When working on the global annual average isotopic composition of oxygen-18 and deuterium (H) in meteoric water, geochemist Harmon Craig observed a correlation between these two isotopes, and subsequently developed and defined the equation for GMWL:
Where δO and δH (also known as δD) are the ratio of heavy to light isotopes (e.g. O/O, H/H).
The relationship of δO  and δH in meteoric water is caused by mass dependent fractionation of oxygen and hydrogen isotopes between evaporation from ocean seawater and condensation from vapor. As oxygen isotopes (O and O) and hydrogen isotopes (H and H) have different masses, they behave differently in the evaporation and condensation processes, and thus result in the fractionation between O and O as well as H and H. Equilibrium fractionation causes the isotope ratios of δO and δH to vary between localities within the area. The fractionation processes can be influenced by a number of factors including: temperature, latitude, continentality, and most importantly, humidity.