1. Chapter 5: Atmospheric Pressure and Wind

2. The Impact of Pressure and Wind on the Landscape
Atmospheric pressure indirectly affects the landscape
Changes manifest primarily by changes in wind and temperature
Wind has a visible component to its activity
Severe storm winds can drastically affect the landscape
Figure 5-2

3. The Nature of Atmospheric Pressure
Gas molecules continuously in motion
Force exerted by gas molecules is called atmospheric pressure
Force exerted on every surface the gas touches
Pressure is approximately 14 lbs per square inch
Figure 5-1

4. The Nature of Atmospheric Pressure
Factors influencing atmospheric pressure
Density
At higher density, particles are closer & collide more frequently, increasing pressure
Temperature
Warmer particles move faster & collide more frequently, increasing pressure
Figure 5-3
Figure 4-15

5. The Nature of Atmospheric Pressure
Figure 5-4
Measurement of Pressure
Barometer – device to measure atmospheric pressure
Average Sea Level Pressure
MILLIBARS
29.92 INCHES OF HG (MERCURY)
14.7 LBS / SQ. INCH
760 MM OF HG
No matter which unit of measurement is used, pressure ALWAYS decreases with altitude
Mapping pressure with isobars
Isobar: line joining points of equal pressure
Pressure gradient: horizontal rate of pressure change
Directly affects wind speed

6. The Nature of Wind Wind: horizontal air movement Origination of wind
Figure 5-5
Wind: horizontal air movement
In theory, why does air move?
Energy imbalances on globe
Why does air actually move?
Differences in horizontal atmospheric pressure
Origination of wind
Uneven heating of Earth’s surface creates temperature & pressure gradients
Direction of wind results from pressure gradient
Winds blow from high pressure to low pressure
Winds are named for the direction they blow from!!!

7. The Nature of Wind Forces which govern the wind
Pressure gradient force
Characterized by wind moving from high to low pressure, always
Winds blow at right angles to isobars
Coriolis force
Turns wind to the right in the N Hemisphere, left in S Hemisphere
Only affects wind direction, not speed, but faster winds turn more
Friction
Wind is slowed by Earth’s surface due to friction, does not affect upper levels
Figure 5-7

8. The Nature of Wind Force balances Geostrophic balance
Balance between pressure gradient force and Coriolis
Winds blow parallel to isobars
Frictional balance
Winds blow slightly towards low pressure and slightly away from high pressure
Winds slowed by friction weaken Coriolis, so pressure gradient force is stronger and turns the winds
Figure 5-6

9. The Nature of Wind
Anticyclones and cyclones
Figure 5-8

10. The Nature of Wind Cyclone: low-pressure cell
Air converges & ascends
Surface convergence & low pressure indicate rising motion
Rising motion results in clouds and storms
Anticyclone: high- pressure cell
Air descends & diverges
Surface divergence & high pressure indicate sinking motion
Sinking motion results in sunny skies
Figure 5-9

11. The Nature of Wind Wind speed Determined by pressure gradients
Tight pressure gradients (isobars close together) indicate faster wind speeds
Wind speeds are gentle on average
Figure 5-10
Figure 5-11

12. The Nature of Wind

13. Vertical Variations in Pressure & Wind
Atmospheric pressure decreases rapidly with height
Atmospheric surface pressure centers lean with height
Winds aloft are much faster than at the surface
Jet streams

14. The General Circulation of the Atmosphere
Atmosphere is in constant motion
Major semi-permanent conditions of wind and pressure—general circulation
Principal mechanism for longitudinal and latitudinal heat transfer
Second only to insolation as a determination for global climate

15. The General Circulation of the Atmosphere
Simple example: Non-rotating Earth
Strong solar heating at equator
Low pressure forms over equator
Ascending air over equator
Little heating at poles
High pressure forms over poles
Descending air over poles
Winds blow equatorward at surface, poleward aloft
Figure 5-12

16. The General Circulation of the Atmosphere
If Earth didn’t rotate
Because Earth rotates

17. The General Circulation of the Atmosphere
Observed general circulation
Earth’s rotation increases complexity of circulation
Hadley cells
Complete vertical circulation cells where warm air rises, cools, moves poleward, & subsides
Around 30° N/S
Figure 5-14
Figure 5-13

18. The General Circulation of the Atmosphere
7 Surface Components
Intertropical convergence zone (ITCZ) (0°)
Trade winds (easterlies) (0° - 30° N/S)
Subtropical highs (30° N/S)
Westerlies (30° - 60° N/S)
Subpolar low (polar front) (60° N/S)
Polar easterlies (60° - 90° N/S)
Polar highs (90° N/S)

19. The General Circulation of the Atmosphere
Components of the general circulation
Subtropical highs (STH’s)
Persistent zones of high pressure near 30° N/S
Result from descending air in Hadley cells
Regions of world’s major deserts
No wind
AKA horse latitudes
Source of 2 of the 3 major wind systems: trade winds & westerlies
Figure 5-16

20. The General Circulation of the Atmosphere
Winds are named for the direction they blow from!
Trade winds (easterlies)
Diverge from equator-ward side of STH’s
About 30°N to 30°S
SE in S Hemisphere,
NE in N Hemisphere
Blow east to west
Most reliable/consistent of wind systems
AKA Winds of commerce
Figure 5-17

21. The General Circulation of the Atmosphere
Trade winds (cont.)
Heavily laden with moisture
Do not produce rain unless forced to rise
If they rise, they produce tremendous precipitation and storm conditions
Figure 5-20

22. The General Circulation of the Atmosphere
Intertropical Convergence Zone (ITCZ)
Zone of rising air from surface heating & convection
Zone of low pressure & convergence toward which trade winds blow
Near equator
Constant rising motion & storminess in this region
Position seasonally shifts (more over land than water)
AKA Doldrums
Figure 5-21

23. The General Circulation of the Atmosphere
Westerlies
Form on poleward sides of STH’s
Wind system of the midlatitudes
Blow west to east
2 cores of high winds – jet streams
Subtropical Jet
Polar Jet
Rossby waves
Figure 5-22
Figure 5-24

24. The General Circulation of the Atmosphere
Figure 5-27
Polar highs
Develop over poles due to extensive cold conditions
Anticyclonic winds
Strong subsidence
Arctic desert
Polar easterlies
Regions north of 60°N & south of 60°S
Winds blow east to west
Cold & dry

25. The General Circulation of the Atmosphere
Polar front
Low pressure around 60° N/S
Air mass conflict between warm westerlies & cold polar easterlies
Rising motion = clouds & precipitation
Very strong winds over oceans
Polar jet stream position typically coincident with the polar front
AKA subpolar low
Figure 5-25

26. The General Circulation of the Atmosphere
7 components of the general circulation
Figure 5-26

27. The General Circulation of the Atmosphere
Vertical wind patterns of the general circulation
Most dramatic differences in surface and aloft winds is in tropics
Antitrade winds
Blow from SW in N Hemisphere, & from NW in S Hemisphere
Figure 5-28

28. Modifications of the General Circulation
Seasonal modifications
7 general circulation components shift seasonally
Components shift northward during Northern Hemisphere summer
Components shift southward during Southern Hemisphere summer
Figure 5-29

30. Modifications of the General Circulation
Monsoons
Seasonal wind shift
Most significant disturbance of general circulation
Offshore: wind movement from land to water
Onshore: wind movement from water to land
Seasonal Winds
Wet summer
Dry winter
Develop due to:
Shift in position of ITCZ
Unequal heating of land & water
Figure 5-31
Figure 5-30

31. Modifications of the General Circulation
Major monsoon systems
Figure 5-32

32. Localized Wind Systems
Figure 5-34
Convectional circulation from differential heating of land & water
Sea breezes
Water heats more slowly than land during the day
Lower pressure over land, higher pressure over sea
Wind blows from sea to land
Land breezes
At night, land cools faster
Higher pressure over land, lower pressure over sea
Wind blows from land to sea

33. Localized Wind Systems
Convectional circulation from differential heating of higher vs. lower elevations
Valley breeze
Mountain top during the day heats faster than valley, creating lower pressure at mountain top
Upslope winds out of valley
Mountain breeze
Mountain top cools faster at night, creating higher spressure at mountain top
Winds blow from mountain to valley, downslope
Figure 5-35

34. Localized Wind Systems
Katabatic winds
Cold winds that originate from cold upland areas
Descend quickly down mountain, can be destructive
Chinook winds
Warm downslope winds
H pressure on windward side, L pressure on leeward side
Warms adiabatically as it moves down leeward slope of Rocky Mountains
Figure 5-36

35. Localized Wind Systems
Santa Ana winds
Hot, dry, high speed winds
Occur in S. California from Oct to Mar
Spread wildfires in dry seasons

36. El Niño-Southern Oscillation
Warming of waters in the eastern equatorial Pacific
Associated with numerous changes in weather patterns worldwide
Typically occurs every 3 to 7 years for about 18 months
El Niño
Reference to the Christ Child
1st signs usually appear around Christmas
Figure 5-37

37. El Niño-Southern Oscillation
Circulation patterns
Normal
El Nino
Upwelling: winds blowing offshore on W side of continents pushes water westward & brings up cold, nutrient-rich water from below
Figure 5-38

38. Upwelling
Phytoplankton shows where upwelling occurs

39. El Niño-Southern Oscillation
Figure 5-40
Patterns associated with El Niño
Teleconnections
Weather effects
La Niña—opposite of El Niño
Circulation is similar to “normal”
Causes of El Niño
Which changes first, the atmosphere or the ocean?

40. El Niño-Southern Oscillation
Sea Levels & Sea Surface Temperatures during normal, El Niño & La Niña years (
Figure 5-39
Figure 5-C
Figure 5-D
Figure 5-E

41. El Niño-Southern Oscillation
Forecasting El Niño
Array of 70 buoys – Galapagos Islands to New Guinea
Forecasts?
Figure 5-B

42. Summary
Atmospheric pressure and wind affect the geographic landscape in several ways
Atmospheric pressure is the force exerted by air molecules on all objects the air is in contact with
Pressure is influenced by temperature, density, and dynamic
Isobars show areas of high pressure and low pressure
Vertical and horizontal atmospheric motions are called wind
Wind is affected by many forces
Geostrophic balance represents a balance between the Coriolis force and the pressure gradient force
Friction slows the wind and turns it towards lower pressure
Wind patterns around high and low pressure systems are anticyclonic and cyclonic, respectively
Areas of divergence at the surface are associated with sinking motion, convergence at the surface with rising motion
Close isobar spacing indicates faster winds

43. Summary
Winds increase rapidly with height, pressure decreases rapidly with height
The global atmospheric circulation is called the general circulation
There are seven components to the general circulation
Each component has associated weather conditions
Seasonal modifications to the general circulation exist, including monsoons
Localized wind systems affect wind direction locally on diurnal time scales
El Niño is a warming of eastern equatorial Pacific water and subsequent switching of the high and low air pressure patterns
El Niño is associated with varied weather patterns in different locations globally