An evaluation of high-frequency temperature observations revealed, that annually there exist 80--100 calm weather days, when a significant part of the nocturnal cooling period can be well fitted with a simple exponential function. Statistics of characteristic time constants indicates a wide, skewed distribution and the lack of an annual cycle. A search for basic meteorological variables determining the magnitude of the cooling rate remained inconclusive. No correlation was found with daily average temperature anomaly, soil moisture, absolute air humidity, wind speed, pressure, nighttime outgoing and daytime incoming radiation. The air temperature is more strongly coupled to the soil temperature, but surprisingly, the cooling rate in the soil seems to be very weakly correlated with the cooling rate of air at 2 m.