Vaisala RS92 radiosonde of a type used in climate research, made by Vaisala Oyj in Helsinki, Finland, 2010
Every day, at hundreds of sites around the world, helium-filled balloons are released to gather important information about the Earth’s atmosphere.
As the balloons rise, they carry small battery-powered instruments known as radiosondes, which measure temperature, pressure and humidity as they ascend to heights of 30 km, transmitting data back to ground stations every couple of seconds.
As they travel upwards the balloons expand – until they reach the size of a double-decker bus – before bursting and falling back to the ground.
During flight the balloons can travel over 120 km downwind. Their journey is tracked by global positioning system (GPS) satellites that convert the radiosonde signals into information about wind speed and direction at different heights throughout the atmosphere.