A weather balloon is an unmanned, high-altitude platform used by meteorologists to gather data about the atmosphere. This system is a complete assembly of components designed to take measurements from the ground up to the stratosphere. The appearance of a weather balloon in flight changes dramatically as it ascends, evolving from a relatively small object at launch to a massive sphere before its mission ends.
The Appearance and Composition of the Balloon Envelope
The envelope is typically made from a thin, highly elastic material like natural latex or a synthetic rubber such as chloroprene. When prepared for launch, the balloon appears as a translucent or white, teardrop-shaped vessel, only partially inflated with a lifting gas like helium or hydrogen. At this stage, the balloon’s diameter is relatively small, often measuring around five feet (1.5 meters) across.
The envelope’s appearance changes because of the atmospheric pressure drop encountered at high altitudes. Since the gas inside cannot escape, the balloon begins to stretch and expand as it rises through the thinning air. This expansion is substantial, with the balloon growing to a diameter of 20 to 35 feet (6 to 8 meters) before reaching its elastic limit. The fully expanded, massive sphere is the final visual form of the envelope just before it bursts, signaling the end of its ascent.
The Instrument Package (Radiosonde)
The instrument package, called the radiosonde, is a small, lightweight box suspended below the envelope. This payload is often constructed from white Styrofoam or cardboard to provide insulation and protection for the sensitive electronics against cold and moisture. The box is generally small, roughly the size of a lunchbox, and is designed to be expendable.
Attached to the exterior of the radiosonde box are various sensors that measure atmospheric parameters during the ascent. For example, the temperature sensor, known as a thermistor, is often mounted on an outrigger to ensure accurate readings away from the box’s heat. The radiosonde also contains a battery-powered radio transmitter and antenna to relay data on pressure, temperature, humidity, and location back to ground stations in real-time.
Visualizing the Full Assembly and Descent
The full assembly in flight presents a noticeable visual contrast between the massive balloon and the small payload below. The radiosonde and its recovery parachute are suspended far beneath the envelope by a suspension line, which can extend between 80 to 115 feet (25 to 35 meters). This long line keeps the instruments away from the thermal interference created by the balloon material, ensuring the accuracy of the measurements.
The flight concludes when the massive, expanded envelope bursts, appearing as a rapid, silent dissipation of the large white sphere in the upper atmosphere. Following the burst, the radiosonde package begins its descent back to Earth, slowed by a small parachute that is often colored a bright orange or red for visibility. This parachute ensures a soft landing for the instrument package, which may drift significant distances from the launch site due to high-altitude winds.
Upon landing, the radiosonde package is typically found as a small, white box attached to a tangle of string and the remnants of the parachute and balloon. If the public encounters a landed package, it is considered safe to handle. However, any partially inflated balloon should be avoided due to the potential presence of flammable hydrogen gas.