当空气温度较高时，含有较重同位素氧-18的蒸汽比含有氧-16的蒸汽更容易凝结，并形成雪状的降水。
When the air was warm, vapor containing the heavier isotope, oxygen-18, condensed and formed precipitation, in the form of snow, more readily than did vapor containing oxygen-16.
水中几乎所有的氧都是氧16，但每一千个分子中就有几个含有较重的同位素18。
Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the heavier isotope 18.
通过测量一束头发上较重的氢和氧同位素的比例，科学家可以构建一个地理时间轴。
By measuring the proportion of heavier hydrogen and oxygen isotopes along a strand of hair, scientists can construct a geographic timeline.
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