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Inversion = cold air is slightly heavier than warm air, making the cold air flow down.Thus cold air gathers near earth's surface, especially to valleys and depressions, on clear and placid winter nights, when the warm air escapes to space (so-called long wave radiation). When this happens it is clearly warmer higher up in the fells. In the winter, it can be several degrees warmer in the upper slopes of the fells than in the valleys.
In the summer, it is usually (especially during the day) colder in the fells than in the valleys.
Cold air is slightly heavier than warm air, therefore the cold air flows down and the warm air rises up. In northern tree-covered hill and fell terrain, 150 metre's difference in altitude, means 2 - 6 degrees differense in temperatures. In fell terrain, hikers should take this into account.
In winter time the warm air above prevents air pollution from leaving the city climate.