Oxygen–18 is a heavier–than–normal isotope of oxygen. In a rain cloud, water molecules containing oxygen–18 are rarer...

Victoria on June 4, 2020

Hi @avif,

Happy to help!

We know that the cloud itself contains more water molecules containing ordinary oxygen than oxygen-18.

However, we only know that the rainfall contains "...a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen."

This does not mean that there are more oxygen-18 molecules than ordinary oxygen molecules in the rainfall. It simply means that a higher proportion of the oxygen-18 molecules descends in the rainfall.

For example, say that 70% of the molecules in the cloud which contain oxygen-18 fall as compared to 30% of the molecules in the cloud that contain ordinary oxygen. If there are 10 times as many ordinary oxygen molecules in the cloud, then there will still be more ordinary oxygen molecules in the rainfall despite the fact that the relative proportion of oxygen-18 molecules is higher.

70% of 10 = 7 oxygen-18 molecules
30% of 100 = 30 ordinary oxygen molecules

Hope this is helpful! Please let us know if you have any further questions.

avif on June 5, 2020

Very much so. Thanks!