  The atmospheric water vapor content above the Roque de los Muchachos
Observatory (ORM) obtained from Global Positioning Systems (GPS) is presented.
GPS measurements have been evaluated by comparison with 940nm-radiometer
observations. Statistical analysis of GPS measurements points to ORM as an
observing site with suitable conditions for infrared (IR) observations, with a
median column of precipitable water vapor (PWV) of 3.8 mm. PWV presents a clear
seasonal behavior, being Winter and Spring the best seasons for IR
observations. The percentage of nighttime showing PWV values smaller than 3 mm
is over 60% in February, March and April. We have also estimated the temporal
variability of water vapor content at the ORM. A summary of PWV statistical
results at different astronomical sites is presented, recalling that these
values are not directly comparable as a result of the differences in the
techniques used to recorded the data.
