accounts  for  the  increases  in  gradients.  In  tropical regions,   the   geostrophic   wind   scales   become   less reliable because pressure gradients are generally rather weak. REVIEW QUESTIONS Q3-1.    Which   two   factors   influence   the   Earth's temperature? Q3-2.    What are the major factors that result in the wide distribution of pressure over the earth's surface? Q3-3.    What   effect   does   Coriolis   force   have   on thermal circulation in the Northern Hemisphere? Q3-4.    According  to  the  3-cell  theory,  how  many circulation belts are there? Q3-5. According  to  the  3-cell  theory,  what  type  of pressure  system  would  you  normally  find  at 30 degrees north latitude? Q3-6. What  is  the  predominant  wind  system  in  the tropics? Q3-7. Name two types of pressure gradient. Q3-8. What  is  the  difference  between  centrifugal force and centripetal force? Q3-9. What is the difference between gradient wind and geostrophic wind? Q3-10. What is the relationship between centrifugal force   and   pressure   gradient   force   around anticyclones? SECONDARY CIRCULATION LEARNING  OBJECTIVE:  Determine  how centers   of   action,   migratory   systems,   and seasonal    variations    affect    secondary    air circulations. Now   that   you   have   a   picture   of   the   general circulation of the atmosphere over Earth, the next step is to see how land and water areas offset the general circulation.  The  circulations  caused  by  the  effect  of Earth’s   surfaces,   its   composition   and   contour,   are known  as   secondary   circulations.  These  secondary circulations give rise to winds that often cancel out the normal effect of the great wind systems. There are two factors that cause the pressure belts of   the   primary   circulation   to   break   up   into   closed circulations of the secondary circulations. They are the non-uniform  surface  of  the  earth  and  the  difference between  heating  and  cooling  of  land  and  water.   The surface   temperature   of   oceans   changes   very   little during   the   year.   However,   land   areas   sometimes undergo extreme temperature changes with the seasons. In  the  winter,  large  high-pressure  areas  form  over  the cold  land  and  the  low-pressure  areas  form  over  the relatively warm oceans. The reverse is true in summer when  highs  are  over  water  and  lows  form  over  the warm land areas. The result of this difference in heating and cooling of land and water surfaces is known as the thermal effect. Circulation    systems    are    also    created    by    the interaction  of  wind  belts  of  pressure  systems  or  the variation in wind in combination with certain distributions  of  temperature  and/or  moisture.  This  is known as the dynamic effect. This effect rarely, if ever, operates alone in creating secondary systems, as most of  the  systems  are  both  created  and  maintained  by  a combination of the thermal and dynamic effects. CENTERS OF ACTION The  pressure  belts  of  the  general  circulation  are rarely  continuous.  They  are  broken  up  into  detached areas of high and low pressure cells by the secondary circulation.    The    breaks    correspond    with    regions showing differences in temperature from land to water surfaces. Turn back to figures 3-2A and 3-2B. Compare the temperature distribution in views A and B of figures 3-2  to  the  pressure  distribution  in  views  A  and  B  of figure 3-3. Note the gradient over the Asian Continent in January. Compare it to the warmer temperature over the ocean and coastal regions. Now look at view A of figure 3-3 and note the strong region of high-pressure corresponding to the area. Now look at the same area in July. Note the way the temperature gradient flattens out and  warms.  Look  at  view  B  of  figure  3-3  and  see  the low-pressure  area  that  has  replaced  the  high-pressure region of winter. These pressure cells tend to persist in a particular area and are called centers of action; that is, they   are   found   at   nearly   the   same   location   with somewhat similar intensity during the same month each year. There is a permanent belt of relatively low pressure along the equator and another deeper belt of low-pressure   paralleling   the   coast   of   the   Antarctic Continent.  Permanent  belts  of  high  pressure  largely encircle  Earth,  generally  over  the  oceans  in  both  the Northern  and  Southern  Hemispheres.  The  number  of centers  of  action  is  at  a  maximum  at  about  30  to  35 degrees from the equator. 3-14