Chapter 5: Atmospheric Pressure, Winds, and Circulation Patterns

Physical Physical GeographyGeographyNinth EditionNinth Edition

Robert E. Gabler

James. F. Petersen

L. Michael Trapasso

Dorothy Sack

Pressure

• Atmospheric Pressure– Variations in

pressure create atmospheric circulation (including wind).

Pressure

• Mercury Barometer– Standard Sea level

pressure is 1013.2 mb. (29.92 inches of mercury)

– When air pressure increases, what happens to the mercury in the tube?

5.1 Variations in Atmospheric Pressure

• Vertical Variation– Air Pressure and air

density decrease rapidly with altitude (height).

– By approximately how much does density drop between 0 and 100 km?

5.1 Variations in Atmospheric Pressure

• Horizontal Variations– Determined by thermal (temp) or dynamic (motion of

atmosphere) conditions.– Thermal

• Warm/hot air is less dense and wants to rise. This creates low pressure near the equator.

• Cold air is more dense and wants to sink, creating high pressure, near the poles.

– Dynamic:• High pressure in the subtropics.

• Low pressure in the subpolar regions (e.g. 40-60o N and S)

5.2 Basic Pressure Systems

• Low (Cyclone) = L– Air is ascending (rising)– Low pressure

• High (anticyclone) = H– Air is descending (subsidence)– High Pressure

5.3 How is Temp related to the Density of Air

• Convergence: wind going toward center (L)

• Divergence: wind going away from center (H)

• Mapping Pressure Distribution– Adjust to sea level pressure– Isobars: lines of equal pressure– strong pressure gradient (isobars close together

causes stronger winds– Weak pressure gradient (isobars farther apart)

causes weaker winds

5.3 How is Temp related to the Density of Air

• Wind is the horizontal movement of air due to pressure differences.

• Pressure Gradient– Where on this figure

would winds be the strongest?

5.4 Wind

• Coriolis Effect – Apparent deflection of

the wind– N. hem: wind is deflected

to the right– S. hem: wind is deflected

to the left.

5.4 Wind

• Friction and WindUpper Levels (no friction):

Ground has little effect. Wind is parallel to isobars. This is a geostrophic wind.

– At or near surface, friction slows the wind and reduces the Coriolis force. Surface wind blows across isobars.

5.4 Wind

• Wind Terminology– Winds are named for where they come from

• Wind from northeast is called NE wind

– Windward– Leeward

5.4 Wind

• Anticyclone (H) – wind moves away from center in a clockwise spiral in N. hem.

• Wind goes form high to low pressure

• Cyclone (L) – wind moves towards center in a counterclockwise spiral in N. hem

5.4 Wind

What do you think might happen to the diverging

air of an anticyclone if there is a cyclone nearby?

5.4 Wind

• Global Pressure Belts– Equator (trough or L)– 30oN and S – subtropical

High– Subpolar low (L)– Polar high (H)

– This idealized pressure pattern is affected by landmasses and topography.

5.5 Global Pressure Belts

• Seasonal Variations in the Pressure pattern– Shift northward in July and

southward in January due to location of sun’s direct rays.

– January:• Icelandic Low• Aleutian low

– July• Bermuda/Azores High• Pacific High

5.5 Global Pressure Belts

What is the difference between the January and July average sea-level pressure at your location? Why do they vary?

5.5 Global Pressure Belts

5.6 Global Surface Wind Systems

5.6 Global Surface Wind Systems

• Latitudinal Zones– Trade Winds (5o-25o)– Doldrums

• A zone of calm and weak winds

• Trade winds converge• ITCZ (Intertropical

Convergence Zone)• ITCZ = region with high

precip. and cloud cover.

5.6 Global Surface Wind Systems

• Subtropical Highs

• Westerlies

• Polar Winds

• Polar Front

5.6 Global Surface Wind Systems

• Effects of Seasonal Migration greatest at transition zone– 5o-15o (ITCZ and

subtropical high)– 30o-40o (subtropical

high and polar front)

5.6 Global Surface Wind Systems

• Longitudinal Differences in Winds

5.7 Upper Air Winds and Jet Streams

• Jet Stream – very strong, narrow band of winds embedded within the upper air westerlies

5.7 Upper Air Winds and Jet Streams

• Polar front Jet Stream

• Subtropical Jet Stream

• Which jet stream is most likely to affect you in January?

5.7 Upper Air Winds and Jet Streams

• Rossby waves

5.7 Upper Air Winds and Jet Streams

5.8 Subglobal Surface Wind Systems

• Monsoons: seasonal shift of the winds– Low pressure (summer) - wet– High pressure (winter) - dry

5.8 Subglobal Surface Wind Systems

• Local Warming Winds– Air is compressed and

heated as it moves downslope

– Chinooks– Santa Ana

• Local Drainage Winds– Katabatic

5.8 Subglobal Surface Wind Systems

• Land-Sea Breeze– Diurnal (daily reversal of

wind)– Differential heating

between land and water

5.8 Subglobal Surface Wind Systems

• Mountain Breeze-valley breeze– Diurnal

5.9 Ocean-Atmosphere Relationships

• Ocean Currents

• Gyres: major surface currents

• Warm currents– Gulf Stream– Kuroshio Current

• How does this map of ocean currents help explain the mild winters in London, England?

5.9 Ocean-Atmosphere Relationships

5.9 Ocean-Atmosphere Relationships

• El Niño: weak warm countercurrent that replaces cold coastal waters off the coast of Peru (equatorial Pacific) .

5.9 Ocean-Atmosphere Relationships

• El Niño Southern Oscillation (ENSO): Easterly surface winds weaken and retreat to the eastern Pacific, allowing central Pacific to warm and the rain area migrates eastward.

• La Niña – opposite of ENSO

5.9 Ocean-Atmosphere Relationships

• El Niño and Global Weather

5.9 Ocean-Atmosphere Relationships

• North Atlantic Oscillation (NAO): relationship between Azores High and Icelandic Low.

• + NAO = larger pressure difference between Azores and Icelandic. Eastern US may be mild and wet during winter.

Physical Geography

End of Chapter 5: Atmospheric Pressure, Winds, and Circulation Patterns