Weather at the surface is only part of the story. The atmosphere extends upward for hundreds of kilometres, and understanding its vertical structure is essential for accurate weather forecasting, aviation safety, and climate monitoring. On land, networks of radiosonde stations launch weather balloons twice daily, building the upper-air dataset that global models depend on. Over the open ocean, those networks simply do not exist.
The Automated Shipboard Aerological Programme (ASAP) was designed to fill that gap. Since the mid-1980s, ASAP has placed automated upper-air sounding systems aboard merchant ships travelling regular ocean routes. From those ships, trained operators — often the ship's navigating officers — launch radiosondes tethered to helium-filled balloons that rise through the atmosphere, transmitting temperature, humidity, pressure, and wind data all the way to altitudes of around 30 kilometres.
The data are encoded and transmitted in near real time to national meteorological services, and then distributed globally on the GTS. Around 5,000 soundings are collected annually, primarily over the North Atlantic.
