This study reveals that moisture transport deficits are the primary cause of droughts, with significant regional variations. The conditional probability of drought given a moisture deficit from oceanic or terrestrial sources is higher than 10% in most land areas, reaching over 20% in some hotspot regions like central-east North America, south-east South America, and east Europe. The research uses Lagrangian techniques to quantify the contribution of moisture sources to precipitation, showing that moisture transport deficits significantly influence drought occurrence. The study highlights that the dominant moisture source's contribution deficit can predict droughts, with implications for hydrological, socioeconomic, and environmental management. The analysis of moisture sources shows that oceanic and terrestrial sources have distinct roles in drought genesis, with oceanic sources dominating in certain regions and terrestrial sources in others. The study also finds that the conditional probability of drought is higher when moisture deficits are from major sources, with values up to five times greater than the independence case. The results suggest that moisture transport deficits can be used to predict droughts, especially in regions where moisture deficits are strongly linked to precipitation deficits. The study also discusses the potential of moisture transport in drought predictability, noting that models can better predict large-scale circulation than precipitation, making moisture transport a more reliable predictor. The findings have implications for understanding the impact of climate change on droughts, as changes in circulation patterns and land–ocean temperature contrasts could alter the relationship between moisture transport and drought occurrence. The study provides a framework for improving drought predictability by focusing on moisture transport deficits, which could help in managing water resources and mitigating drought impacts.This study reveals that moisture transport deficits are the primary cause of droughts, with significant regional variations. The conditional probability of drought given a moisture deficit from oceanic or terrestrial sources is higher than 10% in most land areas, reaching over 20% in some hotspot regions like central-east North America, south-east South America, and east Europe. The research uses Lagrangian techniques to quantify the contribution of moisture sources to precipitation, showing that moisture transport deficits significantly influence drought occurrence. The study highlights that the dominant moisture source's contribution deficit can predict droughts, with implications for hydrological, socioeconomic, and environmental management. The analysis of moisture sources shows that oceanic and terrestrial sources have distinct roles in drought genesis, with oceanic sources dominating in certain regions and terrestrial sources in others. The study also finds that the conditional probability of drought is higher when moisture deficits are from major sources, with values up to five times greater than the independence case. The results suggest that moisture transport deficits can be used to predict droughts, especially in regions where moisture deficits are strongly linked to precipitation deficits. The study also discusses the potential of moisture transport in drought predictability, noting that models can better predict large-scale circulation than precipitation, making moisture transport a more reliable predictor. The findings have implications for understanding the impact of climate change on droughts, as changes in circulation patterns and land–ocean temperature contrasts could alter the relationship between moisture transport and drought occurrence. The study provides a framework for improving drought predictability by focusing on moisture transport deficits, which could help in managing water resources and mitigating drought impacts.