Orvieto is located on a hill top, a windy and relatively unpolluted site. The local microclimate, solar radiation and induced thermal wave, the main thermohygrometric parameters, exchanges of heat and moisture and deposition processes have been considered with reference to stone deterioration and the fall of the tesserae. The thermal pattern of the Cathedral is strongly influenced by direct solar radiation which is only limited by surrounding buildings. The different materials and variety of albedos of the coloured tessarae cause thermal gradients and internal stresses. Relative humidity is quite high, especialy during the night and, in winter, throughout the day. The NNW side of the Cathedral is the coldest and condensation forms in the micropores and on the surface favouring lichen growth. Diurnal heating of the zones affected by direct solar radiation forces marked condensation-evaporation cycles. Droplets condensing on the mosaic grow, run off and are partially absorbed between the tessarae, facilitating their detachment. Thermophoretic deposition and Stefan flow are seldom active; unlike the ever present inertial deposition.