Increasing vapour pressure deficit is not necessarily atmospheric moisture loss: seasonal structure of change in Estonia, 1965–2024

Abstract Rising vapour pressure deficit (VPD) is often read as evidence of atmospheric drying, but under warming VPD increases even at constant relative humidity (RH). We asked when, within the growing season, atmospheric moisture has failed to keep pace with warming in Estonia, using daily data from 20 stations for April–September 1965–2024. As a scaling metric we used the ratio of the trends in actual and saturation vapour pressure (Δea/Δes), where 1 corresponds to constant RH, and we decomposed VPD trends into temperature and humidity components. VPD increased in April (0.17 hPa per decade), July, August and September. From July to September, actual vapour pressure increased at 70–80% of the rate of saturation vapour pressure, not significantly different from constant RH in July and August, so the VPD increase was mainly thermodynamic. In April, the scaling ratio was 0.38 (95% CI 0.13–0.73), RH declined by 0.93% per decade, and declining RH contributed as much to the VPD increase as warming. The April moisture lag was stronger at inland than at maritime stations (median ratio 0.28 vs 0.49). An earlier end of the snow season accounted for about one-fifth of the April RH trend. Results were robust to autocorrelation-corrected tests and to restricting the analysis to RH-homogeneous stations. An increase in VPD is therefore not necessarily a loss of atmospheric moisture: in Estonia, drying beyond the thermodynamic expectation is concentrated in early spring, a signal masked by seasonal means.