Circa mid 1930's.
Water vapor is also a greenhouse gas that plays a critical role in the global climate system.
This role is not restricted to absorbing and radiating energy from the sun, but includes the effect it has on the formation of clouds and aerosols and the chemistry of the lower atmosphere. Despite its importance to atmospheric processes over a wide range of spatial and temporal scales, water vapor is one of the least understood and poorly described components of the Earth's atmosphere. Primarily, water vapor is observed through the use of radiosondes. The Radisonde is a small, lightweight box equipped with weather instruments and a radio transmitter. It is attached to a cord that has a parachute and a gas filled balloon.