Abstract
It is generally believed that at clam nights cold air drainage flows downslope forming cold air lakes at the bottom of the slopes. In this survey, the air temperature, the speed and direction of the wind were measured 50 cm above the ground on the s1Ope of Sugadaira Plateau (Fig. 1) in Nagano from Oct. 31 to Nov. 1, 1969. Results are illsutrated in Fig. 2, 3, 4.
We tried to recognzie the pass of cold air drainage in three manners:
1) The change of wind direction when cold air drainage passes.
2) The low temperature for shrot time and the high value of the standard deviation f the temperature.
3) We assumed that the air temperature rises suddenly after cold air drainage passes. But the results attained did not agree with these hypotheses, for
1) the wind speed is too slow to recognzie the pass of cold air drainage,
2) the high values of the standard deviation are releated to the rise of the air temperature and
3) the rise of air temperature is reasonably explained by sensible and latent heat. Cold air lake is expanded and confirmed as time passes at St. 5 and 4 (Fig. 2). It may be said that cold air lake is formed without cold air drainage. The formation of cold air lake is thought as follows:
At night there is a net loss of heat by radiation. Radiation flux divergence is a major cause of air cooling and the rate is equal in the forest and outside the forest. In the forest no wind blew, and outside the forest wind blew all night (Fig. 4). The eddy transfer component must be greater outside the forest than in the forest. So it gets cooler relatively in the forest.
