Figure 12-5. Three-cell circulation pattern due to the rotation of  the Earth.  Figure 12-4.  Atmosphere weights.  correct in theory, the circulation of air is modified by several  forces, the most important of which is the rotation of the Earth.  the surface. Then, it flows southward along the surface back  toward the equator. Coriolis force bends the flow to the right,  thus creating the northeasterly trade winds that prevail from  30° latitude to the equator. Similar forces create circulation  cells that encircle the Earth between 30° and 60° latitude and  between 60° and the poles. This circulation pattern results in  the prevailing upper level westerly winds in the conterminous  United States.  The force created by the rotation of the Earth is known as  the Coriolis force. This force is not perceptible to humans as  they walk around because humans move slowly and travel  relatively short distances compared to the size and rotation  rate of the Earth. However, the Coriolis force significantly  affects motion over large distances, such as an air mass or  body of water.  The Coriolis force deflects air to the right in the Northern  Hemisphere, causing it to follow a curved path instead of a  straight line. The amount of deflection differs depending on  the latitude. It is greatest at the poles and diminishes to zero  at the equator. The magnitude of Coriolis force also differs  with the speed of the moving body—the greater the speed,  the greater the deviation. In the Northern Hemisphere, the  rotation of the Earth deflects moving air to the right and  changes the general circulation pattern of the air.  Circulation patterns are further complicated by seasonal  changes, differences between the surfaces of continents and  oceans, and other factors such as frictional forces caused  by the topography of the Earth’s surface that modify the  movement of the air in the atmosphere. For example, within  2,000 feet of the ground, the friction between the surface and  the atmosphere slows the moving air. The wind is diverted  from its path because of the frictional force. Thus, the wind  direction at the surface varies somewhat from the wind  direction just a few thousand feet above the Earth.  The Coriolis force causes the general flow to break up into  three distinct cells in each hemisphere.  [Figure 12-5]  In  the Northern Hemisphere, the warm air at the equator rises  upward from the surface, travels northward, and is deflected  eastward by the rotation of the Earth. By the time it has  traveled one-third of the distance from the equator to the  North Pole, it is no longer moving northward, but eastward.  This air cools and sinks in a belt-like area at about 30°  latitude, creating an area of high pressure as it sinks toward  Atmospheric pressure historically was measured in inches of  mercury ("Hg) by a mercurial barometer.  [Figure 12-6]  The  barometer measures the height of a column of mercury inside a  glass tube. A section of the mercury is exposed to the pressure  of the atmosphere, which exerts a force on the mercury. An  increase in pressure forces the mercury to rise inside the tube.  When the pressure drops, mercury drains out of the tube  decreasing the height of the column. This type of barometer is  typically used in a laboratory or weather observation station,  is not easily transported, and difficult to read.  Measurement of Atmosphere Pressure  12-4  18,000 feet 14.7 lb  1 7.4 lb  1 Sea level  1 Square Inch 1 Square Inch 1 Square Inch 1 Square Inch