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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. Procedures are given to correct the results of both methods for topography, local sheltering and the type of building and wall. The methods included in this part of ISO 15927 do not apply in a) mountainous areas with sheer cliffs or deep gorges, b) areas in which more than 25 % of the annual rainfall comes from severe convective storms, c) areas and periods when a significant proportion of precipitation is made up of snow or hail.