Toxicol.andEnviron.Chem., Vol. 84, No. 1–4, pp. 53–67 2001 OPA (Overseas Publishers Association) N.V.Reprints available directly from the publisher Published by license underPhotocopying permitted by license only the Gordon and Breach SciencePublishers imprint,a member of the Taylor & Francis Group.

Precipitable water (W), defined as the total amount of water vapor in the zenith direction between the surface of the earth and the top of the atmosphere, is a critical variable for climate studies. Variations and trends of precipitable water values time series at Hellinicon Station (37 540 N, 23 440 E) in Athens, Greece are examined (00:00 UTC and 12:00 UTC radiosondes) for the time period 1967–1995. Time series were examined using regression analysis and the Mann-Kendall Rank Statistic Test. For 00:00 UTC, an annual downward trend is detected for the period 1977–1995; in addition 1977 is the approximate year, according to the sequential version of the Mann-Kendall Rank Statistic, when the decrease of the annual and seasonal mean precipitable water starts. For 12:00 UTC no statistically significant annual trend is detected, whereas a statistically significant downward trend for winter, spring and summer for the period 1977–1983, 1976–1982 and 1976–1985 respectively is recognized.