Travel across a mountain range and the landscape frequently changes completely between one side and the other. Forest on the near side, dry scrub on the far side, within a few kilometres. The mountain itself is doing it.
Air carrying moisture from the sea meets the slope and has nowhere to go but up. Rising air expands and cools, and cool air cannot hold as much water vapour as warm air, so the moisture condenses into cloud and falls as rain on the windward slope. By the time the air crosses the summit it has lost most of its water.
Coming down the far side it warms again, which has the opposite effect: warm air can hold more moisture, so instead of releasing water it takes it, drying the ground it passes over. This is why the sheltered side of a range is often arid, and the effect has a name: a rain shadow.
Some of the driest places on earth sit in rain shadows a short distance from some of the wettest. The same wind that drowns one valley starves the next.
The practical consequence shapes where people live and what they plant, often within sight of each other. Two villages on opposite sides of one ridge may grow entirely different crops, speak about the weather in entirely different terms, and never think of the mountain as the reason.
Vocabulary in this text
- condense /kənˈdɛns/ — to turn from gas to liquid, or to make shorter
- instead of /ˌɪnˈstɛd ˈʌv/ — in place of something else
- arid /ˈæɹəd/ — having very little rain
- rain shadow /ɹˈeɪn ˈʃæˌdoʊ/ — the dry area on the far side of a mountain range
- consequence /ˈkɑnsəkwəns/ — a result of an action
- reason /ɹˈizən/ — why something happens