This part of ISO 15927 specifies two procedures for providing an estimate of the quantity of water likely to impact on a wall of any given orientation. It takes account of topography, local sheltering and the type of building and wall. The first method, given in Clause 3 and based on coincident hourly rainfall and wind data, defines a means of calculating - the annual average index, which influences the moisture content of an absorbent surface, such as masonry, and - the spell index, which influences the likelihood of rain penetration through masonry and joints in other walling systems. The second method, given in Clause 4 and based on average wind data and a qualitative recording of the presence and intensity of rain (the present weather code for rain), defines a means of calculating the spell length during which an absorbent material such as masonry is moistened, which has a 10 % probability of being exceeded in any year (commonly referred to as having a mean return period of 10 years). A comparison between the two methods is given in informative Annex D.