Occasionally, an American homeowner wakes up to find a spent weather balloon in his or her backyard. It's a strange sight: tattered strips of neoprene, tangled cords, a crumpled parachute and a small cardboard box. It's no surprise that weather balloons are often mistaken for extra-terrestrial spacecraft.
The core component of the whole assembly is the radiosonde, a shoebox-sized cardboard box packed with three basic atmospheric instruments:
- Thermistor. A ceramic-covered metal rod that acts as a rudimentary thermometer.
- Hygristor. A small slide that acts as a humidity sensor. The slide is coated with film of lithium chloride (LiCl), the electrical resistance of which changes based on the surrounding humidity.
- Aneroid barometer. A small metal canister filled with air that measures air pressure. As the air pressure around it decreases at higher altitudes, the canister expands, triggering a sensor.
The radiosonde also has a low-powered radio transmitter to relay data from all three instruments back to receivers on the ground. A small battery provides power to the radiosonde.
The advantage of a radiosonde is that scientists don't need to retrieve the device to obtain weather data. In the 1920s and '30s, when meteorologists used kites or aircraft to measure upper-atmosphere weather data, specialists would have to wait until the aircraft touched down or the kite was reeled in before they could start making weather calculations.
Holding the whole assembly aloft is a large balloon made of neoprene, a synthetic rubber. The balloons are filled either with helium or hydrogen depending on the preferences of the individual launch station. Hydrogen is cheaper, has better lifting capacity, and can be easily extracted from water. However, hydrogen is also very flammable -- a fact that has prompted many explosion-shy weather stations to adopt helium instead.
Altogether, a complete weather balloon assembly costs about a few hundred dollars. A high-altitude rocket, on the other hand, can cost several hundred thousand dollars for just a single flight. Even a high-altitude aircraft flight can cost up thousands of dollars per hour. The relative cheapness of weather balloons is what has kept them the go-to device for recording weather data for more than six decades.