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