fundamentals of physical geography 1e petersen sack gabler chapter 4: atmospheric pressure, winds,...

Fundamentals of Physical Geography 1e

Petersen

Sack

Gabler

Chapter 4: Atmospheric Pressure, Winds, and Circulation

Representations of Earth: San Francisco Bay Region and AirportThe swirling circulation patterns in Earth’s atmosphere are created by changes in pressure

and winds.

Pressure

• Atmospheric Pressure– Variations in

pressure create atmospheric circulation (including wind).

Variations in Atmospheric Pressure

• Atmospheric Pressure– Standard Sea level

pressure is:• 1013.2 millibars (mb)• 29.92 inches of mercury

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

Variations in Atmospheric Pressure

• Air Pressure and Altitude– Air Pressure decreases with

increasing elevation (altitude).

– Mount Everest (29,028 feet) has only 1/3 of the pressure at sea level.

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

Variations in Atmospheric Pressure

• Cells of High and Low Pressure– Low (Cyclone) = L

• Air is ascending (rising)• Low pressure• Convergence at surface

– High (anticyclone) = H• Air is descending

(subsidence)• High pressure• Divergence at surface

Fig. 4-3, p. 78

Stepped Art

Low pressure(converging air)

CYCLONE ANTICYCLONE

High pressure(diverging air)

Variations in Atmospheric Pressure

• Horizontal Pressure 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)

• Mapping Pressure Distribution– Adjust to sea level pressure– Isobars: lines of equal pressure– Pressure Gradient

• Strong pressure gradient (isobars close together causes stronger winds

• Weak pressure gradient (isobars farther apart) causes weaker winds

Variations in Atmospheric Pressure

• Pressure Gradients and Wind– Wind

• Horizontal movement of air due to pressure differences.

• High to Low• Corrects radiational

imbalances between N and S pole

Where on this figure would winds be the strongest?

Wind

• Wind Terminology– Windward– Leeward– Winds are named for

where they come from• Wind from NE is called

NE wind

– Prevailing windsHow might vegetation differ on

the windward and leeward sides of an island?

Wind

Fig. 4-5, p. 79

Stepped Art

LeewardWindward

• The Coriolis Effect and Wind– Coriolis Effect

• Apparent deflection of the wind

• N. hem: wind is deflected to the right

• S. hem: wind is deflected to the left.

If no Coriolis effect exists at the equator, where would the maximum Coriolis effect be located?

Wind

• The Coriolis Effect and Wind– Surface Wind

• Pressure Gradient Force (PGF)

• Friction force• Coriolis effect• Wind crosses isobars

– Geostrophic Wind• PGF• Coriolis force • Wind parallel to isobars

Wind

• Cyclones, Anticyclones, and Wind Direction– 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

Wind

What do you think might happen to the diverging air of an anticyclone if there is a cyclone nearby?

Wind

• A Model of Global Pressure– Equator low (trough)– Subtropical High -30oN

and S– Subpolar low (L)– Polar high (H)– This idealized pressure

pattern is affected by landmasses and topography

Global Pressure and Wind Systems

• A Model of Global Pressure

Global Pressure and Wind Systems

• Seasonal Variations in the Pressure pattern– January

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

• Icelandic Low• Aleutian low

Global Pressure and Wind Systems

• Seasonal Variations in the Pressure Pattern– July

• Shift northward due to location of sun’s direct rays.

• Bermuda/Azores High

• Pacific High

Global Pressure and Wind Systems

– What is the difference between the Jan. and July average sea-level pressure at your location?

– Why do they vary?

Global Pressure and Wind Systems

• A Model of Atmospheric Circulation– Convergence and

Divergence– Surface Winds (H L)– Differential heating,

Earth’s rotation, Coriolis force, and atmospheric dynamics

Global Pressure and Wind Systems

• Polar Easterlies• Westerlies• Trade Winds (5o-25o)

– Northeast trades– Southeast trades

Global Pressure and Wind Systems

• Trade Winds (5o-25o)– Northeast trades– Southeast trades– Tropical easterlies

Global Pressure and Wind Systems

• The Intertropical Convergence Zone (ITCZ)– Equatorial low– Strong

convergence, rising air, heavy rain, and calm winds

– Doldrums

Global Pressure and Wind Systems

• Subtropical Highs• Westerlies• Polar Winds• Polar Front

– Relatively warm air (westerlies) meets cold air (polar)

– Subpolar low

Global Pressure and Wind Systems

• Latitudinal Migration with the Seasons– 5o-15o

• ITCZ and subtropical high

– 30o-40o • Subtropical high in

summer• Wetter westerlies in

winter along polar front

Global Pressure and Wind Systems

• Longitudinal Variation in Pressure and Wind– Subtropical Highs

• West coasts – Subsidence and

divergence– Stable– Relatively dry

• East coasts – Unstable and moist

Global Pressure and Wind Systems

• Jet Stream– Polar front

• Very strong, narrow band of winds embedded within the upper air westerlies

– Subtropical

Which jet stream is most likely to affect your home state?

Upper Air Winds and Jet Streams

• Rossby Waves How are Rossby waves

closely associated with the changeable weather of the central and eastern United States?

Upper Air Winds and Jet Streams

• Monsoon Winds– Monsoon

•Seasonal shift of the winds

•Low pressure (summer) – wet

•High pressure (winter) – dry

•Cherapunji, India

Regional and Local Wind Systems

• Local Winds– Chinook (Foehn)

• The term Chinook means “snow eater.” Can you offer an explanation for how this name came about?

– Santa Ana– Katabatic (drainage)

winds

Regional and Local Wind Systems

• Local Winds– Land-Sea Breeze

• Diurnal (daily reversal of wind)• Differential heating between land and water• What is the impact on daytime coastal

temperatures of the land and sea breeze?

Regional and Local Wind Systems

• Local Winds– Mountain breeze-valley breeze

• How might a green, shady valley floor and a bare, rocky mountain slope contribute to these changes?

Regional and Local Wind Systems

• Ocean Currents– Gyres: major surface

currents– What influences the

direction of these gyres?

Ocean-Atmosphere Interactions

• Ocean Currents– Warm Currents

• Gulf Stream• Kuroshio Current

– Cold Currents• California Current• Labrador Current

– Upwelling• California Current• Humboldt (Peru)

Current

Ocean-Atmosphere Interactions

• Ocean Currents– How does this map of ocean currents help

explain the mild winters in London, England?

Ocean-Atmosphere Interactions

• El Niño– Weak warm countercurrent that replaces

cold coastal waters off the coast of Peru (equatorial Pacific)

Ocean-Atmosphere Interactions

• 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

Ocean-Atmosphere Interactions

• El Niño and Global Weather– Past few

decades on average every 2.2 years

– 1982-83

Ocean-Atmosphere Interactions

• North Atlantic Oscillation (NAO)– Relationship between

Azores High and Icelandic Low

– Positive NAO• Larger pressure

difference between Azores and Icelandic

• Eastern US may be mild and wet during winter

Ocean-Atmosphere Interactions

• North Atlantic Oscillation (NAO)– Negative NAO

• Weak Azores High and a weak Icelandic Low

• Eastern US may be colder and snowier

Ocean-Atmosphere Interactions

Fundamentals of Physical Geography 1e

Petersen

Sack

Gabler

End of Chapter 4: Atmospheric Pressure, Winds, and

Circulation