The high altitudes and extremely low temperatures of the Antarctic plateau result in an exceedingly low atmospheric water vapor content. In this article we estimate the precipitable water vapor at Dome A using optical spectra of the diffuse solar irradiance in the zenith direction. The spectra were obtained from the Nigel spectrometer at Dome A during 2009. We find that the Nigel spectra contain sufficient information to extract daily average water vapor estimates, which agree with satellite observations to within ±0:22 mm. Finally, we use these water vapor estimates to model the optical and near-IR transmission (between 700 nm and 2.5 μm) of Dome A to demonstrate significant advantages gained in this wavelength range and compare it with that of Paranal and Chajnantor, two midlatitude observatories.