The water balance in urban areas is quite different from that of rural areas, because most of the earth's surface in urban areas is occupied by buildings and paved roads. The environmental differences affect the content of the soil moisture in the two areas. However, the annual variationn in ground temperatures depend not only on the heat exchange at the ground surface but also on the soil moisture.<br> Generally, the vertical profile of the mean annual ground temperatures is uniform in Tokyo. Therefore, the heat flux in the ground becomes zero. This results also zero in a heat balance when the heat exchange at the ground surface is integrated over the whole year. This situation is expressed by the following equation:<br> _??__N-_??_₀-_??_₀=0<br> where two terms other than the net radiation _??__N may be expressed in the following forms, respectively:<br> _??_₀=h(_??__s-_??__a)<br> _??_₀=αh{f (w)e_s(_??__a) -e_a}<br> h; sensible heat transfer coefficient<br> w; ratio of latent heat transfer coefficient to sensible heat transfer coefficient<br> _??__s; annual mean of ground temperature<br> _??__a; annual mean of air temperature<br> f(w); fractional constant dependent on the soil moisture (w)<br> e_s (_??__a); saturation vapor pressure at _??__a<br> e_a; annual mean of actual vapor pressure The annual mean of ground temperature is given as follows:<br> _??_<br> _??_; saturation deficit<br> _??_; slope of saturation vapor pressure at _??__a<br> If the amount of soil moisture is sufficient throughout the year, the fractional constant becomes unity and the annual mean of the ground temperature reaches the minimum value. Contrary to this situation, if the amount of soil moisture is insufficient, the annual mean of the ground temperature becomes higher. As shown in Fig. 5, the annual mean ground temperature in the central area is higher than in the surrounding areas. This is because the amount of soil moisture in the central portion is smaller than that in the surrounding areas.<br> Soil moisture influences the thermal diffusivity. The thermal diffusivity in turn affects the amplitude ratio of annual variations, and the phase difference at two depths. These two situations are expressed respectively by the following equations:<br> A_Z2/A_z1=e^{-(z2-z1) (ω/2k)1/2} and (α, _z2-α_z1)=(z₂-z₁) (l/2ωk)^1/2<br> A_z; amplitude of annual variation at the depth z<br> α_z; phase angle at the depth z<br> k; thermal difrusivity<br> ω; angular frequency of annual variation<br> As shown in Fig. 9, the amplitude ratio at two depths becomes larger in the central area than in the surrounding places. In contrast, the difference of phase angles between two depths becomes smaller in the central area compared to the surrounding areas. These are also caused by the decrease of soil moisture.<br> Finally, it is concluded that the heat island in the ground, in central Tokyo, is formed by the decrease of soil moisture.