| Summary: | This paper seeks for an answer to a question why a north-facing shady slope may provide equal or even higher daily temperatures than a south-facing sunny slope. Findings are based on data collected in 1998-2000 with the aid of data loggers (type Minilog-TX) which were located on opposite sides of an over 200 metres deep and three kilometres wide, east-west oriented Lake Paanajärvi valley. Due to the high-latitude position (66o15’) near the Arctic Circle sun is to be seen during the summer solstice day also at night-time on tops of the highest hills, while during the midwinter sunshine does not reach the valley bottom for many weeks. Comparison of thermal data with the official daily weather records made at the valley bottom showed that the sun warmed up the north-facing shady slope more than the south-facing sunny slope from evening till morning on the cloudless days of June and July. Winds of the same days were mostly orientated along the valley axis. Fair-weather conditions coinciding with along-valley winds evoked an idea of a helical flow. Summing up, the prime mover for equal daily valley temperatures is the midsummer sun which, firstly, keeps the shady side warmer than the sunny side at night time and, secondly, warms up the sunny slope at daytime. The latter event results in a local low pressure, rising air masses and compensating air flow across the valley bottom, with a concurrent wind along the valley due to the difference in warming capacities between the uplands and the lowlands. The combined action of these two winds generates a spiral-like helical flow which adds to the equality of temperatures in that it moves the daytime warmth of the sunny slope to the opposite side of the valley.
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