Lisø PhD Dissertation Manuscript - NTNU

1452
the Norwegian building practice. However, wood is the
most common cladding material for dwellings and
smaller buildings, and a great percentage of the building
stock is built with wooden exterior cladding separated
from the interior construction bya ventilated and
drained air gap (two-stage tightening). A two-stage
watertight fac-ade is a design consisting of an outer rain
protection layer, a ventilated and drained space and an
airtight inner layer. The outer rain protection layer, i.e.
the cladding, can be different kinds of wood panelling,
metal sheeting or board claddings. In houses with
ventilated wooden cladding, moisture migration by
capillarityis certainlynot the major form of moisture
ingress into the inner construction from driving rain.
Therefore, an index that yields information about the
relative occurrence of weather conditions relevant for
capillarymoisture transfer into a masonrywall is not
appropriate for determining the risk from driving rain of
moisture uptake in an inner wall construction behind the
air gap in a building with ventilated cladding. A
different type of approach is needed.
Rather, byusing a multi-year sample interval, enough
observations of wind-plus-rain are accrued to obtain a
representative picture of the relative frequencyof
driving rain from different directions at a weather
observing station. Normalizing the frequencydistribution
bythe annual rainfall amount at a station then
allows for quantitative comparison of directional driving
rain between stations. A wall index can then be
calculated using the usual method of summing directional
driving rain contributions from directions with
wind blowing against a wall having a specific orientation
[3,4].
3. Methodology and discussion
Synoptic observations from most weather stations in
Norwayinclude the 10-min average wind speed and
direction at the time of observation as well as a
numerical code (standardized internationallybythe
World Meteorological Organization [5]) identifying the
state of the weather at the time of the observation.
Observations are done four times daily(1, 7 AM, 1 and
7 PM local time) at the larger stations and three times
daily(7 AM, 1 PM and 7 PM local time) at the more
rural stations. Electronic records of synoptic observations
go back to at least 1960 for most stations. As the
present-weather codes are numerical, records of observations
from many years are easily analysed in
spreadsheet programs. There are six separate codes for
rain and three for rain showers, listed in Table 1 below.
Clearlythere is an element of subjectivityand thus
uncertaintyin analysing codes representing the observer-determined
type of rain that was occurring at a
particular time and location.
ARTICLE IN PRESS
J.P. Rydock et al. / Building and Environment 40 (2005) 1450–1458
Table 1
Codes for rain or rain showers used to select rain events in the synoptic
observation data
CodeSynoptic weather description
60 Rain, not freezing, intermittent, slight at time of observation
61 Rain, not freezing, continuous, slight at time of observation
62 Rain, not freezing, intermittent, moderate at time of observation
63 Rain, not freezing, continuous, moderate at time of observation
64 Rain, not freezing, intermittent, heavyat time of observation
65 Rain, not freezing, continuous, heavyat time of observation
80 Rain shower(s), slight
81 Rain shower(s), moderate or heavy
82 Rain shower(s), violent
The strategyemployed here was quite simple. We
began byselecting all observations in the analysis period
that coded for some type of rain (codes 60–65 or 80–82),
the thought being that while the determination of
whether a rainfall event was light, moderate or heavy,
continuous or intermittent or showerywas subjective,
the appearance of one of these codes could confidently
be construed as objective evidence that a rainfall event
was associated with that observation time. Next, the
selected observations were grouped according to the
wind direction (in 101 increments) observed at the time
of the rainfall event. The analysis period chosen was
January1, 1974 to December 31, 2003, the 30-year
period representing the most recent climate in Norway.
Data from four observing stations in different parts of
the countryand with different climates are considered
here as examples to illustrate the methodology. The
stations are located in the cities of Oslo, Bergen and
Tromsø, and at the airport serving Trondheim (about
30 km east of Trondheim). These four cities are shown
on the map of Norwayin Fig. 1. Average precipitation
and wind speeds for the four cities are shown in Table 2.
Oslo has a relativelyprotected location on the Oslo
Fjord, and the wind speeds are generallylow (average
annual wind speed 2.7 m/s). Bergen, Trondheim and
Tromsø are more exposed to mid-latitude cyclones
coming in off the Atlantic Ocean, and have average
annual wind speeds between 3.5 and 4 m/s. Bergen has
byfar the largest annual precipitation, with 2250 mm/yr.
From this table alone one would be able to conclude
that Bergen has byfar the largest driving rain, Oslo the
least, and Trondheim and Tromsø somewhere in
between.
Table 3 lists the total average annual number of rain
events for each station as well as the percentage of
moderate/heavy observations for the 30-year analysis
period. Not surprisingly, Bergen, with the highest
average annual precipitation, had the highest number
of annual rain observations with wind (245 out of a total
of 365 4 ¼ 1460 possible per year, or about 17%) and
the highest average number of moderate or heavy