1. Prentice Hall EARTH SCIENCE
Tarbuck Lutgens 

2. Chapter 16
Air Pressure and Wind
Who is Stan Hatfield and Ken Pinzke

3. 16.1 Understanding Air Pressure
Air Pressure Defined
16.1 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.

5. 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.

6. A Mercury Barometer
Makes no sense without caption in book

7. 16 Understanding Air Pressure
Factors Affecting Wind
16 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.

8. Unequal heating and wind

9. Wind flow

10. 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.

11. Isotherms/Isobars
://

12. Isobars
Makes no sense without caption in book

13. 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.

14. The Coriolis Effect
Makes no sense without caption in book

15. 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.

17. 16 Pressure Centers and Winds
Highs and Lows
16 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.

18. 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.

19. Cyclonic and Anticyclonic Winds
Makes no sense without caption in book

20. 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.

21. Airflow Patterns, Surface and Aloft
Makes no sense without caption in book

22. 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.

23. Circulation on a Non-Rotating Earth
Makes no sense without caption in book

24. 16 Pressure Centers and Winds
Global Winds
16 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.
• 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.

25. 16 Pressure Centers and Winds
Global Winds
16 Pressure Centers and Winds
 Rotating Earth Model
• 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.

26. Circulation on a Rotating Earth
Makes no sense without caption in book

27. 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. How will this effect precipitation?

28. Surface Pressure
Makes no sense without caption in book

29. 16 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.

30. Sea and Land Breezes
Makes no sense without caption in book

31. 16 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.

32. Valley and Mountain Breezes
Makes no sense without caption in book

33. 16 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.

34. 16 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.

35. Normal Conditions
Makes no sense without caption in book

36. El Niño Conditions
Makes no sense without caption in book

37. 16 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.

38. 19.3 Regional Wind Systems Global Distribution of Precipitation
 Global precipitation can be explained if knowledge of global winds and pressure systems are applied.

39. 16 Water in the Atmosphere
Water’s Changes of State
16 Water in the Atmosphere
 Precipitation is any form of water that falls from a cloud.
 When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.

40. 16 Water in the Atmosphere
Water’s Changes of State
16 Water in the Atmosphere
 Solid to Liquid
• The process of changing state, such as melting ice, requires that energy be transferred in the form of heat.
• Latent heat is the energy absorbed or released during a change in state.
 Liquid to Gas
• Evaporation is the process of changing a liquid to a gas.
• Condensation is the process where a gas, like water vapor, changes to a liquid, like water.

41. 16 Water in the Atmosphere
Water’s Changes of State
16 Water in the Atmosphere
 Solid to Gas
• Sublimation is the conversion of a solid directly to a gas without passing through the liquid state.
• Deposition is the conversion of a vapor directly to a solid.

42. Changes of State
Makes no sense without caption in book

43. 16 Water in the Atmosphere
Humidity
16 Water in the Atmosphere
 Humidity is a general term for the amount of water vapor in air.
 Saturation
• Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure.
• When saturated, warm air contains more water vapor than cold saturated air.

44. 16 Water in the Atmosphere
Humidity
16 Water in the Atmosphere
 Relative Humidity
• Relative humidity is a ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure.
• To summarize, when the water-vapor content of air remains constant, lowering air temperature causes an increase in relative humidity, and raising air temperature causes a decrease in relative humidity.

45. 16 Water in the Atmosphere
Humidity
16 Water in the Atmosphere
 Dew Point
• Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation.
 Measuring Humidity
• A hygrometer is an instrument to measure relative humidity.
• A psychrometer is a hygrometer with dry- and wet-bulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb’s measurement. The two temperatures are compared to determine the relative humidity.

46. Dew on a Spider Web
Makes no sense without caption in book

47. Convergence and Localized Convective Lifting
Makes no sense without caption in book

48. 16 Cloud Formation Condensation
 For any form of condensation to occur, the air must be saturated.
 Types of Surfaces
• Generally, there must be a surface for water vapor to condense on.
• Condensation nuclei are tiny bits of particulate matter that serve as surfaces on which water vapor condenses when condensation occurs in the air.

49. 16 Cloud Formation Condensation
 For any form of condensation to occur, the air must be saturated.
 Types of Surfaces
• Generally, there must be a surface for water vapor to condense on.
• Condensation nuclei are tiny bits of particulate matter that serve as surfaces on which water vapor condenses when condensation occurs in the air.

50. 16 Cloud Types and Precipitation
Types of Clouds
16 Cloud Types and Precipitation
 Clouds are classified on the basis of their form and height.
• Cirrus (cirrus = curl of hair) are clouds that are high, white, and thin.
• Cumulus (cumulus = a pile) are clouds that consist of rounded individual cloud masses.
• Stratus (stratus = a layer) are clouds best described as sheets or layers that cover much or all of the sky.

51. Cirrus Clouds
Makes no sense without caption in book

52. 16 Cloud Types and Precipitation
Types of Clouds
16 Cloud Types and Precipitation
 High Clouds
• Cirrus clouds are high, white, and thin.
• Cirrostratus clouds are flat layers of clouds.
• Cirrocumulus clouds consist of fluffy masses.
 Middle Clouds
• Altocumulus clouds are composed of rounded masses that differ from cirrocumulus clouds in that altocumulus clouds are larger and denser.
• Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot.

53. 16 Cloud Types and Precipitation
Types of Clouds
16 Cloud Types and Precipitation
 Low Clouds
• Stratus clouds are best described as sheets or layers that cover much or all of the sky.
• Stratocumulus clouds have a scalloped bottom that appears as long parallel rolls or broken rounded patches.
• Nimbostratus clouds are the main precipitation makers.

54. Cloud Classification
Makes no sense without caption in book

55. 16 Cloud Types and Precipitation
Fog
16 Cloud Types and Precipitation
 Fog is defined as a cloud with its base at or very near the ground.
 Fog Caused by Cooling
• As the air cools, it becomes denser and drains into low areas such as river valleys, where thick fog accumulations may occur.
 Fog Caused by Evaporation
• When cool air moves over warm water, enough moisture may evaporate from the water surface to produce saturation.

56. 16 Cloud Types and Precipitation
Forms of Precipitation
16 Cloud Types and Precipitation
 The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lower few kilometers of the atmosphere.
 Rain and Snow
• In meteorology, the term rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm.
• At very low temperatures (when the moisture content of air is low) light fluffy snow made up of individual six-sided ice crystals forms.

57. 16 Cloud Types and Precipitation
Forms of Precipitation
16 Cloud Types and Precipitation
 Rain and Snow
• Sleet is the fall of clear-to-translucent ice.
• Hail is produced in cumulonimbus clouds.
• Hailstones begin as small ice pellets that grow by collecting supercooled water droplets as they fall through a cloud.

58. Largest Recorded Hailstone
Makes no sense without caption in book

59. Types of precipitation

60. Hail formation