**4.3 The effect of thermo physical properties**

Even though orientation (see 4.1) and colour (see 4.2) affect the surface temperatures of the materials exposed to solar radiation, so also their thermo physical properties. This fact is evident from the maximum surface temperatures and the daily temperature variation of certain groups of materials, such as loose, earthen materials, light-weight concrete and wood.

The high surface temperatures (Tables 1 and 2) of loose earthen materials directly depend on their thermo physical properties. Dry earth has low thermal conductivity (0.25 to 0.30 W/mK) and density (300 to 1600 kg/m3) (Oke, 1995) values, depending on the composition of the soil (Oke, 1995). The values for wood are similar (about 0.09 W/mK for softwood and 0.19 W/mK for hardwood) (Oke, 1995). Consequently, the ability of these materials to diffuse heat through their mass (Oke, 1995; Givoni, 1998) and their ability to store heat (Oke, 1995; Szokolay, 1980), which are expressed by the theoretical properties of diffusivity and admittance, respectively, are relatively low. As a result, earthen materials and wood have a rather contradictory thermal behaviour. They heat up considerably until noon, but cool down rapidly (3 to 4 oC every half hour) after 13:00 to 14:00, with their surface temperatures dropping below the air temperature by sunset (19:30 to 20:00).
