ch.19 notes mc neely 2009
TRANSCRIPT
Ch. 19 Air Pressureand Wind
Earth-Space Science
Bremen High School
Teacher: Aaron McNeely
http://www.etplanet.com/download/wallpaper/Windows%20XP%20Wallpaper/wind.jpg
Air PressureSec 19.1
Air pressure: The pressure exerted by the weight of air
overhead, exerted in all directions At sea level 14.7 pounds per square
inch, (1kg/cm2) Responsible for earth’s winds Exerted in all directions, reason that
objects aren’t crushed
Mercury Barometer
Increased air pressure causes Mercury to rise, decrease to sink
http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter09/Text_Images/FG09_03.JPG
Air Pressure and Wind
Wind is the horizontal movement of air Wind results from horizontal differences in
air pressure Air flows from areas of high pressure to
areas of low pressure (H to L) The unequal heating of earth’s surface
results in pressure differences Ultimately the sun is the source of wind
Mapping Pressure Differences
Areas of equal pressure are connected using isobars
Isobars resemble contour lines on a topographic map
Weather MapIsobars are curved lines
Pressure Gradients
Refers to spacing between isobars Closer the lines, the greater the
pressure difference Closely-spaced isobars = strong winds Widely spaced isobars = light winds
Other Factors
Ground friction Coriolis effect
Coriolis Effect
How earth’s rotation affects moving objects
All free moving objects are deflected to the right of their path of motion in the northern hemisphere
To the left in the southern hemisphere Occurs because the earth has moved
underneath a freely moving object
Coriolis Effect Example
A missile fired straight south from the north pole toward a target on the equator would appear to move to the right
Friction Effects
Without friction, winds are balanced between pressure regimes and the Coriolis effect
Near the surface, ground friction causes wind to cross isobars from high to low pressure regimes
Factors Affecting Wind
Factor Cause Effect on Wind
Pressure Differences
Unequal surface heating
Air moves from high to low pressure
Coriolis Effect
Earth’s rotation
Air deflected to the right in northern
hemisphere
Friction Contact between air and surface
Wind crosses isobars from high
to low
Assessment
7. Why don’t objects such as a table collapse under the weight of the air above them?
A table doesn’t collapse because air pressure is exerted in all directions.
8. Suppose the height of a column in a mercury barometer is decreasing. What is happening?
Air pressure is decreasing.
Assessment
9. What is the ultimate energy source for most wind?
The sun is the ultimate energy source for most wind.
10. How does the Coriolis effect influence motion of free-moving objects?
The Coriolis effect deflects free-moving objects to the right in the northern hemisphere, and to the left in the southern hemisphere.
High and LowSec 19.2
Pressure systems are important items on weather maps and are used for weather prediction
The two basic pressure systems are high (anticyclonic) and low (cyclonic)
In the northern hemisphere: Winds blow outward and clockwise around a
high pressure system (anticyclone) Winds blow inward and counterclockwise around
a low pressure system (cyclone)
What type of system is this?
Anticyclone and Cyclone
Anticyclone: A clockwise spiral
Cyclone: A counter-clockwise spiral
Pressure and Air
Rising air is associated with cloud formation and precipitation
Sinking air is usually clear and sunny Low pressure causes air to rise in the
atmosphere High pressure cause air to sink
towards the surface
Moving Air
High and Low Summary
Anticyclones High pressure Winds blow
clockwise Sinking air Clear skies, fair
(good) weather
Cyclones Low pressure Winds blow
counterclockwise Rising air Cloudy, rainy skies,
stormy (bad) weather
H L
N Hemisphere Pressure Systems
Name Pressure Symbol Wind Direction Weather
Cyclone Low L Counterclock-wise
Rain, storms
Anticyclone High H Clockwise Fair
Global Winds
Unequal heating of the earth is the ultimate cause of winds
The equator receives more solar energy than the poles
The atmosphere moves warm air toward high latitudes, and cold toward low latitudes
The Nonrotating Earth (NRE)
Hypothetical: The earth does not rotate and is completely covered with water
Winds would flow from high to low pressure (no Coriolis effect)
Low pressure at the, high pressure at the poles
Wind would rise at the equator move toward the poles and sink
Also termed Hadley cells
NRE
•Imaginary model of earth’s winds
•Warm at equator, cold at poles
•Air rises at equator, sinks at poles
•Surface flows from poles to equator
• The NRE creates two giant Hadley (convection) cells
Rotating Earth
When rotation is added to the NRE model, the two cell convection system breaks down into many individual cells
Rotating Earth
Areas of Interest: Equatorial Low (ITCZ,
doldrums) Subtropical Highs
(Horse latitudes) Trade Winds Westerlies
Subpolar Lows Polar Easterlies
Polar High
Earth’s Major Winds
http://www.ux1.eiu.edu/~cfjps/1400/FIG07_007.jpg
Prevailing Winds Map
Assessment
14 Describe how winds blow around pressure centers in the northern hemisphere.
In the northern hemisphere, wind in a high pressure rotates clockwise, and wind in a low pressure system rotates counterclockwise.
Assessment
15. Compare the air pressure for a cyclone with an anticyclone.
Air experiences high pressure in a cyclone and low pressure in an anticylone.
Assessment
16. Describe how the atmosphere balances the unequal heating of earth’s surface.
Sinking and rising air near the earth’s surface helps to regulate surface temperatures.
Assessment
17. What is the only truly continuous pressure belt? Why is it continuous?
The subpolar low in the southern hemisphere is the only continuous pressure belt. The subpolar low is uninterrupted by continental land masses.
Assessment
18. In general, what type of weather can you expect if a low-pressure system is moving into your area?
Low pressure systems usually deliver clouds and precipitation.
Local WindsSec 19.3
Two factors Surface features (topographic effects) Differences in surface composition (land
vs. water) Ex: Land & Sea breezes, Valley &
Mountain breezes
Land & Sea
Sea Breeze
Warm land, cool water
Low pressure over land, high pressure over water
Winds blow in from sea
Land Breeze
Cool land, warm water
High pressure over land, low pressure over water
Winds blow out from land
Valley & Mountain
Valley Breeze
Warm valley, cooler air overhead
Low pressure in valley causes winds to flow upward
Mountain Breeze
Cool valley, warmer air overhead
High pressure in valley causes winds to flow downward
Wind Direction
Prevailing Wind: When the wind consistently blows more often from one direction
In the United States, the prevailing westerlies consistently blow from west to east
Wind Speed
An anemometer measures wind speed
Anemometer
http://www.pfmt.org/standman/images/anameter1.gif
Assessment
29. What are local winds, and how are they caused?
Local winds are small in scale and are caused by differences in air pressure. These pressure differences result from the shape and unequal heating of the land.
Assessment
30. Describe the general movement of weather within the United States.
Weather in the United States generally moves from west to east.
Assessment
31. What two factors mainly influence global precipitation?
The two factors that influence precipitation are the amount of moisture in air and the location of land and water.
Assessment
32. The mercury barometer was invented by
a) Galileo b) Newton c) Torricelli d) Watt
Assessment
33. The force exerted by air above is called?
a) air pressure b) convergence c) divergence d) the Coriolis effect
Assessment
34. What are centers of low pressure called?
a) air masses b) anticyclones c) cyclones d) jet streams
Assessment
35. Variations in air pressure from place to place are the principal cause of
a) clouds b) lows c) hail d) wind
Assessment
36. What is the pressure zone that is associated with rising air near the equator?
a) equatorial low b) equatorial high c) subtropical low d) subtropical high
Assessment
37. Where is the deflection of wind due to the Coriolis effect the strongest?
a) near the equator b) in the midlatitudes c) near the poles d) near the westerlies
Assessment
38. In what stormy region do the westerlies and polar easterlies converge?
a) equatorial low b) subpolar high c) polar front d) subtropical front
Assessment
39. In what stormy region do the westerlies and polar easterlies converge?
a) equatorial low b) subpolar high c) polar front d) subtropical front