The structure of the precipitable water (PW) field over mountains is difficult to observe and can have a complex form. We demonstrate a method to analyse this structure with a case study at the Etna volcano in Sicily. The method involves decomposition of the PW field, power spectral analysis and high-resolution numerical modelling including an experiment to show the sensitivity to solar radiative forcing which can drive the mesoscale circulations and consequently the complex patterns of water vapour advection in complex terrain near a coastline. The PW decomposition and power spectral analysis were applied to both the model data and remotely sensed MODIS data. The PW field has two structural components: a horizontal mean component and a horizontal perturbation component that possesses a wave number k dependence of in the mesoscale range. For our case example (summer, mid-morning) we show that the horizontally perturbed component is largely driven by the combined land-sea and upslope breeze effects.