The quantification of water vapor in tropical regions like Central America is necessary to estimate the influence of climate change on its distribution and the formation of precipitation. This work reports daily estimations of precipitable water vapor (PWV) using Global Positioning System (GPS) delay data over the Pacific region of Costa Rica during 2017. The GPS PWV measurements were compared against atmospheric sounding and Moderate Resolution Imaging Spectrometer (MODIS) data. When GPS PWV was calculated, relatively small biases between the mean atmospheric temperatures (T m ) from atmospheric sounding and the Bevis equation were found. The seasonal PWV fluctuations were controlled by two of the main circulation processes in Central America: the northeast trade winds and the latitudinal migration of the Intertropical Convergence Zone (ITCZ). No significant statistical differences were found for MODIS Terra during the dry season with respect GPS-based calculations (p > 0.05). A multiple linear regression model constructed based on surface meteorological variables can predict the GPS-based measurements with an average relative bias of −0.02 ± 0.19 mm/day (R 2 = 0.597). These first results are promising for incorporating GPS-based meteorological applications in Central America where the prevailing climatic conditions offer a unique scenario to study the influence of maritime moisture inputs on the seasonal water vapor distribution.Climate 2019, 7, 63 2 of 18 of atmospheric water vapor content are needed to improve the predictability of rainfall and the understanding of and feedback in climate related processes [5,6].A quantifiable parameter useful for studying water vapor is the precipitable (or integrated) water vapor (PWV). Precipitable water vapor mainly comprises tropospheric water vapor and the less abundant stratospheric water and can be used to analyze water vapor variability and its contributions to climate change [6]. The classical approach to gather information about PWV is using atmospheric sounding based on radiosonde profiles [7]. However, due to high costs, radiosonde networks lack spatial and temporal resolutions and, thus, provide limited information to carry out detailed studies of weather and climate. For example, radiosondes are usually launched 1-2 times per day in monitoring stations spaced several hundred kilometers from each other. In recent years, the fast development of ground-based GPS networks allows a new source of water vapor information. As atmospheric water changes the atmospheric refractivity, satellite-receiver path delays provide a unique information on the total water vapor within the troposphere and stratosphere. Therefore, GPS has become a standard technique for measuring PWV with some noticeable advantages over radiosondes. For instance, GPS can be used in all weather conditions and has low operation costs, allowing for a high temporal resolution with numerous records throughout the daytime and nighttime [8][9][10]. In Costa Rica, there are 14 Global Navigation Satellite Syst...