We have developed a reliable powerful method to measure the atmospheric column of water vapour (PWV) down to very low levels. For this purpose we use and off-the-shelf cheap spectrometer to measure the equivalent width of the H2O bands at 940nm. In order to calibrate the measurements we use the radiative transfer model included in the package SCIATRAN to produce theoretical solar spectra as observed on the ground, based on simultaneous measurements of the atmospheric profile obtained by means of balloon soundings; we then use these calibrated theoretical spectra to calibrate our spectrometer measurements. In this way we have measured above the Observatorio de Sierra Nevada (OSN) precise values of PWV down to 0.17 mm(H2O), the mean value at DomeC. We also present the comparative statistics of PWV as measured from ballon soundings at three observatories: OSN, Canarias, and Hawaii.