There   are   also   regions   where   the   pressure   is predominantly low or high at certain seasons, but not throughout the year.  In the vicinity of Iceland, pressure is low most of the time. The water surface is warmer (due  to  warm  ocean  currents)  than  the  surface  of Iceland or the icecaps of Greenland. The Icelandic low is most intense in winter, when the greatest temperature contrast   occurs,   but   it   persists   with   less   intensity through the summer. Near Alaska, a similar situation exists with the Aleutian low. The Aleutian low is most pronounced when the neighboring areas of Alaska and Siberia are snow covered and colder than the adjacent ocean. These lows are not a continuation of one and the same   cyclone.   They   are,   however,   regions   of   low pressure  where  lows  frequently  form  or  arrive  from other  regions.  Here  they  remain  stationary  or  move sluggishly for a time, then the lows move on or die out and are replaced by others. Occasionally these regions of low pressure are invaded by traveling high-pressure systems. Two  areas  of  semi  permanent  high-pressure  also exist. There is a semi permanent high-pressure center over  the  Pacific  westward  of  California  and  another over the Atlantic, near the Azores and of the coast of Africa. Pressure is also high, but less persistently so, west of the Azores to the vicinity of Bermuda. These subtropical highs are more intense and cover a greater area in summer than winter. They also extend farther northward summer. In winter, these systems move soul toward the equator, following the solar equator. The   largest   individual   circulation   cells   in   the Northern Hemisphere are the Asiatic high in winter and the   Asiatic   low   in   summer.   In   winter,   the   Asiatic continent is a region of strong cooling and therefore is dominated  by  a  large  high-pressure  cell.  In  summer, strong  heating  is  present  and  the  high-pressure  cell becomes a large low-pressure cell. (See fig. 3-3A and fig. 3-3B.) This seasonal change in pressure cells gives rise to the monsoon flow over India and Southeast Asia. Another cell that is often considered to be a center of  action  is  the  polar  high.  Both  Arctic  and  Antarctic highs  have  considerable  variations  in  pressure,  and these   regions   have   many   traveling   disturbances   in summer. For example, the Greenland high (due to the Greenland icecap) is a persistent feature, but it is not a well-defined  high  during  all  seasons  of  the  year.  The Greenland high often appears to be an extension of the polar  high  or  vice  versa. Other  continental  regions show seasonal variations, but are generally of small size and  their  location  is  variable.  Therefore,  they  are  not considered to be centers of action. An   average   annual   pressure   distribution   chart (figure 3-14) reveals several important characteristics. First, along the equator there is a belt of relatively low pressure encircling the globe with barometric pressure of about 1,012 millibars. Second, on either side of this belt  of  low  pressure  is  a  belt  of  high  pressure.  This high-pressure  area  in  the  Northern  Hemisphere  lies mostly  between  latitudes  30°  and  40°N  with  three well-defined centers of maximum pressure. One is over the eastern Pacific, the second over the Azores and the third  over  Siberia;  all  are  about  1,020  millibars.  The belt  of  high  pressure  in  the  Southern  Hemisphere  is roughly  parallel  to  30°S.  It,  too,  has  three  centers  of maximum  pressure.  One  is  in  the  eastern  Pacific,  the second  in  the  eastern  Atlantic,  and  the  third  in  the Indian  Ocean;  again,  all  are  about  1,020  millibars.  A third  characteristic  to  be  noted  from  this  chart  is  that, beyond the belt of high pressure in either hemisphere, the   pressure   diminishes   toward   the   poles.   In   the Southern Hemisphere, the decrease in pressure toward the South Pole is regular and very marked. The pressure decreases   from,   an   average   slightly   above   1,016 millibars  along  latitude  35°S  to  an  average  of  992 millibars along latitude 60°S In the Northern Hemisphere, however, the decrease in pressure toward the North Pole is less regular and not as great. This is largely  due  to  the  distribution  of  land  and  water:  note the extensive landmass in the Northern Hemisphere as compared to those of the Southern Hemisphere. While   the   pressure   belts   that   stand   out   on   the average  annual  pressure  distribution  chart  represent average  pressure  distribution  for  the  year,  these  belts are   rarely   continuous   on   any   given   day.   They   are usually  broken  up  into  detached  areas  of  high  or  low pressure by the secondary circulation of the atmosphere. In either hemisphere, the pressure over the land  during  the  winter  season  is  decidedly  above  the annual    average.    During    the    summer    season,    the pressure is decidedly below the average, with extreme variations occurring such as in the case of continental Asia.  Here  the  mean  monthly  pressure  ranges  from about  1,033  millibars  during  January  to  about  999 millibars during July. Over the northern oceans, on the other   hand,   conditions   are   reversed;   the   summer pressure there is somewhat higher. Thus in January the Icelandic  and  Aleutian  lows  intensify  to  a  depth  of about 999 millibars, while in July these lows fill and are almost obliterated. 3-15