Global Pattern of Surface
Pressure and Wind
Global Pattern of Surface Pressure
If we averaged the sea level pressures over
the Earth throughout the year we would
see broad regions of high and low
pressure.
If we ignore the locations of continents and
oceans and major mountain ranges, the
broad regions would fall in latitudinal belts
around the Earth.
North Pole
Polar High Pressure
Polar Front – low pressure
Subtropical High Pressure
Intertropical Convergence Zone – low pressure
Subtropical High Pressure
Polar Front – low pressure
Polar High Pressure
South Pole
Pressure and Wind Patterns
These latitudinal bands of high and low
pressure regions help to determine the
average wind directions that we see at the
surface of the Earth.
North Pole
Polar High Pressure
Polar Front – low pressure
Subtropical High Pressure
Intertropical Convergence Zone – low pressure
Subtropical High Pressure
Polar Front – low pressure
Polar High Pressure
South Pole
Intertropical Convergence Zone
The InterTropical Convergence Zone (ITCZ)
is a zone of low pressure near the equator
where the trade winds from the northern
and southern hemispheres meet.
Trade Winds
The trade winds are the relatively steady
northeasterly or southeasterly winds
between the subtropical high pressure
system and the ITCZ that were used by
sailing ships to travel across the oceans.
Circulation In the Tropics
Most of the air converging on the ITCZ rises,
creating an area of relatively calm winds at
the surface called the doldrums.
Most of the air is sinking near the centers of
the subtropical highs, which also creates
an area of relatively calm winds at the
surface called the horse latitudes.
Hadley Cells
The circulation cell formed by the sinking air
in the subtropical high, the trade winds
blowing toward the ITCZ, the air rising at
the ITCZ and the diverging poleward flow
just below the tropopause is called a
Hadley cell.
There are two Hadley cells, one in the
northern hemisphere and one in the
southern hemisphere.
Precipitation Patterns in the Hadley
Cells
The rising motion in the region of the ITCZ
produces clouds and precipitation.
The heavy rains near the ITCZ are sufficient
to support the growth of tropical
rainforests.
The sinking of hot, dry air in the subtropical
highs leads to the formation of deserts.
The Hadley Cells and the Seasons
The subtropical highs and the ITCZ shift
toward the hemisphere that is having its
warm seasons.
Thus, in January the pattern shifts toward
the southern hemisphere and the ITCZ is
south of the equator.
In July the pattern is shifted toward the north
and the ITCZ is found north of the equator.
ITCZ
The Pattern Outside the Tropics
The average wind directions outside the
tropics are determined by the polar high
pressure system, the polar front, and
the subtropical high pressure system.
North Pole
Polar High Pressure
Polar Front – low pressure
Subtropical High Pressure
Intertropical Convergence Zone – low pressure
Subtropical High Pressure
Polar Front – low pressure
Polar High Pressure
South Pole
The Polar Front
The polar front moves north and south as it
is pushed by the polar and subtropical
high pressure systems.
Many of the low pressure systems that we
find on the surface weather map form
along the polar front.
North Pole
Polar High Pressure
Polar easterlies
Polar Front – low pressure
Surface westerlies
Subtropical High Pressure
Northeast trade winds
Intertropical Convergence Zone – low pressure
Southeast trade winds
Subtropical High Pressure
Surface westerlies
Polar Front – low pressure
Polar easterlies
Polar High Pressure
South Pole
Effects of Continents and Oceans
Continents, oceans, major mountain ranges,
and the formation of transient weather
systems all disrupt this nice neat and tidy
pattern of winds to produce the actual
winds we see at the Earth’s surface.
Air Masses
An air mass is a large body of air that has
similar thermal and moisture
characteristics in the horizontal directions.
Air Masses
Air masses form over source regions, which
are large, relatively flat areas over the
Earth’s surface with relatively uniform
characteristics.
Air masses typically have a surface high
pressure system at their center.
Source Regions
Air sinks and
spreads out from
the same
geographic
region.
H
The air takes on the characteristics of the surface.
Typical Air Mass Types
Arctic – very cold and very dry
Continental polar – cold and dry
Maritime polar – cool and moist
Maritime tropical – warm and moist
Continental tropical – hot and dry
Winter Pressure Pattern
The average sea level pressures are
generally higher over the continents
(Asia and North America) and lower over
the oceans (the Pacific and Atlantic)
during the winter.
This pattern is less evident in the southern
hemisphere because there is so much less
land in the southern hemisphere.
Summer Pattern
The average sea level pressures are
generally higher over the ocean (Pacific
and Atlantic) and lower over the
continents (Asia and North America)
during the summer.
Again, this pattern is also less evident in the
southern hemisphere.
The Monsoon
Since Asia is the largest landmass on the
surface of the Earth, it has the largest
impact on the pattern of pressure and
winds at the surface.
The most well known manifestation of the
impact of Asia is known as the monsoon.
The word monsoon means a large-scale
shift of wind directions with the seasons.
January Pattern
The surface pattern over Asia in January is
dominated by a large high pressure
system sometimes called the Siberian
high.
The Siberian High
This large high pressure system produces
diverging clockwise flow that spreads
cold, dry air over large portions of Asia.
Thus, winter is the dry season over Asia
July Pattern
The surface pattern over Asia in July is
dominated by low pressure over
southern and eastern Asia.
July Pattern (Cont.)
Warm moist air flows into Asia from the
Indian and Pacific oceans.
The warm moist air rises in low pressure
systems and as it flows up mountain
ranges like the Himalayas, producing
clouds and precipitation.
Rainy Season
Thus, summer is the rainy season over
much of Asia.
North American Monsoon
Although North America is not as large as
Asia, it is large enough to produce a
monsoon over the southwest.
January
The average sea level pressure over the
southwestern portion of the U.S. is
generally dominated by high pressure in
January.
July
Low pressure develops at the surface in
July.
If the low pressure is strong enough, it is
able to pull in moist air from the Gulf of
California into Arizona and summertime
thunderstorms can develop.