PRESSURE OVER THE GLOBE The unequal heating of Earth’s surface due to its tilt, rotation, and differential insolation, results in the wide   distribution   of   pressure   over   Earth’s   surface. Study figures 3-3A and 3-3B. Note that a low-pressure area   lies   along   the   intertropical   convergence   zone (ITCZ)  in  the  equatorial  region.  This  is  due  to  the higher temperatures maintained throughout the year in this region. At the poles, permanent high-pressure areas remain    near    the    surface    because    of    the    low temperatures  in  this  area  throughout  the  entire  year. Mainly the "piling up" of air in these regions causes the subtropical high-pressure areas at 30°N and S latitudes. Relatively high or low pressures also dominate other areas during certain seasons of the year. ELEMENTS OF CIRCULATION Temperature differences cause pressure differences,  which  in  turn  cause  air  movements.  The following sections show how air movements work and how they evolve into the various circulations—primary, secondary, and tertiary. To   explain   the   observed   wind   circulation   over Earth,  three  basic  steps  are  used.  The  first  step  is  to assume Earth does not rotate and is of uniform surface; that is, all land or all water. The second step is to rotate Earth, but still assume a uniform surface. The third step is  to  rotate  Earth  and  assume  a  non-uniform  surface. For now, we deal with the first two steps, a non-rotating Earth   of   uniform   surface   and   a   rotating   Earth   of uniform surface. Static Earth The circulation on a non-rotating Earth is referred to as the thermal circulation because it is caused by the difference in heating. The air over the equator is heated and rises (low pressure); while over the poles the air is cooled and sinks (high pressure). This simple circulation was shown in figure 3-1. Rotating Earth In  thermal  circulation,  the  assumption  was  made that  the  Earth  did  not  rotate,  but  of  course  this  is  not true.  The  rotation  of  Earth  causes  a  force  that  affects thermal   circulation. This   rotation   results   in   the deflection  to  the  right  of  movement  in  the  Northern Hemisphere,  and  to  the  left  of  the  movement  in  the Southern Hemisphere. This force is called the Coriolis force.   The  Coriolis  force  is  not  a  true  force.  It  is  an apparent force resulting from the west-to-east rotation of Earth. The effects, however, are real. Arctic  rivers  cut  faster  into  their  right  banks  than their  left  ones.  On  railroads  carrying  only  one-way traffic,  the  right  hand  rails  wear  out  faster  than  the left-hand  rails.  Artillery  projectiles  must  be  aimed  to the  left  of  target  because  they  deflect  to  the  right. Pendulum  clocks  run  faster  in  high  latitudes  than  in lower latitudes. All these phenomena are the result of the Coriolis force, which is only an apparent force. The most  important  phenomena  are  that  this  force  also deflects winds to the right in the Northern Hemisphere. Therefore, it is important to understand how this force is produced. As Earth rotates, points on the surface are moving eastward (from west to east) past a fixed point in space at  a  given  speed.  Points  on  the  equator  are  moving  at approximately 1,000 miles per hour, points on the poles are not moving at all, but are merely pivoting, the points somewhere  between  are  moving  at  speeds  between 1,000  and  zero  miles  per  hour  depending  upon  their relative position. Refer to view A in figure 3-4. 3-5 A B C AG5f0304 Figure 3-4.—Coriolis force.