balloons into the atmosphere. They are referred to as Dropsondes.

For this article we will be discussing more on the radiosondes that are usually carried by weather balloons, getting to know their mode of operation and how they work.

Brief History

The first flights of aerological devices were done in the 19th century with kites and meteographs. The Meteograph is a recording device that measures the pressure and temperature that are always gotten and recovered after each experiment. Using the kites to fly the meteograph proved to be a difficult task because the kites were linked to the ground and were very difficult to move together in gusty conditions.

Over the years, various efforts were made at remotely sensing the atmosphere with aerological instruments. In 1924, Colonel William Blaire came up with the first experiment of weather measurements from balloons, making use of the temperature dependence of radio circuits. The first radiosonde that gave a precise encoded telemetry from weather sensors was invented in France by Robert Bureau. 

On January 7, 1929, Bureau flew the device and coined the name “radiosonde”. A year later, it was single-handedly developed by Pavel Molchanov and he flew the radiosonde on January 30, 1930. Molchanov’s model and design became a popular standard because of its simplicity and because it converted sensor readings to Morse code, making it easy to use without special equipment or training. Over the years, the radiosondes kept being modified up to date.

Mode Of Operation

The radiosonde is launched into the atmosphere by a latex or rubber balloon filled with either hydrogen or helium. As the balloon ascends through the atmosphere, the pressure decreases, causing the balloon to expand, and eventually, it reaches the extent where the skin will break which in turn terminates the ascent. After bursting, a small parachute on the radiosonde’s support line carries it to Earth. 

The radiosondes are equipped with a thermometer, humidity sensor, pressure sensor, and a GPS sensor for position determination before being launched into the atmosphere so as to measure the temperature, pressure, relative humidity, wind speed and wind direction. These obtained data are transmitted immediately to the ground station by a radio transmitter located within the instrument package. 

Weather balloons carrying radiosondes are one of the major ways weather-predicting companies like the Kanda weather group make forecasts about future weather.

Whenever Kanda launches its weather balloons, the ground-based radio direction finding antenna equipment tracks the motion of the radiosonde during its ascent through the air and aids with the communication between the computer that stores all the variables and the radiosonde. These variables include all the data that has been gathered by the radiosonde during its flight in the atmosphere. The data that is gotten is then assimilated into the weather forecast model before a weather forecast is made. 

Radiosondes can be launched in almost any type of weather except for thunderstorms. The disadvantage here is that severe thunderstorms and heavy precipitation may cause instrument failure or radio interference.

Components of a Radiosonde

The radiosonde is an electronic device that is made up of a suite of sophisticated meteorological sensors and other major components such as the Oscillator radio transmitter, the Battery, Balloon and Parachute.

The Meteorological Sensors include;

• THERMISTOR This is a white ceramic-covered metallic rod that serves as a temperature sensor on radiosonde. The electrical resistance of this rod changes with a change in the air temperature. The thermistor is white so as to minimize heating by sunlight. The temperature range for the thermistor lies between 40° C to 90° C.

• HYGRISTOR This is a humidity sensor consisting of a glass slide or in some cases a plastic strip covered with a moisture-sensitive film of lithium chloride (LiCl) and a binder. The electrical resistance of the chemical (LiCl) keeps changing once there’s a change in the atmospheric humidity. The hygristor on most radiosondes is built to record the ambient relative humidity ranging from 15% to 100%.

• ANEROID BAROMETER

Pressure is measured in the radiosonde by means of an aneroid barometer. It consists of a small partially evacuated metal cylindrical or rectangular container called a canister. The volume of the canister expands as the radiosonde ascends, in response to a reduction in the atmospheric pressure. The aneroid is designed to register pressures from 1040 MB to 10 MB or less.

OSCILLATOR RADIO TRANSMITTER

This is a radio transmitter that relays the measurements and data gotten back to a receiver at the radiosonde launch station. Two frequency bands are used in transmitting meteorological data from the radiosonde to the radiosonde ground station; there are, 400.15–406 MHz and 1668.4–1700 MHz.

BATTERY

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.

BALLOON & PARACHUTE

The radiosonde package is carried by a spherically shaped balloon. The balloon is made of a film of natural or synthetic rubber. As the balloon ascends, it expands in size from approximately 6 feet to a diameter between 24 and 32 ft before it bursts. An attached parachute returns the instrument package safely to the ground.