Weather balloons, officially known as radiosondes, are a foundational tool for atmospheric science, providing a vertical snapshot of the air above us. These high-altitude vehicles carry sophisticated instrument packages into the upper reaches of the atmosphere to gather real-time meteorological data. Launched simultaneously from hundreds of locations worldwide, the collected data is a direct input into the numerical models that generate daily weather forecasts. The extreme atmospheric changes encountered during the ascent ultimately dictate the final moments of the balloon’s mission.
The Science Behind Weather Balloons
A weather balloon assembly consists of three primary components: the balloon itself, the cable, and the radiosonde instrument package. The balloon is typically made of highly flexible latex or synthetic rubber, such as neoprene, designed to stretch significantly as it rises. It is filled with a lighter-than-air lifting gas, usually hydrogen or the more expensive helium, which dictates the balloon’s buoyancy and ascent rate. The balloon is only partially inflated at launch to allow for the massive expansion that will occur later in the flight.
Radiosonde Instrumentation
The small, lightweight radiosonde hangs below the balloon and is packed with sensors to measure atmospheric properties continuously. These sensors precisely track temperature, relative humidity, and atmospheric pressure throughout the flight. Advanced models also include a Global Positioning System (GPS) receiver, which allows ground stations to calculate wind speed and direction by tracking the balloon’s movement across the sky. Data is transmitted back to a ground receiver via a radio signal several times per second, providing a continuous profile of the atmosphere.
Navigating the Earth’s Atmospheric Layers
The weather balloon’s flight path takes it through the lowest two layers of the atmosphere, beginning its journey in the troposphere. This layer extends from the surface up to about 12 to 20 kilometers, depending on the latitude, and is where nearly all weather phenomena occur. As the balloon ascends through the troposphere, the air temperature generally decreases with height.
Above the troposphere lies the stratosphere, a more stable layer that can extend up to approximately 50 kilometers. Balloons spend the majority of their mission time traversing the stratosphere, a region with much lower air density. Crucially, as the balloon climbs, the atmospheric pressure surrounding it drops dramatically.
The Specific Altitude and Cause of the Burst
The layer where weather balloons burst is the stratosphere, typically at altitudes ranging from 29 to 34 kilometers (about 95,000 to 112,000 feet). This altitude is reached after a flight time of approximately 90 minutes to two hours. The physical reason for the rupture is the extreme pressure differential between the gas inside the balloon and the thin air outside. As the external atmospheric pressure decreases with altitude, the gas inside the flexible envelope expands significantly.
This expansion is governed by gas physics, where the volume of a gas is inversely proportional to the pressure exerted on it. A balloon that is 1.5 meters in diameter at launch can swell to 6 to 8 meters before failure. The balloon’s material, stretched to its elastic limit, can no longer contain the expanding gas, causing it to rupture explosively.
Post-Flight Data Collection and Retrieval
Immediately following the burst, the radiosonde package begins its descent back to Earth. A small, brightly colored parachute, attached to the balloon assembly at launch, immediately deploys to slow the fall of the instrument. This controlled descent is necessary to prevent damage and minimize any hazard on the ground. The radiosonde continues to transmit data throughout its parachute-assisted fall, providing scientists with a secondary profile of the atmosphere.
The instrument package is designed to withstand the landing and is often made of environmentally considerate materials. While the National Weather Service does not typically track every radiosonde for recovery, the devices are sometimes found by the public for recycling. Specialized research flights often use GPS trackers to pinpoint the landing site, enabling ground teams to recover the valuable instrumentation.