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Such a behavior is explained by the humidity content of the test walls. For porous building materials like red brick or adobe, a great part of the water content was evaporated during the 60 minute warming process. For a concrete wall,the evaporation was more sluggish because of the wall's low porosity. Consequently, for "high" wind velocities the evaporated water was not yet detected by the top sensors, but for "low" velocities the air stream was not capable of dragging the evaporated water coming out from the test wall. This hypothesis is supported by the high thermal conductivity of the concrete compared with the other building materials and the correspondingly larger amount of heat coming out from the concrete wall towards the air. Therefore, the temperature of air near the wall surface rises quickly, the density and Reynolds number (Re) diminish, and the viscosity increases, causing an augmentation of the Prandtl number (Pr) and the thickness of the hydrodynamic and thermal boundary layers. This augmentation is supported by the low speed of the heat carriers (air particles) that results in an "overheating" of such heat carriers as they absorb more heat during their time of travel along the surface of the wall. Thus, Eq. (4) was not used as an alternative method to calculate the convective heat transfer coefficients of the test walls since the thicknesses of the thermal boundary layer estimated from the temperature profiles were distorted by the evaporated water coming out of the test wall.