Chapters 6, 7 and 8. Read the assigned textbook pages and review the circulation diagrams and wind patterns below.
Chapter 7 Videos
Chapter 7 Lecture
Chapters 7 and 8 Lecture
Scales of Atmospheric Motion
Atmospheric motion is classified by the size and lifespan of the circulation. The largest patterns include planetary winds; smaller circulations include weather systems, thunderstorms and localized gusts.
Macroscale / planetary
Global wind belts and longwaves in the westerlies.
Synoptic scale
Weather-map-scale cyclones and anticyclones.
Mesoscale
Thunderstorms, tornadoes, land breezes and sea breezes.
Microscale
Turbulence, individual wind gusts and dust devils.
Local and Regional Wind Systems
Unequal heating creates temperature and pressure differences that drive local winds. Land heats and cools faster than nearby water, creating daily coastal circulations.
Sea Breeze
During the day, cooler air moves from the water toward warmer land near the surface.
Land Breeze
At night, land cools more quickly and a near-surface breeze may blow from land toward water.
Mountain and Valley Breezes
Daytime upslope flow and nighttime downslope flow result from heating and cooling of mountain slopes.
Other Winds
Chinook/foehn winds are warm, dry downslope winds; katabatic winds are gravity-driven cold-air drainage; country breezes flow toward warmer urban areas.
Three-Cell Model and Global Wind Belts
The idealized three-cell model describes circulation in each hemisphere: the Hadley cell from 0° to 30°, the Ferrel cell from 30° to 60°, and the Polar cell from 60° to 90°. Their associated surface wind belts include the trade winds, prevailing westerlies and polar easterlies.
| Latitude (Northern Hemisphere) | Pressure / Wind Pattern |
|---|---|
| 90° N | Polar high |
| 60°–90° N | Polar easterlies |
| Near 60° N | Subpolar low / polar front |
| 30°–60° N | Prevailing westerlies |
| Near 30° N | Subtropical high / horse latitudes |
| 0°–30° N | Northeast trade winds |
| Near 0° | Equatorial low / ITCZ / doldrums |
| 0°–30° S | Southeast trade winds |
Global Pressure Belts and Precipitation
The idealized pressure belts include the equatorial low (ITCZ), subtropical highs (roughly 20°–35°), subpolar lows (roughly 50°–60°), and polar highs. Rising air and convergence near lows often support cloud formation and precipitation. Sinking air near highs tends to suppress clouds and favor drier conditions.
Global rainfall patterns are influenced by these belts: wet equatorial areas, midlatitude storm tracks, dry subtropical deserts and relatively dry polar regions. Ocean currents and topography also influence precipitation.
Seasonal Pressure Patterns and Monsoons
Actual circulation is more complex than the idealized model because continents and oceans heat differently. Winter brings prominent features such as the Siberian high and the Aleutian and Icelandic lows. In summer, the subtropical North Atlantic high is often called the Bermuda high.
Monsoons involve pronounced seasonal changes in winds and rainfall. The Asian monsoon is particularly strong, while the North American monsoon brings increased summer rainfall to portions of the Southwest and northwestern Mexico.
Jet Streams, Rossby Waves and Upper-Air Flow
The polar jet stream is a narrow band of strong upper-level westerly winds associated with large horizontal temperature contrasts near the polar front. The jet shifts seasonally and can steer weather systems.
Rossby waves are large-scale waves in the midlatitude westerlies. Their troughs and ridges affect storm tracks. Jet streaks are localized maxima in jet-stream wind speed; associated upper-level convergence and divergence can influence vertical air motion and surface pressure systems.
Ocean Currents, El Niño and La Niña
Large-scale winds help drive surface ocean currents and gyres, which transport heat and affect nearby climates. Coastal upwelling can bring colder deep water toward the surface.
El Niño and La Niña are opposite phases of the El Niño–Southern Oscillation (ENSO), involving tropical Pacific sea-surface temperatures and atmospheric circulation. These changes can influence weather patterns far beyond the Pacific.
What to Know for Test 3
- Know the four scales of atmospheric motion, examples and relative lifespans.
- Explain sea and land breezes and other local winds.
- Identify Hadley, Ferrel and Polar cells and the global surface wind belts.
- Know pressure systems and wind directions at 0°, 30°, 60° and 90°.
- Explain surface and upper-level wind patterns, jet streams and Rossby waves.
- Connect rising/sinking air with wet and dry climate regions.
Reading: Pages 176–180, 182–193 (especially 183–185), and 190–195.