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. This animation shows the atmospheric surface 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 major changes in pressure occur over land where the surface altitude varies, but the sharp, moving low pressures areas for Frances and Songda can be clearly seen in the oceans. Since changing surface pressure areas over land are hard to see in these images due to the strong altitude variations, plots of the atmospheric surface pressure are almost never used to study the weather. A different plot, of sea-level pressure, is used instead.
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