An aside on the Earth's reference frame for pressure: the geoid
{"Image DocumentName": "An aside on the Earth\u2019s reference frame for pressure: the geoid", "Image ImageDescription": "When we calculate horizontal pressure gradients, they are relative to a level surface along which the pressure gradient vanishes (no force, no motion) Earth\u2019s mass is not distributed evenly AND Earth is not a perfect ellipsoid. That is, the level surface is not locally flat . The surface of constant pressure, i.e. along which the gravitational force has no changes, is the geoid Good geoid site  Talley SIO 210 (2014)", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}