AbstractKnowledge of the diurnal cycle of precipitable water vapor (PWV) is very limited owing to the lack of data with sufficient temporal resolution. Currently, GPS receivers have proven to be a suitable technique to determine PWV diurnal variations. In this study, the annual and seasonal diurnal cycles of PWV have been obtained from GPS data for 10 locations over Spain. The minimum value of PWV is reached approximately at the same time at all the stations, ∼0430–0530 UTC, whereas the maximum is reached in the second half of the day, but with a larger dispersion of its occurrence between stations. The annual sub‐daily variability ranges from 0.41 to 1.35 mm (3–7%). The highest values are recorded at the stations on the Mediterranean coast, with a doubling of the values of the stations on the Atlantic coast or inland. The winter cycle is quite similar at all locations, whereas in summer local effects are felt strongly, making the diurnal cycle quite different between stations. The PWV mean diurnal cycle is strongest in summer and weakest in spring, with a sub‐daily variability of 1.34 and 0.66 mm respectively. Harmonic analysis shows that the first two harmonics can explain 97% of the variance. The diurnal (24 h) harmonic explains 85% of the variance, has mean amplitude of 0.40 mm, and the peak time is from early afternoon to evening. The semi‐diurnal (12 h) harmonic is weaker, with an amplitude of 0.13 mm, and peak time between 0400 and 1000 UTC. The diurnal cycle of temperature alone would be a proxy for PWV cycle during the night, but not during the daytime. The breeze regime is the main factor responsible for the phase lag between PWV and temperature cycles during daytime. No clear correlation between the daily cycle of precipitation and PWV has been found. Copyright © 2011 Royal Meteorological Society