Climbing 100 metres feels like travelling 80 kilometres toward the pole
Walk 100 metres uphill and, in climate terms, you have moved about 80 kilometres closer to the nearest pole. That rough rule explains why a tropical peak can wear permanent snow, and why a single mountainside can host forests, meadows and bare frozen rock stacked one above another.
The cooling comes from rising air. Sunlight warms the ground, the ground warms the air above it, and warm air expands and climbs. Air conducts heat poorly, so a rising pocket barely trades warmth with its surroundings; as the pressure on it falls, it simply gets colder. For dry air the drop is about 9.8 degrees Celsius per kilometre. Moist air cools more slowly, around 5.5 degrees per kilometre, because condensing vapour releases heat as clouds form. Higher still, snow replaces rain and winds strengthen.
Ecologists capture the effect with a measure devised by Leslie Holdridge in 1947 called biotemperature: the average temperature with every sub-zero reading counted as zero, since dormant plants do not care how cold it gets. On permanently snowy summits it can fall below 1.5 degrees. Mountain ice is now shrinking at an accelerating rate, making slopes less stable and landslides more frequent, and nearly half of mountain areas supply important water to populations downstream.
What counts as a mountain is surprisingly unsettled. Britain and Ireland usually require a summit of at least 2,000 feet, about 610 metres. The United States Board on Geographic Names once drew the line at 1,000 feet but dropped that definition in the 1970s, and the US Geological Survey now says the words have no technical meaning there. Meanwhile ranges averaging 1,500 to 2,000 metres, such as the Black Hills and the Sivalik Hills, are still called hills. By the United Nations Environment Programme's broader classes, which weigh slope as well as height, about 24 percent of Earth's land is mountainous, including a third of Eurasia.
Most mountains come in three kinds, all driven by moving plates. Volcanic peaks such as Mount Fuji build up from magma; fold mountains like the Jura crumple where plates collide, with thickened crust sinking into the mantle beneath them like the hull of a loaded boat; and block mountains rise along faults. Erosion never stops working on them, which is why a mountain's surface is younger than its rock. Even the question of the tallest has two answers: Everest reaches highest above sea level, but Mauna Kea stands 9,330 metres from its base on the ocean floor.
Source: Mountain
