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Dew is one of the important moisture sources in many arid and semiarid regions. The knowledge of moisture origin of dew under various climatic conditions is still lacking. Isotopic variations can preserve information about moisture origin and formation mechanisms. Therefore, the isotopic compositions of dew and precipitation (δ2H, δ18O, δ17O, d‐excess, lc‐excess and 17O‐excess) were investigated at three sites with different climatic conditions (i.e., Gobabeb with extremely dry climate, Nice with Mediterranean climate and Indianapolis with humid continental climate). The results showed that there were three types of dew at Gobabeb: advective dew, groundwater‐derived dew, and shallow soil water‐derived dew, accounting for 27.3%, 45.4% and 27.3% of the dew events, respectively. The ultimate moisture sources of advective dew and the other two types of dew at Gobabeb were from the South Atlantic Ocean and a mixture of the Indian and South Atlantic Oceans, respectively. Dew in Nice included ocean‐derived dew from the North Atlantic Ocean with local evapotranspiration replenishment, and local‐derived dew, mainly from the continental Europe and Mediterranean Sea, accounting for 39.1% and 60.9% of the dew events, respectively. All the Indianapolis dew were likely local‐derived dew. Based on the moisture origins, the future dew trends were speculated under global warming. Dew frequencies at Gobabeb and Indianapolis under future climates are uncertain due to the concurrent increases in atmospheric water vapour and temperature. The local‐derived dew in Nice would likely decrease due to the decreasing precipitation and increasing drought, and the ocean‐derived dew under future climates is uncertain. This study provides a practical method to distinguish dew moisture sources, and such information is useful for future prediction of dew trends under climate change. [ABSTRACT FROM AUTHOR]

• 17O-excess was used to distinguish dew equilibrium or kinetic fractionation. • Dew underwent kinetic fractionation associated with evaporation in Gobabeb. • Dew with temperature over 14.7 °C was influenced by evaporation in Indianapolis. • Dew did not experience significant evaporation in Nice. • Relative humidity is a main factor explaining different dew evaporation processes. With increasing aridity in many regions, dew is likely to play an increasingly important role in the ecohydrological processes in many ecosystems, especially in arid and semiarid regions. Few studies investigated the role of evaporation during dew formation and how it varies under different climate settings. 17O-excess, as a new tracer, could be used to extract information of evaporation dynamics from natural water samples (e.g., precipitation, river, and lake). Therefore, to fill the knowledge gap in evaporation mechanisms during dew formation, we report the isotopic variation (δ2H, δ18O, δ17O, and 17O-excess) of dew and precipitation from three distinct climatic regions (i.e., Gobabeb in the central Namib Desert, Nice in France with Mediterranean climate, and Indianapolis in the central United States with humid continental climate). We examined whether dew formed in different climate settings was affected by different degree of evaporation using observed isotopic values and evaporation models during the formation processes, and modeled the effects of key meteorological variables (i.e., temperature and relative humidity) on 17O-excess variations. The results showed that dew in Gobabeb experienced kinetic fractionation associated with evaporation under non-steady state conditions during dew formation with enriched δ18O and low 17O-excess values. Dew formations with temperatures over 14.7 °C in Indianapolis were also influenced by evaporation under non-steady state conditions. However, dew formation in Nice did not experience significant evaporation. Evaporation processes (equilibrium or kinetic fractionation) occurring during nights with heavy dew under three climate settings were mainly related to the variation of atmosphere relative humidity. The 17O-excess tracer provides a new method to distinguish the different evaporation processes (equilibrium or kinetic fractionation) during dew formation and our result provides an improved understanding of dew formation. [ABSTRACT FROM AUTHOR]