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When moisture-laden air is forced to rise and cool as it blows over a hill or mountain range, it can become saturated, resulting in cloud and, sometimes, rain. Having lost moisture, as the air descends the other side of the high ground, it is drier. Drier air warms at a faster rate as it descends, compared with saturated air. As a result, on the sheltered (lee) side of hills, the weather can often be drier and several degrees warmer than on the exposed (windward) side. The word ‘Foehn’ comes from Alpine countries, where this phenomenon is common. In North America, the same effect across the Rockies is called the ‘Chinook’ (which is native American for ‘Snow-Eater’).
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