chabriac wrote:Is it normal to never get to 100%? ( a particularity of Wufi) or should I keep my local data.
Hi chabriac,
this has nothing to do with WUFI; it probably depends on how the weather data have been measured.
I, too, have noticed that sometimes in weather data for locations from the same region, one file may have no relative humidities exceeding, say, 98%, while another file may often have humidities of 100%. I think the main reason for this is simply the limited accuracy of the usual humidity sensors. One of our technicians once told me that the capacitive sensors which are used in most meteorological stations can easily be off by several percent, in particular in very humid or very dry conditions. So I think that in most cases these differences at humidities close to 100% are simply slight miscalibrations of the sensors (these sensors tend to have some calibration drift, as Manfred has already pointed out.)
I'm not sure if the humidity routinely reaches 100% when it rains. When the rain water (coming from cooler regions of the atmosphere) is cooler than air temperature, its vapor pressure must be lower than the saturation vapor pressure of the ambient air, and the relative humidity must remain below 100%. On the other hand, if the rain water is falling on a warm surface (e.g. an asphalt road), it may become warmer than the ambient air and thus even supersaturate the air (leading to small vapor plumes above the road surface). However, if relative humidity always reached 100% during a rain event, you would always see some condensation on all surfaces which are slightly cooler than the ambient air. This does not seem to be the case usually, so I suppose that the humidity, while indeed rising strongly, rarely reaches 100%.
You
will have 100% in fog.
As Manfred suggests, you may use the weather data with the higher humidity to be on the safe side. However, I'm a bit surprised that the results for your wall react so sensitively when a relatively small change in the weather data is made. Yes, the water content of your wall at 100% RH will be very different from the water content at 97% (the moisture storage function being very steep in this region), but it should also take your wall a long time to reach free saturation (the water content at 100% RH) if the humidity can only be added by vapor diffusion (the relative humidity of the ambient air drives vapor diffusion, not capillary transport). Therefore, it should not make much of a difference whether the wall is exposed to 100% or only 97% for a few hours during the nights. I suspect that there might be another reason why the weather files lead to such different results.
Regards,
Thomas
