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Precipitable water vapor (PWV or precipitable water, PW) is a measure of the water vapor in an atmosphere, consisting of the equivalent depth of water in the liquid state. Such a number might be cited for the whole atmosphere from top to bottom (total precipitable water, TPW), or might be cited as that between two specified altitudes. The average over the entire Earth is on the order of 2 to 3 cm and Earth regions and times can vary from near zero to 60 cm or more. Reliably determining measurements by region and time is challenging. One method uses performance data of microwave-communications to/from communications and global positioning satellites. PWV is of interest in the effort to determine the future of Earth's climate.
PWV is of obvious interest to planetary science in the study of other atmospheres but only a small percentage of bodies have any appreciable amount of water vapor. It is of very much interest regarding astronomical observation from the ground because water vapor is a significant element in determining atmospheric windows: the less water vapor, the less opaque the atmosphere and the windows offer transparency to wider bands and greater transparency at each frequency, in the microwave and millimeter regions of the spectrum. Observatories are often located at locations that generally have a low PWV.