variable.   These   areas   are   referred   to   as   the   horse latitudes.   The prevailing westerlies, which are on the poleward side of the subtropical high-pressure belt, are persistent throughout the mid-latitudes. In the Northern Hemisphere,   the   direction   of   the   westerlies   at   the surface   is   from   the   southwest.   In   the   Southern Hemisphere, westerlies are from the northwest. This is due to the deflection area resulting from the Coriolis effect as the air moves poleward. Poleward  of  the  prevailing  westerlies,  near  60°N and  60°S  latitudes,  lies  the  belt  of  low-pressure  basic pressure    known    as    the    polar    front    zone.    Here, converging  winds  result  in  ascending  air  currents  and consequent poor weather. WIND THEORY Newton’s  first  two  laws  of  motion  indicate  that motion  tends  to  be  in  straight  lines  and  only  deviates from such lines when acted upon by another force or by a combination of forces. Air tends to move in a straight line  from  a  high-pressure  area  to  a  low-pressure  area. However,  there  are  forces  that  prevent  the  air  from moving in a straight line. Wind Forces There    are    four    basic    forces    that    affect    the directional    movement    of    air    in    our    atmosphere: pressure   gradient   force   (PGF),   the   Coriolis   effect, centrifugal  force,  and  frictional  force.  These  forces, working together, affect air movement.  The forces that are  affecting  it  at  that  particular  time  determine  the direction that the air moves. Also, the different names given  to  the  movement  of  the  air  (geostrophic  wind, gradient   wind,   etc.)   depends   on   what   forces   are affecting it. Pressure Gradient The   rate   of   change   in   pressure   in   a   direction perpendicular to the isobars is called pressure gradient. Pressure  applied  to  a  fluid  is  exerted  equally  in  all 3-7 POLAR FRONT POLAR FRONT WESTERLY WESTERLY WIND WIND BELT BELT POLAR EASTERLIES POLAR EASTERLIES 60^O NORTHEAST TRADEWINDS SOUTHEAST TRADEWINDS INTERTROPICAL CONVERGENCE ZONE DOLDRUM  BELT DRUM 30^O 60^O 30^O UB UB DOL- POLAR HIGH CUMULONIMBUS TOPS AS HIGH AS 60,000 FT CEILING BELOW 1,000 FT WINDS EASTERLY AT SURFACE WESTERLY ALOFT WINDS WESTERLY AT ALL ELEVATIONS WINDS EASTERLY TO 5,000 FT THEN WESTERLY WINDS VARIABLE WINDS EASTERLY TO 25,000 FT, THEN WESTERLY 60,000 FT. 25,000 FT. AG5f0306 Figure 3-6.—Idealized pattern of the general circulation.  (The 3-cell theory).