Water moves continuously
between the atmosphere, ocean, and land. In much of the world, the differences in seasonal precipitation—and in how and where that water gets stored on land—are big enough to affect Earth’s local gravity field.
These maps show changes in water storage in the Western Hemisphere from July 2009 through June 2010; that is, they show how much more or less water is tied up in lakes and rivers, groundwater aquifers, soil moisture, snow, and glaciers. The measurements were made by the Gravity Recovery and Climate Experiment (GRACE)
, twin satellites that measure subtle changes in Earth's gravity over time. In this case, the satellites measured how Earth's gravity field changed as water piled up or was depleted from different regions at different times of year.