The weight of the Earth's atmosphere exerts pressure on the surface of the Earth. This pressure varies from place-to-place due the variations in the Earth's surface since higher altitudes have less atmosphere above them than lower altitudes. Atmospheric pressure also varies from time-to-time due to the uneven heating of the atmosphere by the sun and the rotation of the Earth, causing weather. In order to see the changes in pressure which affect the weather, the variation due to altitude is removed from the surface pressure, creating a quantity called sea level pressure. This animation shows the atmospheric sea level pressure for the whole globe from September 1, 2004, through September 5, 2004, during the period of Hurricane Frances in the western Atlantic Ocean and Typhoon Songda in the western Pacific Ocean. The sharp, moving low pressures areas for Frances and Songda can be clearly seen in the oceans. Even with the direct effect of altitude removed, cold high-altitude regions such as the South Pole and the Himalayan Plateau still exhibit lower-than-normal pressures, probably due to the interaction of cold air over those regions with the warmer air in the surrounding regions.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 22.214.171.124.0