global average barometric pressure: january

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 on 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

• 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 Equator-Polar 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