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) Compare january and july for northern hemisphere: 1. Remember that maps are of average pressures, so they just show general tendencies 2. Aleutian Low not evident in July (but note that 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 +

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

Sail Around the World Alone race, in the screaming sixties

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, 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