A powerful new method to measure the atmospheric water vapour column.

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.