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