δD (isotopic ratio of HDO to H2O) observations derived from IASI measurements above the North Atlantic reveal the influence of the surface heating over Sahara in controlling the humidity budget above the Atlantic Lacour et al. (2017). The added information contained in the isotopic ratio of water vapour allows to clearly distinguish between three different sources of water vapour and their interactions. This is the first time such detailed water isotopic ratio maps are realized thank to IASI sampling.