Development of Temperature Patterns during Clear Nights

This paper examines nocturnal air temperature structure development in complex terrain. Clear nights were studied in order to compare the cooling rate in different topographical areas with a variety of land cover. It was found that large variations in temperature developed over a very short time period after sunset and that in situ cooling was the dominant factor causing this in valleys and other wind-sheltered locations. Shelter can be provided both from valley sides and from nearby trees with the main effect being to reduce the vertical mixing. The nighttime increase in temperature difference between valley bottoms and nearby reference areas was interpreted to be due to cold airflows. This was also shown by the increasing lateral extension of cold air accumulating in valleys. This development was found only in open valley locations. Sheltered areas cooled at a much faster rate than exposed sites during early evening. Further cooling did not increase the lateral extension of cold air or result in larger temperature differences. This is attributed to lack of cold airflow. The rapid cooling was supported due to reduced mixing of warmer air from above. This hypothesis was further confirmed in this study by analysis of clear nights with low regional wind speed when it was found that sheltered locations differed significantly in temperature compared to wind-exposed areas.