Atmospheric water vapour plays a paramount role in the climate system, as demonstrated by some facts listed below:

• 60% of the natural greenhouse effect can be explained with water vapour opacity in the atmosphere (Kiehl and Trenberth, 1997).
• Water vapour plays an amplifying role in global warming through a strong positive climate feedback loop as evident in climate predictions (Held and Soden, 2000).
• The water vapour content in the lower troposphere increases with increasing temperature. This results in changes of the hydrological cycle, in particular an increased likelihood of intense precipitation events (IPCC, 2007; Allan et al., 2010).
• The spatial distribution of water vapour and precipitation and their seasonal cycle exhibit clear similarities at least in the tropics (Trenberth, 2011).
• With increasing temperature and unchanged winds, wet areas will get wetter and dry areas will get drier (Chou and Neelin, 2004; Allan and Soden, 2007). This likely leads to intensified droughts in divergence zones of the subtropics and floods in the convergence zones of the tropics.
• Eventually, the combined effect of more intense precipitation events and increased water vapour will affect atmospheric circulation (Sherwood et al., 2010; Trenberth, 2011).

It is thus essential for the climate community to base their work on high quality information about the temporal and spatial distribution of the various parameters characterising atmospheric water vapour in the atmosphere.