(left) Difference from the average altitude at which the pressure is 500 millbars (mb). Pressure is close to 1000 mb near the Earth's surface and drops at higher altitudes. A 150-meter geopotential height anomaly means you have to go 150 meters higher into the atmosphere than usual to reach the altitude where the pressure drops to 500 mb. (right). Difference from average wind speed and direction at the altitude where the pressure is 500 mb. Map by NOAA Climate.gov, based on NCEP Reanalysis data from NOAA ESRL. large pressure image | large wind image