Download Air Masses and Pressure Centers

The Effect on Climate of
Seasonal Variation in Air Masses
and Global Circulation
Figures are from:
Lutgens, F. (2007). The Atmosphere, 10ed. Upper Saddle River, NJ:
Pearson Education.
Air Mass Source Regions for
North America
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Source regions are mostly confined to subtropical and subpolar locations
Middle latitudes are the site where cold and warm air masses collide
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Due to converging winds of a traveling cyclone
This zone lacks the conditions needed to be a source region (stagnant air)
Winter and Summer maps show that the extent and temperature characteristics
fluctuate.
Semipermanent Pressure and Wind Systems
Average Surface Pressure and Global Winds for January and July
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On both maps, the pressure regimes are elongated, not zonal.
Subtropical highs, centered between 20o and 35o over subtropical oceans,
are the most prominent features.
Some pressure cells, such as subtropical highs are year-round features.
Others are seasonal
Little pressure variation occurs from midsummer to midwinter in the
Southern Hemisphere due to the large amount of water in the hemisphere.
January Pressure and Wind Patterns
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Polar Highs
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Azores High
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Prominent features of winter circulation over northern continents
Siberian High over frozen landscape of northern Asia
A weaker polar high is located over North America
Positioned near the northwest coast of Africa
Semipermmanent lows (Aleutian and Icelandic lows)
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Not present in July and are not stationary. They are the composite of many cyclonic storms that move
across the regions.
Regions affected by these lows experience cloudy conditions with abundant winter precipitation.
Aleutian low’s cyclones
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Produced by frigid air and directed by the Siberian high (flows off the continent of Asia and overrun comparatively warm air
over the Pacific
This generates cyclonic storms and travel eastward, bringing abundant preicipitation to coastal areas of N. America.
Note the position of the ITCZ
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Results in cool, dry air over Asia due to a strong high that develops.
Subtropical highs with subsiding drier air dominate Asia
July Pressure and Wind Patterns
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Increased insolation on North American landmasses
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Cause high surface temperatures
Lows replace winter highs
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Tropical highs in the Northern Hemisphere
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Dominate summer circulation over oceans
Pump warm moist air onto the continents west of the highs resulting in increased precipitation over
parts of eastern North America and Southeast Asia.
The Bermuda High is positioned near Bermuda (hence, its name)
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Thermal lows have warm ascending air that causes inward surface flow of air
Strongest low is over southern Asia
Weakest is over SW United States
A Subtropical high that migrates to the east during the winter and is then called the Azores high.
ITCZ moves to the north and draws warm, moist air onto the Asian continent
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The summer monsoon with heavy rainfall over the Indian subcontinent
Eastern Asia and Southeastern Asia experience high precipitation from frequent
thunderstorms.