Continuous, in situ measurements of water vapor isotopic composition have been conducted in the North Atlantic, at the Bermuda Islands (32.26° N, 64.88° W), between November 2011 and June 2013, using a cavity ring-down spectrometer water vapor isotope analyzer and an autonomous self-designed calibration system. Meticulous calibration allows us to reach an accuracy and precision on 10 min average of δ18O, δ D, and d-excess of, 0.14, 0.85, and 1.1‰, verified using two parallel instruments with independent calibration. As a result of more than 500 days with 6-hourly data the relationships between deuterium excess, relative humidity (RH), sea surface temperature (SST), wind speed, and wind direction are assessed. From the whole data set, 84 % of d-excess variance is explained by a strong linear relationship with relative humidity. The slope of this relationship (−42.6 ± 0.4‰ % (RH)) is similar to the theoretical prediction of Merlivat and Jouzel (1979) for SST between 20 and 30 °C. However, in contrast with theory, no effect of wind speed could be detected on the relationship between d-excess and relative humidity. Separating the data set into winter, spring, summer, and autumn seasons reveals different linear relationships between d-excess and humidity. Changes in wind directions are observed to affect the relationships between d-excess and humidity. The observed seasonal variability in the relationship between d-excess and relative humidity underlines the importance of long-term monitoring to make accurate conclusions.