Wind-driven rain impinging on monuments and mountain slopes

For general climatic purposes, rain is measured in terms of amount, intensity and frequency. In this paper, the problem of wind-driven rain impinging on monuments and mountain slopes is considered because the normal meteorological stations measure the precipitation falling on the horizontal plane, but miss the vector dimension (i.e. the vertical and horizontal components). The rain impinging on vertical surfaces remains unknown. The convenience of representing rainfall as a vector is discussed, especially when it is necessary to know the impact of rain in urban environments, on stone monuments (e.g. stone erosion) or mountain slopes (landslide risk assessment). An analysis is made of: wind driven rain; its trajectory; the effective catch areas that funnels with horizontal and vertical openings have when the drop trajectory is inclined; and the precipitation amounts that can be collected using a combination of a horizontal and a vertical funnel. Fixed and wind-orientated vectopluviometers with vertical funnels are commented. From these inputs, it is possible to calculate the precipitation falling at any inclination angle, or impinging on any slant surface. As wind-driven rain is a vector, the two components collected by the combination of a horizontal and a vertical gauge should be added between them in vector form. Useful formulae are presented and commented.