Global Average Barometric Pressure: January
Figure 4.11
Global Average Barometric Pressure: July
Figure 4.11
Primary High Pressure & Low Pressure Areas
• Equatorial low pressure zone: (aka Intertropical
Convergence Zone) band of cloudiness &
abundant precipitation. Thermally induced
• Polar high pressure cells: subsiding air, stronger
in winter. When strong, are associated with
minimal precipitation. Thermally induced
• Subtropical high pressure cells: areas of
subsiding air, dry climates when near west side of
continents. Dynamically induced
• Subpolar low pressure cells: mean locations are
Iceland & Aleutian Islands. Cause winter precip.
(mid-latitude cyclones) Dynamically induced
• All of above pressure cells move north and south
in response to seasonal changes in the subsolar
point. Same w/ wind systems.
OPTIONAL Comparing July & January
Isobar Maps, Northern Hemisphere
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Remember that maps are of average pressures, so
they just show general tendencies
Aleutian Low not evident in July (but it is a weak
feature)
High pressure cells are more developed (have more
isobars) over oceans in July
High pressure cells are located farther north in July
(both location and strength driven by land-sea
temperature difference, with land warming in summer)
Note presence of continental high pressure over N
Asia and North America in January (thermal highs)
Other pressure cell terms
• Doldrums
• Horse Latitudes
Primary Wind Systems
• Trade Winds: most constant of all wind systems.
Zone also source region for tropical storms and
hurricanes.
• Westerlies: intensity positively related to EquatorPolar temperature differences. What season are they
strongest? Which hemisphere the most persistent?
• Polar Easterlies: velocity related to strength of polar
high; stronger in winter
• Polar Front: (not a wind) boundary where polar
easterlies and the westerlies converge, bad weather
zone
• Actual global wind circulation is modified by the
effects of continents and their different levels of
heating and cooling as compared with the oceans
General Atmospheric Circulation: This is
similar to your model to know!
Figure 4.13
Upper Atmospheric Circulation
• Jet Stream: the core of upper-level
westerly geostrophic winds (parallel to
isobars) , generally of high velocity.
• Rossby Waves are found in the upper-air
westerly circulation of middle and high
latitudes. See “Jet Streams” slide below.
Cross Section of General Atmospheric Circulation
Figure 4.13
Jet Streams
Figure 4.17
Local Winds
• defined
Monsoonal Wind Systems
• Seasonal wind changes that happen at a
regional/continental scale due to seasonal
reversals of onshore-offshore temperature
differences.
• Summer: onshore circulations, rain!
• Winter: offshore circulations, dry
• Where? S & SE Asia, Eq. Africa, a bit of
S. America