At the ALMA site on Llano de Chajnantor the amount of precipitable water vapour (PWV) is typically 1.0 mm and falls below 0.5 mm up to 25% of the time. The image below shows the atmospheric transmission between 70 and 960 GHz for a range of common PWV values.
The linked atmospheric transmission calculator can be used in order to see details of the atmospheric transparency. It is based on the ATM model written by Juan Pardo and covers the frequency range 10 to 1010 GHz with a spectral resolution of 0.1GHz.
The tool plots the zenith atmospheric transmission on Llano de Chajnantor, for a specified frequency range (limits 10 to 1010 GHz) and for up to 6 different PWV values in the range 0 to 20 mm.
The table below shows the amount of time (in percent) below a certain pwv