Hydrogen is the most abundant element in the universe. It’s also an important component of water and of organic materials.
Hydrogen has three isotopes: hydrogen (mass = 1 [1 proton]), deuterium (mass = 2 [1 proton, 1 neutron]), and tritium (mass = 3 [1 proton, 2 neutrons]). Tritium is radioactive and is used in ‘hydrogen bombs’. Hydrogen and deuterium are both stable, and because deuterium is twice the mass of hydrogen, there is substantial fractionation between the two.
Water has two hydrogen atoms (in addition to one oxygen). Evaporation and condensation of water causes fractionation of hydrogen that we can measure (alongside oxygen) to determine the temperature at which condensation and/or evaporation took place. With this knowledge, we can determine, for example, the pathways by which moisture (clouds) moves across continents.
These fractionated isotopic ratios are then preserved in rocks from which we can later extract the ratios and learn about ancient weather patterns as well.