Water vapor is a key in the hydrological cycle and the main driver of atmospheric events. Remote sensing of water vapor using the Global Positioning System (GPS) is essential for weather and climate research. Advances in GPS application has allowed the measurements of ionospheric total electron content (TEC) and precipitable water vapor (PWV) in all weather conditions and on a global scale with fine temporal and spatial resolution. Using TEC as a measure of solar activity and PWV as the terrestrial response, a new approach for determining and quantifying the solar influence on PWV by indirectly correlating the PWV and TEC variations on a time basis is presented in this book.