1. The Atmosphere in Motion
Chapter 19

2. Air Pressure & Wind section 1
Horizontal movement of air
Helps moderate surface temperatures
Distributes moisture
“cleanses” the atmosphere
Several forces affect direction of movement
Caused by differences in air pressure

3. What is Air Pressure?
Weight of the air pushing down on Earth’s surface
At sea level = 14.7 pounds per square inch (psi)
At sea level, the barometric pressure is inches or millibars (mb).
As increase elevation, pressure decreases b/c less air above
Decreases ~50% every 5 km
Exerted in all directions

4. Air Pressure With Altitude

5. Mercury Barometer
Weight of column of mercury is balanced by the pressure exerted on the dish of mercury from air above

6. Aneroid Barometer & Barograph
Without liquid
Partially evacuated metal chamber that compresses or expands based on outside pressure

7. Recording Air Pressure
Different units can be used to express air pressure
With a mercury barometer
Inches
Millimeters
Meteorologists use:
millibars

8. You can easily change from inches to millibars

9. Why Does Air Pressure Change?
Elevation
As altitude increases, pressure decreases
Temperature
As temperature increases, pressure decreases
Molecules move further apart as air is heated
So fewer air molecules than in same volume of cool air
Warm air  lower pressure, cold air  high pressure
Humidity
As humidity increases,
pressure decreases
Water molecules have
less mass than oxygen
or nitrogen molecules

10. Why Does Air Pressure Change?
Changes in air pressure can aid in predicting the weather
A decrease in pressure often indicates approaching warmer, more humid, air along w/ rain or snow
Less dense air  less pressure exerted
An increase in pressure often indicates approaching cooler, drier air & fair weather
More dense air  more pressure exerted

11. Why Does Air Pressure Change?
Meteorologists analyze air pressure by plotting isobars on weather maps
Isobar: line that joins points of equal barometric (air) pressure
A “closed” isobar forms a loop on a weather map
High-pressure area (“high”)
Air pressure steadily increases toward the center of a set of closed isobars
~Think of a hill
Low-pressure area (“low”)
Air pressure steadily decreases toward the center of a set of closed isobars
~Think of a valley

12. Isobars

13. Pressure Gradient Pressure Gradient = change in pressure
change in distance
The closer the isobars, the steeper the gradient
Pressure changes quickly
Faster, stronger winds
The further the isobars, the more gentle the slope
Pressure changes slowly
Slower, weaker winds

14. What Makes the Wind Blow?
Differences in pressure caused by unequal heating of Earth’s surface -Winds blow from areas of High to Low pressure
H L
WIND
Falling air Rising air
(cold, dry = more dense) (warm, moist = less dense)
Fair weather stormy weather
High Pressure Low Pressure

16. Measuring Surface Wind Direction & Speed
Wind vanes  instrument to determine the direction of wind
Broad tail
Resists wind
“Points away” from where the wind is blowing from
Arrowhead
Points into the wind (where the wind is blowing from)
Winds are named for the direction from which they blow from
Examples
Blow from west (to east) = westerly (or west) wind
Nor’easter = winds from the northeast

17. westerly wind

18. Measuring Surface Wind Direction & Speed
Anemometer  instrument used to measure wind speeds 10 meters above ground
Effects on water, smoke, trees, & other objects can also be used as estimates of wind speed.

19. Factors Affecting Winds section 2
The Coriolis Effect: the tendency of an object (wind, ocean current, plane, etc.) moving freely over the earth’s surface to curve away from its path of travel
Due to Earth’s rotation
Northern Hemisphere  deflect to right (from perspective of object)
Blow clockwise out of areas of high pressure
Blow counterclockwise into areas of low pressure
*****Diagrams
Southern Hemisphere  deflect to left (from perspective of object)
Does not depend on object’s direction of movement
Only noticeable on large scale (winds, planes, ocean currents)
Greatest near poles, least near equator
Increases if speed of object increases

21. Coriolis Effect Animation
Coriolis Effect & Wind
Direction Visualiziation

22. Friction Friction between the air & ground slows winds
Changes the impact of the Coriolis effect on surface winds
More friction  less “deflection”/curving
Ex. rough land
Less friction  more “deflection”/curving
Ex. smooth land or water
Winds at higher altitudes  less friction  stronger Coriolis effect

23. Friction Jet Stream Video
Internet Investigation: How Does the
Jet Stream Change Through the Year?
Jet stream: Band of very fast winds ( km/hr) near the top of the troposphere (hardly affected by friction)
Typically 1000s of km long, 100s of km wide, & about 1 km from top to bottom
Polar-front jet stream: cool polar air joins w/ warmer air to the south
Generally flows west to east
Great effect on weather in the U. S.
Energy for storms, directs storms’ paths
Can reach to central FL in winter, generally over Canada and northern U.S. in summer
Speed depends on pressure gradient in upper troposphere (depends on surface temps)
Fastest in winter
Tropical-easterly jet stream: warm air in tropics of N. Hemisphere
Weaker than polar-front jet stream

25. Global Wind Patterns sec 3
Affected by:
***Unequal heating of Earth by sunlight (temp diff btw equator & poles)
***Earth's rotation (spin) (& Coriolis effect)
Location of continents
Time of year
Local topography (landforms)

26. Global Wind Patterns
What would happen if Earth did not rotate & there was no Coriolis effect?
The unequal heating:
makes the tropical equatorial regions warmer than the polar regions.
lower pressure at the (warmer) equator
Air rises & moves toward poles
higher pressure at the (cold) poles
Polar air moves toward equator
~heats, rises, & continues cycle
Would result in one large circulation cell in each hemisphere

28. Effects of Earth’s Rotation
B/c Earth rotates:
Coriolis effect prevents air from flowing straight from equator to poles
Air flowing northward from equator is deflected to right
Air flowing southward from equator is deflected to left
Air cools & sinks long before reaches polar regions
Air circulation is better represented w/ 3-cells in each hemisphere
Idealized, not 100% accurate, but helpful in understanding global wind patterns

29. Effects of Earth’s Rotation
Three-Celled Circulation Model
3 circulation cells in each hemisphere
0 (equator)-30°
30-60°
60-90° (pole)
Direction of circulation changes from each cell to the next
Caused by alternating bands of high & low pressure at Earth’s surface
Polar front = boundary at 60° where air flows away from high pressure poles & collides w/ warmer air moving up from lower latitudes
As winds blow from high to low pressure, they are deflected by the Coriolis effect
To right in northern hemisphere
To left in southern hemisphere

31. Weaknesses of the Three-Celled Model
3 main weaknesses
Gives a simplified view of circulation btw 30 & 60°
Referred to as middle latitudes or mid-latitudes
Most of the U. S.
Surfaces winds determined by locations of transient high- & low-pressure systems
Change often
Does not take into account the effects of the continents (heat & cool more rapidly than oceans) or seasons
Based on a simplified view of upper-level winds
Impression that generally travel N  S
Primarily westerly (except near equator, Coriolis is weak)

32. Strength of the Three-Celled Model
Fairly accurate image of general surface winds & pressures outside the mid-latitudes
Gives a picture of wind patterns & pressure systems that is useful for climate studies
b/c involves averaging patterns over long periods

33. Description of Wind & Pressure Belts
Intertropical Convergence Zone (ITCZ) or doldrums: a low pressure belt at the equator where winds from both hemispheres come together
Little to no wind, hot & humid, rain is common
Tradewinds: blow from the NE (N. Hemi) & SE (S. Hemi) are found at about 30º N & S
Polar highs: high-pressure regions where cold air sinks at the poles
Polar easterlies: surface winds at poles that blow from east
Prevailing winds: winds that usually blow from same direction
Tradewinds
Polar easterlies
Prevailing westerlies which blow (SW in N. Hemi & NW in S. Hemi) in the mid-latitudes.

34. Polar Front
Polar Front

36. Continental & Local Winds Sec 4
Because of tilt of Earth, relative position of sun changes over year
Causes seasons
Temperature changes
Changes in global winds
Also affected by positions of continents
Highest temps in N. Hemi often N of equator
Sun causes air to heat, rise, & flow toward poles

37. Effects of Seasons & Continents
Summer
continents hotter than oceans
heats up (& cools down) faster than water b/c absorbs (& radiates) heat better
hot land heats air above it, becomes less dense, rises, causing low pressure
Oceans cooler than land
Heats up (& cools down) slower than land b/c does not absorb (& radiate) heat quickly
Cool water, air above cooler, more dense, higher pressure
Highs & lows determine direction of prevailing winds at various locations
Winter  opposite from summer

38. Direction of winds change seasonally  monsoons
Most dramatic in southern Asia
Winter  cold, dry winds
Summer  warm, moist winds & heavy rains

39. Local Winds Extends 100 km or less
Caused mostly by differences in temperature
Examples:
Land & sea breezes
Mountain & valley breezes

40. Developing a Sea Breeze
Daytime
Land heats faster creating warm air above it.
decreases the pressure (& density)
air rises.
low pressure develops over land
Water heats slower, so it has cooler air above it.
Increases pressure (& density)
Air sinks
High pressure develops over water causing a difference in pressure.
Wind blows from high (sea) to low (land)

41. Developing a Land Breeze
Nighttime
Water stays warm longer creating warm air above it.
decreases the pressure (& density)
air rises.
low pressure develops over water
Land cools faster, so it has cooler air above it.
Increases pressure (& density)
Air sinks
High pressure develops over land causing a difference in pressure.
Wind blows from high (land) to low (sea)