Abstract A one-dimensional numerical model of heat transfer from open air into a forest is discussed. Diurnal variations of computed forest temperature show good agreement with the observed data. Rate of heat transfer q into a forest is assumed to be proportional to the temperature difference ΔT between the outside and inside of the forest, and the relation q = h ( ΔT) holds. The heat exchange constant h at the level of the tree crowns and on the forest floor is estimated at 6–20 W m −2 K −1, throughout the year studied, except in summer days. In these daylight hours, the h-value decreases remarkably due to increasing static stability of the forest air. Streamflow temperature indicates smaller amplitude than that of ground surface temperature, and it is computed by applying a smaller value to h than that on the forest floor, excepting the summer when water temperature is much lower than forest air temperature. Heat transport by running water plays an important role in determining streamflow temperature, and low temperature during the summer is closely related to the annual variations.