There are many parameters accounting for the quality of an astronomical site, namely seeing, cloud cover, ground winds, high-altitude winds, etc. The water vapor content is the main parameter affecting the IR quality of astronomical sites. The fraction of nights with good IR conditions (small column of water vapor) as a function of the epoch of the year will allow an optimal scheduling of telescope observing time. Global Positioning System (GPS) is an increasingly operational tool for measuring the precipitable water vapor (PWV). In this seminar, we briefly describe the procedure to estimate the PWV through GPS and we present the statistical results derived from a 7.5-year long time series of PWV estimations derived from GPS at the Roque de los Muchachos Observatory.