Foehn Winds When air flows downhill from a high elevation, its temperature is raised by adiabatic compression. Foehn winds are katabatic winds caused by adiabatic heating of  air  as  it  descends  on  the  lee  sides  of  mountains. Foehn winds occur frequently in our western mountain states  and  in  Europe  in  the  late  fall  and  winter.  In Montana and Wyoming, the Chinook is a well-known phenomenon; in southern California, the Santa Ana is known particularly for its high-speed winds that easily exceed 50 knots. For the purpose of illustrating a Foehn wind, the Santa Ana is used. The   condition   producing   the   Foehn   wind   is   a high-pressure   area   with   a   strong   pressure   gradient situated near Salt Lake City, Utah. This gradient directs the wind flow into a valley leading to the town of Santa Ana near the coast of California. As the wind enters the valley,  its  flow  is  sharply  restricted  by  the  funneling effect of the mountainsides. This restriction causes the wind   speed   to   increase,   bringing   about   a   drop   in pressure  in  and  near  the  valley.  The  Bernoulli  effect causes this pressure drop in and near a valley. Generally  speaking,  when  the  Santa  Ana  blows through   the   Santa   Ana   Canyon,   a   similar   wind simultaneously  affects  the  entire  southern  California area.    Thus,    when    meteorological    conditions    are favorable,  this  dry  northeast  wind  blows  through  the many  passes  and  canyons,  over  all  the  mountainous area,  including  the  highest  peaks,  and  quite  often  at exposed   places   along   the   entire   coast   from   Santa Barbara  to  San  Diego.  Therefore,  the  term  Santa  Ana refers to the general condition of a dry northeast wind over southern California. In  the  Rocky  Mountain  states,  the  onset  of  Foehn winds have accounted for temperature rises of 50°F or more in only a few minutes. In southern California, the temperature, though less dramatically, also rises rapidly and is accompanied by a rapid decrease in humidity (to 20  percent  or  less)  and  a  strong  shift  and  increase  in wind speeds.   Although these winds may on occasion reach  destructive  velocities,  one  beneficial  aspect  is that   these   winds   quickly   disperse   the   severe   air pollutants that plague the Los Angeles Basin. REVIEW QUESTIONS Q3-15. What is the cause of monsoon winds? Q3-16. What causes land and sea breezes? Q3-17. Describe Bernoulli's theorem. Q3-18. When  does  a  valley  breeze  usually  reach  its maximum? Q3-19. What causes eddies? Q3-20. What causes Foehn winds? SUMMARY In this chapter, we studied the primary, secondary and tertiary circulation of the atmosphere.  We learned about  large-scale  circulations,  worldwide  locations  of major pressure systems, horizontal and vertical pressure  systems.   We  studied  how  pressure  systems, temperature, and world winds relate to each other, and finally  we  studied  small-scale  effects,  due  to  local features. A   good   understanding   of   atmospheric circulation   is   essential   in   order   to   understand   the characteristics of air masses and fronts. 3-29