Köppen’s climatic types are still considered valid today. His climatic zones, like others, are by no means static.  Climatic  zones  shift  with  long-range  weather patterns. The most noticeable shifts in these climatic zones have been observed over the northern portions of North America and Asia and over Africa. Russia and Canada,   for   example,   have   been   able   to   conduct farming at higher latitudes over the past 200 years due to   milder   temperatures.   Recent   studies,   however, indicate a general return of cooler temperatures at high latitudes,  and  now  the  growing  region  is  gradually moving southward again where temperatures are more moderate. In Africa, desert regions have made notable shifts southward due to decreasing precipitation. Trewartha  is  the  most  recent  classifier  of  climate. Initially,   his   climatic   classifications   were   based   on Köppen’s;   however,   over   the   years,   he   has   made significant changes and is now recognized for developing  his  own  six  climatic  groups.  These  six groups are tropical, dry, subtropical, temperate, boreal, and    polar.    Five    of    these    groups    are    based    on temperature  and  one  is  based  on  precipitation  (see Table 6-2). Trewartha’s climatic groups, like Köppen’s, are  also  further  broken  down  into  climatic  types  and subtypes. REVIEW QUESTIONS Q6-9.    List    the    five    climatic    belts    and    their boundaries. Q6-10.     Name  the  three  classifications  of  climatic types. Q6-11.     What are the five climatic types according to Köppen? CLIMATIC CONTROLS LEARNING OBJECTIVE: Identify the controlling factors that affect climate. The  variation  of  climatic  elements  from  place  to place and from season to season is due to several factors called  climatic  controls.  The  same  basic  factors  that cause  weather  in  the  atmosphere  also  determine  the 6-7 NORTH    POLAR    ZONE NORTH   TEMPERATE    ZONE SOUTH   TEMPERATE    ZONE HOT   BELT SOUTH    POLAR    ZONE 60 40 20 0 20 40 180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 120 50 F  ISOTHERM (WARMEST MONTH) MEAN  ANNUAL ISOTHERM OF 68  F MEAN  ANNUAL ISOTHERM  OF  68  F 50  F  ISOTHERM (WARMEST MONTH) AGf06001 Figure 6-1.—Temperature zones.