These have been getting brighter and more common in recent years, and also seem to be moving to lower altitudes.

这些云层在近年中变得颇为明亮，出现得也更加频繁，并且这些云层似乎也正向着低处移动。

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Mountain air contains less oxygen than air at lower altitudes, so breathing it causes the heart to beat faster and the body to burn more energy.

高山上的空气含氧量小于比低海拔地区，所以呼吸时心脏会跳动地更快，身体也会消耗更多能量。

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At lower altitudes, they complained of mosquitoes that had never before infested the area, the team reports online today in Biology Letters.

他们说这个地区从来没受到过这么大一群蚊子的侵扰。研究小组在今天的《生物学快报》在线版上报道了这则消息。

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Nearly 200, 000 people are in makeshift camps, mainly of tents, at lower altitudes.

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After sunset, the plane slowly glides to lower altitudes on the stored power until dawn, when the process starts again.

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The proportion of oxygen in the air 21% is constant whether at sea level or on the highest peak, but at lower altitudes there is more air pressure to force oxygen into tissue.

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