Weather balloon (also called sounding balloon) is a high altitude gas balloon that carries meteorological instruments whose role is to collect or send back information about weather like atmospheric pressure, temperature, humidity, and wind speed. Measuring device that measures these atmospheric parameters is called “radiosonde”. Wind speed is measured by tracking the speed of a weather balloon with radar, radio direction finding, or navigation systems like GPS. A subtype of weather balloon that is designed to stay at a constant altitude for long periods of time is called “transosonde”. Weather balloons are used because it is nearly impossible to predict the weather without knowing the weather conditions of the upper atmosphere.
Léon Teisserenc de Bort, the French meteorologist, is one on the inventors of weather balloons. He started using them in 1896 and since then he launched hundreds of weather balloons from his observatory in Trappes, France. These balloons helped him conduct experiments that lead to his discovery of the tropopause and stratosphere. Tropopause is the boundary in the Earth's atmosphere between the troposphere and the stratosphere where air ceases to cool with height, and becomes almost completely dry. Stratosphere is the next layer of the atmosphere which has a characteristic that it is cooler as further it is from the Earth. Transosondes were invented in 1958 as an experiment in measuring of radioactive debris from atomic fallout. Early weather balloons had to be retrieved in order to collect the data while today’s can send back the information. These data gathering and transmitting devices were developed in the 1930s.
Weather balloons are usually made of a highly flexible latex material or Chloroprene which is usually filled with hydrogen, which costs less, or helium, if it can be obtained. Speed by which the balloon is rising is controlled by the amount of gas with which the balloon is filled. Radiosonde, which measures atmospheric parameters and sends them by means of radio transmissions, is tied bellow the balloon and it hangs at the lower end of the string. Different radiosondes measure different parameters. Weather balloons can rise 40 km in the air. If higher altitude is needed, radiosondes are sent into the air with “sounding rockets”, rockets designed to take measurements and perform scientific experiments during sub-orbital flights. For heights greater than 1,500 kilometers, weather measurements are carried by satellites.
Some of measuring instruments that weather balloons usually carry in its radiosonde are: thermistor (ceramic-covered metal rod that works as a simple thermometer); hygristor (a slide coated with film of lithium chloride that works as a humidity sensor whose electrical resistance changes based on the humidity); and aneroid barometer (small metal canister that has air inside and a membrane that reacts to the outside pressure). If the weather balloon bursts because of too low atmospheric pressure, radiosonde will start falling to Earth. Because of that, radiosonde is equipped with a parachute which will bring it safely to the ground.
Zhuzhou Research & Design Institute of China, Totex and Cosmoprene of Japan and Pawan Rubber Products of India are the greatest manufacturers of weather balloons.