Temperature–Humidity Interactions and Mood Polarity in Bipolar Disorder: Evidence from a Beijing Cohort

Background Meteorological conditions have long been implicated in the seasonal variability of psychiatric disorders, yet polarity-specific effects in bipolar disorder (BD) remain poorly characterized. Methods We retrospectively analyzed 11,244 first-admission BD inpatients at Beijing Anding Hospital (2013–2021). Episode polarity at admission (mania/hypomania vs. depression) was the primary outcome. Meteorological exposures—including ambient temperature (T2M), relative humidity (RH2M), and sunshine duration (SSD, as a proxy for photoperiod)—were assigned as monthly averages for the admission month, with alternative moving-average and seasonal definitions tested in sensitivity analyses. Logistic regression and generalized additive models (GAMs) were applied to assess linear, nonlinear, and interaction effects, adjusting for age, sex, and marital status. Sex- and age-stratified analyses were performed to identify susceptible subgroups. Results No consistent linear main effects were observed for temperature, humidity, or sunshine duration. By contrast, the temperature–humidity interaction (T2M × RH2M) was robustly significant (logistic regression OR = 0.999, 95% CI: 0.998–1.000, p = 0.001; GAM ti[T2M × RH2M] p < 0.001), with improved model fit (ΔAIC = –5.05). Response surfaces demonstrated non-additive effects, whereby the impact of temperature on polarity shifted with ambient humidity. Stratified analyses indicated stronger effects in men (p = 0.013) and in younger (11–24 years) and middle-aged (32–46 years) subgroups. Sensitivity analyses using moving averages and seasonal means yielded directionally consistent findings. Conclusions This large-scale monsoon-climate study highlights the polarity-specific influence of meteorological interactions in BD. Temperature–humidity coupling, rather than individual factors, emerges as a key environmental determinant, with sex- and age-related heterogeneity suggesting vulnerable subgroups. These findings provide polarity-focused evidence for climate-sensitive early warning systems and targeted clinical management in BD.