a) Scatter plot of the PWV estimated from GOES/FNL versus the PWV from GOES/GFS at the APEX site from 15 August to 25 October 2009, over the range 0–1.2 mm. The 1:1 line is also shown in the plot. b) CDF of PWV absolute relative errors between GOES/FNL, GOES/GFS, and observations at the APEX site (solid and dashed lines), and between GOES/GFS and observations at the RHUBC-II site (dotted line).