The  center  of  low  pressure
forming  an  extrartropical  cyclone
is  alwajTS  associated  with  one  or
two  centers  of  high  pressure.  In
Figs.  744,  745,  746,  747  and  748,
is  contrasted  the  atmospheric  cir-
culation about  centers  of  low
pressure  and  high  pressure.  Figs.
E744  to  746  show  the  wind-move-
ment at  three  different  levels
about  the  center  of  low  pressure.
The  wind  directions  as  ^own  by
the  arrows  have  been  determined
from  a  large  number  of  observa-
tions of  the  cloud  movements  at
several  levels  in  the  atmosphere
over  broad  areas  of  the  northern
hemisphere.  A  careful  study  of
the  first  three  figures  shows  that  while  the  sur-
face winds  about  the  low  pressure  exhibit  a  well-
marked  right-hand  deflection  from  the  radial  lines
indicating  the  steepest  barometric  slopes  toward
the  center,  yet
all  the  winds
blow  distinctly
inward.  Fig.
745,  giving
the  direction
of  the  lower
clouds,  shows
that  this  right-
hand  deflec-
tion is  of  yet
greater  mag-
nitude at  a
moderate  ele-
vation, so  that

the  winds  have

1       f  Fl£*  748.   Movement  of  Ugb  dooda  about

an  almost  Cir-  ^j^^  ^  ^^  piessuie  in  northan  bem-

CUlar    move-  Isphere.    (Pica.  744-748 after  Hann.)


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THE  ATMOSPHERE  AND  ITS  PHENOMENA


581


ment  around  the  center  and  the  inward  compo-
nent becomes  almost  nothing.  The  directions  of
the  highest  clouds  over  the  center  of  low  pres-
sure (Fig.  746)  indicate  very  distinctly  a  spiral
movement  away  from  the  center  of  low  pressure.
