Water vapor is one of the most important greenhouse gases affecting Earth's radiation budget. A sound understanding of its distribution and variability in space and time is necessary to monitor and interpret climate change. It is important also in numerical weather prediction (NWP). Still, it is one of the most difficult quantities to measure accurately.

Recently, microwave sounders such as the AMSU-B, aboard polar-orbiting satellites, started measuring water vapor globally and continuously. Microwave data are less affected by clouds than infrared data and have already yielded large benefits in operational NWP. AMSU-B data can also be used to evaluate upper tropospheric humidity (UTH) which is important for climate studies.

The talk gives an overview of the AMSU data, tools for the comparison of AMSU data to radiosonde data, and the scaling of AMSU data to UTH.

The methodology and applications of the comparison will be discussed in detail.