Oxygen-18 is a heavier-than-normal isotope of oxygen  In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen  But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth
Consequently, scientists were surprised when measurements along the entire route of rain clouds' passage from above the Atlantic Ocean, the site of their original formation, across the Amazon forests, where it rains almost daily, showed that the oxygen-18 content of each of the clouds remained fairly constant
Rain clouds above tropical forests are poorer in oxygen-18 than rain clouds above unforested regions.