Water isotopes trace the history of an air parcel. Lighter isotopes preferentially evaporate and heavier isotopes preferentially condense thus more condensation leads to more isotope depletion. The TES measurements show that in the tropics, re-evaporation of precipitation is an important process controlling cloud formation. Up to 70% of precipitation is re-evaporated into the cloud.
High H2O and HDO/H2O ratio over land indicates strong evapo-transpiration as the water vapor source
Relatively Low HDO/H2O ratio with high H2O indicates re-evaporation of precipitation in tropical cloud systems
Lower HDO/H2O ratio with latitude due to condensation along with poleward transport