The isotopic signature of meteoric water, particularly the ratio of oxygen-18 to oxygen-16 (δ¹⁸O), is a key tracer in hydrology and paleoclimatology, primarily linked to temperature. However, variations in δ¹⁸O in precipitation and groundwater cannot be fully explained by temperature alone. This study shows that interactions between climate-induced changes in local parameters during precipitation events contribute significantly to observed deviations. These effects are superimposed on a large-scale climate pattern based on temperature. The unique 40-year dataset from Austria reveals the influence of relative humidity and precipitation type on δ¹⁸O variations. The study highlights the importance of considering local meteorological factors, such as sub-cloud evaporation and relative humidity, which affect the isotopic composition of precipitation. The results indicate that the δ¹⁸O pattern in Vienna drinking water is influenced by temperature, the North Atlantic Oscillation (NAO), and the snow-to-precipitation ratio (S/P). The S/P ratio and relative humidity are in anti-phase with δ¹⁸O, indicating that higher δ¹⁸O values occur with more rain and lower relative humidity. The analysis of variance (ANOVA) confirms that temperature, relative humidity, S/P ratio, and NAO index collectively explain a significant portion of the δ¹⁸O variance. The study also shows that local factors, such as sub-cloud evaporation, play a crucial role in δ¹⁸O variations, especially in mountainous regions. The results emphasize the importance of considering local meteorological parameters in understanding climate impacts on the water cycle. The study concludes that stable isotopes in precipitation and groundwater provide valuable information about climate changes, beyond just temperature, and that future research should focus on larger geographical areas to better understand these variations.The isotopic signature of meteoric water, particularly the ratio of oxygen-18 to oxygen-16 (δ¹⁸O), is a key tracer in hydrology and paleoclimatology, primarily linked to temperature. However, variations in δ¹⁸O in precipitation and groundwater cannot be fully explained by temperature alone. This study shows that interactions between climate-induced changes in local parameters during precipitation events contribute significantly to observed deviations. These effects are superimposed on a large-scale climate pattern based on temperature. The unique 40-year dataset from Austria reveals the influence of relative humidity and precipitation type on δ¹⁸O variations. The study highlights the importance of considering local meteorological factors, such as sub-cloud evaporation and relative humidity, which affect the isotopic composition of precipitation. The results indicate that the δ¹⁸O pattern in Vienna drinking water is influenced by temperature, the North Atlantic Oscillation (NAO), and the snow-to-precipitation ratio (S/P). The S/P ratio and relative humidity are in anti-phase with δ¹⁸O, indicating that higher δ¹⁸O values occur with more rain and lower relative humidity. The analysis of variance (ANOVA) confirms that temperature, relative humidity, S/P ratio, and NAO index collectively explain a significant portion of the δ¹⁸O variance. The study also shows that local factors, such as sub-cloud evaporation, play a crucial role in δ¹⁸O variations, especially in mountainous regions. The results emphasize the importance of considering local meteorological parameters in understanding climate impacts on the water cycle. The study concludes that stable isotopes in precipitation and groundwater provide valuable information about climate changes, beyond just temperature, and that future research should focus on larger geographical areas to better understand these variations.