The Earth's atmosphere exerts pressure based on the weight of the air above, so the pressure reduces with rising altitude. This rate of pressure reduction with altitude is based on the temperature of the air, with the pressure of colder air reducing faster with altitude than warmer air. Therefore, a surface of constant pressure has a lower altitude at the poles than the equator. This animation shows the altitude above sea level (the geopotential height) of the 300 hectopascal (hPa) pressure surface 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. This pressure is about one-third of the normal pressure at sea level. The largest downward slope of this surface occurs in the mid-latitudes and is shown in yellow in the animation. At this region, air is trying to flow from the equator towards the poles to reduce the slope, but the rotation of the Earth forces the flow to divert to the east, forming the strong west-to-east jet stream flows in these regions. Frances and Songda can be seen as sharp yellow dots of reduced height in their respective locations.
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 188.8.131.52.0