1. Chapter 19
Air Pressure and Wind
Who is Stan Hatfield and Ken Pinzke

2. 19.1 Understanding Air Pressure
Air Pressure Defined
19.1 Understanding Air Pressure
 Air pressure is the pressure exerted by the weight of air.
Air pressure is exerted in all directions— down, up, and sideways. The air pressure pushing down on an object exactly balances the air pressure pushing up on the object.
Watch this can crushing example of air pressure. air pressure is a monster

3. 19.1 Understanding Air Pressure
Measuring Air Pressure
19.1 Understanding Air Pressure
 A barometer is a device used for measuring air pressure.
When air pressure increases, the mercury in the tube rises. When air pressure decreases, so does the height of the mercury column.
This guy can show you how a barometer works and has a cool voice.

4. A Mercury Barometer
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5. 19.1 Understanding Air Pressure
Factors Affecting Wind
19.1 Understanding Air Pressure
Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure.
 The unequal heating of Earth’s surface generates pressure differences. Solar radiation is the ultimate energy source for most wind.
 Three factors combine to control wind: pressure differences, the Coriolis effect, and friction.

6. 19.1 Understanding Air Pressure
Factors Affecting Wind
19.1 Understanding Air Pressure
 Pressure Differences
• A pressure gradient is the amount of pressure change occurring over a given distance.
• Closely spaced isobars—lines on a map that connect places of equal air pressure—indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure gradient and light winds.

7. Isobars
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Copy the activity on the following page into paint. Read the info and complete the activities.

9. 19.1 Understanding Air Pressure
Factors Affecting Wind
19.1 Understanding Air Pressure
 Coriolis Effect
• The Coriolis effect describes how Earth’s rotation affects moving objects. In the Northern Hemisphere, all free-moving objects or fluids, including the wind, are deflected to the right of their path of motion. In the Southern Hemisphere, they are deflected to the left.
The coriolis effect can be a difficult concept to understand. To get more information about the coriolis effect, click on the link below.
Understand the Coriolis Effect
Video demonstrating result of Coriolis Effect

10. The Coriolis Effect
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Use the cards, scissors, markers, etc. at your lab station.
Follow Dr. C as he helps you understand Coriolis Effect.

11. 19.1 Understanding Air Pressure
Factors Affecting Wind
19.1 Understanding Air Pressure
 Friction
• Friction acts to slow air movement, which changes wind direction.
• Jet streams are fast-moving rivers of air that travel between 120 and 240 kilometers per hour in a west-to-east direction.
How do Jet Streams Work???
More about jet streams
Even more about jet streams in the United States

12. Effect of Friction
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13. 19.2 Pressure Centers and Winds
Highs and Lows
19.2 Pressure Centers and Winds
 Cyclones are centers of low pressure.
 Anticyclones are centers of high pressure.
 In cyclones, the pressure decreases from the outer isobars toward the center. In anticyclones, just the opposite is the case—the values of the isobars increase from the outside toward the center.

14. 19.2 Pressure Centers and Winds
Highs and Lows
19.2 Pressure Centers and Winds
 Cyclonic and Anticyclonic Winds
• When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, winds blow counterclockwise around a low. Around a high, they blow clockwise.
• In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow of air outward around an anticyclone.

15. Cyclonic and Anticyclonic Winds
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16. 19.2 Pressure Centers and Winds
Highs and Lows
19.2 Pressure Centers and Winds
 Weather and Air Pressure
• Rising air is associated with cloud formation and precipitation, whereas sinking air produces clear skies.
 Weather Forecasting
• Weather reports emphasize the locations and possible paths of cyclones and anticyclones.
• Low-pressure centers can produce bad weather in any season.

17. Airflow Patterns, Surface and Aloft
Convergence
HIGH =
Divergence
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Lows move west to east.
Their paths can be somewhat unpredictable, making accurate estimation of their movement difficult.
It is important to understand total atmosphere circulation….

18. 19.2 Pressure Centers and Winds
Global Winds
19.2 Pressure Centers and Winds
 The atmosphere balances these differences by acting as a giant heat-transfer system. This system moves warm air toward high latitudes and cool air toward the equator.
 Non-Rotating Earth Model
• On a hypothetical non-rotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form.

19. Circulation on a Non-Rotating Earth
Heated air at the equator would rise until it reached the Tropopause.
Eventually the upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator.
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20. 19.2 Pressure Centers and Winds
Global Winds
19.2 Pressure Centers and Winds
 Rotating Earth Model
• If the effect of rotation were added to the global circulation model, the two-cell convection system would break down into smaller cells.
• Near the equator, rising air produces a pressure
Zone known as the equatorial low – a region characterized by abundant precipitation.
• degrees, north or south latitude, the sinking air and its associated heating due to compression produce hot, arid conditions – the center of this zone of sinking dry air is the subtropical high.

21. • Trade winds are two belts of winds that blow almost constantly from easterly directions and are located on the north and south sides of the subtropical highs.
• Westerlies are the dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.
• Polar easterlies are winds that blow from the polar high toward the subpolar low. These winds are not constant like the trade winds.
• A polar front is a stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin.

22. Circulation on a Rotating Earth
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global air circulation

23. 19.2 Pressure Centers and Winds
Global Winds
19.2 Pressure Centers and Winds
 Influence of Continents
• The only truly continuous pressure belt is the subpolar low in the Southern Hemisphere. In the Northern Hemisphere, where land masses break up the ocean surface, large seasonal temperature differences disrupt the pressure pattern.
• Monsoons are the seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea. In summer, the wind blows from sea to land.

24. Surface Pressure
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The Intertropical Convergence Zone (ITCZ) is a zone of unstable air that forms along the equator where the northeast and southeast trade winds converge.

25. What are centers of low pressure called?
Air masses
Anticyclones
Cyclones
Jet streams

26. Variations in air pressure from place to place are the principal caused of
clouds.
Lows.
Hail.
Wind.

27. In the winter, large landmasses often develop a seasonal
high-pressure system
Low-pressure system
Typhoon
Trade wind

28. What is the pressure zone that is associated with rising air near the equator?
Equatorial low
Equatorial high
Subtropical low
Subtropical high

29. In what stormy region do the westerlies and polar easterlies converge?
Equatorial low
Subpolar high
Polar front
Subtropical high

30. 19.3 Regional Wind Systems Local Winds
 The local winds are caused either by topographic effects or by variations in surface composition—land and water—in the immediate area.
 Land and Sea Breezes
• In coastal areas during the warm summer months, the land surface is heated more intensely during the daylight hours than an adjacent body of water is heated. As a result, the air above the land surface heats, expands, and rises, creating an area of lower pressure. At night the reverse takes place.

31. Sea and Land Breezes
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32. 19.3 Regional Wind Systems Local Winds  Valley and Mountain Breezes
• In mountainous regions during daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air on the mountain slopes is less dense, it glides up along the slope and generates a valley breeze. After sunset the pattern may reverse.

33. Valley and Mountain Breezes
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34. 19.3 Regional Wind Systems How Wind Is Measured  Wind Direction
• The prevailing wind is the wind that blows more often from one direction than from any other.
• In the United States, the westerlies consistently move weather from west to east across the continent.

35. 19.3 Regional Wind Systems How Wind Is Measured  Wind Speed
• An anemometer is an instrument that resembles a cup and is commonly used to measure wind speed.

36. 19.3 Regional Wind Systems El Niño and La Niña  El Niño
• El Niño is the name given to the periodic warming of the ocean that occurs in the central and eastern Pacific.
• At irregular intervals of three to seven years, these warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters.
• A major El Niño episode can cause extreme weather in many parts of the world.

37. Normal Conditions
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38. El Niño Conditions
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39. 19.3 Regional Wind Systems El Niño and La Niña  La Niña
• Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered that has a distinctive set of weather patterns.

40. 19.3 Regional Wind Systems Global Distribution of Precipitation
 Global precipitation can be explained if knowledge of global winds and pressure systems are applied.

41. Valley and mountain breezes are examples of ____.
global winds
trade winds
local winds
jet streams

42. A sea breeze usually originates during the ____.
a. evening and flows toward the land
day and flows toward the land
evening and flows toward water
day and flows toward the water

43. A land breeze usually originates during the ____.
a. evening and flows toward the land
day and flows toward the land
evening and flows toward the water
d. day and flows toward the water

44. A wind that consistently blows more often from one direction than from any other is called a ____.
a. local wind
prevailing wind
trade wind
jet stream

45. Winds are labeled according to which of the following?
whether they originate on land or water
the direction from which they blow
the region in which they can be found
none of the above

46. Which phenomenon is associated with surface temperatures in the eastern Pacific that are colder than average?
a. La Niña
El Niño
global warming
local winds

47. Which of the following is a warm countercurrent that periodically flows southward along the coasts of Ecuador and Peru?
a. mountain breeze
La Niña
El Niño
d. none of the above