Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter.  After you complete the chapter, you should be able to:

13.1 Define air pressure and describe the instruments used to measure this weather element.

13.2 Discuss the three forces that act on the atmosphere to either create or alter winds.

13.3 Contrast the weather associated with low-pressure centers (cyclones) and high-pressure centers (anticyclones).

13.4 Summarize Earth’s idealized global circulation. Describe how continents and seasonal temperature changes complicate the idealized pattern.

13.5 List three types of local winds and describe their formation.

13.6 Describe the instruments used to measure wind. Explain how wind direction is expressed using compass directions.

13.7 Discuss the major factors that influence the global distribution of precipitation.

Concept Checks

 13.1 Describe air pressure in your own words. What is standard sea-level pressure in millibars, in inches of mercury, and in pounds per square inch?Describe the operating principles of the mercury barometer and the aneroid barometer.

13.2 List three factors that combine to direct horizontal airflow (wind).Write a generalization relating the spacing of isobars to wind speed. Briefly describe how the Coriolis effect and friction influence air movement. Unlike winds aloft, which blow nearly parallel to the isobars, surface winds generally cross the isobars. Explain what causes this difference.

13.3 Prepare a diagram with isobars and wind arrows that shows the winds associated with surface cyclones and anticyclones in both the Northern and Southern Hemispheres. For a surface low-pressure center to exist for an extended period, what condition must exist aloft? What general weather conditions are to be expected when the pressure tendency is rising? When the pressure tendency is falling?

13.4 Referring to the idealized model of atmospheric circulation, in which belt of prevailing winds is most of the United States? The trade winds diverge from which pressure belt? Which prevailing wind belts converge in the stormy region known as the polar front? Which pressure belt is associated with the equator? Explain the seasonal change in winds associated with India. What term is applied to this seasonal wind shift?

13.5 What is a local wind? In what way are land and sea breezes similar to mountain and valley breezes? What are chinook winds?

13.6 What are the two basic wind measurements? What instruments are used to make these measurements? From what direction does a northeast wind blow? Toward what direction does a south wind blow?

13.7 With which global pressure belt are the rain forests of Africa’s Congo basin associated? Which pressure system is linked to the Sahara Desert? What factors, in addition to the distribution of wind and pressure, influence the global distribution of precipitation?

Concepts In Review The Atmosphere in Motion

13.1 Understanding Air Pressure

Define air pressure and describe the instruments used to measure this weather element.

Key Terms: wind, air pressure, mercury barometer, aneroid barometer

• Air has weight: At sea level, it exerts a pressure of 1 kilogram per square centimeter (14.7 pounds per square inch), or 1 atmosphere.

• Air pressure is the force exerted by the weight of air above. With increasing altitude, there is less air above to exert a force, and thus air pressure decreases with altitude—rapidly at first and then much more slowly.

• The unit meteorologists use to measure atmospheric pressure is the millibar. Standard sea-level pressure is expressed as 1013.2 millibars. Isobars are lines on a weather map that connect places of equal air pressures.

• A mercury barometer measures air pressure using a column of mercury in a glass tube sealed at one end and inverted in a dish of mercury. It measures atmospheric pressure in inches of mercury, the height of the column of mercury in the barometer. Standard atmospheric pressure at sea level equals 29.92 inches of mercury. As air pressure increases, the mercury in the tube rises, and when air pressure decreases, so does the height of the column of mercury.Aneroid (“without liquid”) barometers consist of partially evacuated metal chambers that compress as air pressure increases and expand as pressure decreases.

13.2 Factors Affecting Wind

Discuss the three forces that act on the atmosphere to either create or alter winds.

Key Terms: pressure gradient force (PGF), isobar, pressure gradient, Coriolis effect, geostrophic wind, jet stream

• Wind is controlled by a combination of (1) the pressure gradient force, (2) the Coriolis effect, and (3) friction. The pressure gradient force, which results from pressure differences, is the primary force that drives wind. It is depicted by the spacing of isobars on a map. Closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate a weak pressure gradient and light winds.

• The Coriolis effect, which is due to Earth’s rotation, produces deviation in the path of wind (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). Friction, which significantly influences airflow near Earth’s surface, is negligible above a height of a few kilometers.

• Above a height of about 1.5 kilometers, the Coriolis effect is equal to and opposite the pressure gradient force, which results in geostrophic winds. Geostrophic winds flow in a nearly straight path, parallel to the isobars, with velocities proportional to the pressure gradient force.

• A prominent feature of upper-level flow are jet streams, fast-moving currents that travel in a west-to-east direction.

QUESTION: These diagrams show surface winds at two locations. All factors in both situations are identical except that one surface is rugged land and the other is water. Which diagram likely represents winds over the land? Explain your choice.

13.3 Highs and Lows

Contrast the weather associated with low-pressure centers (cyclones) and high-pressure centers (anticyclones).

Key Terms: cyclone (low), anticyclone (high), convergence, divergence, pressure (barometric) tendency

• The two types of pressure centers are (1) cyclones, or lows (centers of low pressure), and (2) anticyclones, or highs (centers of high pressure). In the Northern Hemisphere, winds around a low (cyclone) are counterclockwise and inward. Around a high (anticyclone), they are clockwise and outward. In the Southern Hemisphere, the Coriolis effect causes winds to be clockwise around a low and counterclockwise around a high.

• Because air rises and cools adiabatically in a low-pressure center, cloudy conditions and precipitation are often associated with their passage. In a high-pressure center, descending air is compressed and warmed; therefore, cloud formation and precipitation are unlikely in an anticyclone, and “fair” weather is usually expected.

QUESTION: Refer to Figure 13.4. Assume that you are an observer checking this aneroid barometer several hours after it was last checked. What is the pressure tendency? How did you figure this out? What does the tendency indicate about forthcoming weather?

13.4 General Circulation of the Atmosphere

Summarize Earth’s idealized global circulation. Describe how continents and seasonal temperature changes complicate the idealized pattern.

Key Terms: equatorial low, intertropical convergence zone (ITCZ), subtropical high, trade winds, westerlies, polar easterlies, subpolar low, polar front, polar high, monsoon

• If Earth’s surface were uniform, four belts of pressure oriented east to west would exist in each hemisphere. Beginning at the equator, the four belts would be the (1) equatorial low, also referred to as the intertropical convergence zone (ITCZ), (2) subtropical high at about 25° to 35° on either side of the equator, (3) subpolar low, situated at about 50° to 60° latitude, and (4) polar high, near Earth’s poles.

• Particularly in the Northern Hemisphere, large seasonal temperature differences over continents disrupt the idealized, or zonal, global patterns of pressure and wind. In winter, large, cold landmasses develop a seasonal high-pressure system from which surface airflow is directed off the land. In summer, landmasses are heated, and a low-pressure system develops over them, which permits air to flow onto the land. These seasonal changes in wind direction are known as monsoons.

• In the middle latitudes, between 30° and 60° latitude, the general west-to-east flow of the westerlies is interrupted by the migration of cyclones and anticyclones. The paths taken by these cyclonic and anticyclonic systems are closely correlated to upper-level airflow and the polar jet stream. The average position of the polar jet stream, and hence the paths followed by cyclones, migrates southward with the approach of winter and northward as summer nears.

13.5 Local Winds

List three types of local winds and describe their formation.

Key Terms: local wind, sea breeze, land breeze, valley breeze, mountain breeze, chinook, Santa Ana

• Local winds are small-scale winds produced by a locally generated pressure gradient. Sea and land breezes form along coasts and are brought about by temperature contrasts between land and water. Valley and mountain breezes occur in mountainous areas where the air along slopes heats differently than does the air at the same elevation over the valley floor. Chinook and Santa Ana winds are warm, dry winds created when air descends the leeward side of a mountain and warms by compression.

QUESTION: With which local wind are the clouds in this photo most likely associated? Would you expect clouds such as these to form at night? Explain.

13.6 Measuring Wind

Describe the instruments used to measure wind. Explain how wind direction is expressed using compass directions.

Key Terms: wind vane, cup anemometer, prevailing wind

• The two basic wind measurements are direction and speed. Winds are always labeled by the direction from which they blow. Wind direction is measured with a wind vane, and wind speed is measured using a cup anemometer.

QUESTION: When designing an airport, it is important to have planes take off into the wind. Refer to the accompanying wind rose and describe the orientation of the runway and the direction planes would usually travel when they took off. Where on Earth might you find a wind rose like this?

13.7 Global Distribution of Precipitation

Discuss the major factors that influence the global distribution of precipitation.

• The general features of the global distribution of precipitation can be explained by global winds and pressure systems. In general, regions influenced by high pressure, with its associated subsidence and divergent winds, experience dry conditions. Regions under the influence of low pressure, with its converging winds and ascending air, receive ample precipitation.Air temperature, the distribution of continents and oceans, and the location of mountains also influence the distribution of precipitation.

QUESTION: This satellite image was produced with data from the Tropical Rainfall Measuring Mission (TRMM). Notice the band of heavy rainfall shown in reds and yellows that extends east–west across the image. With which pressure zone is this band of rainy weather associated? Is it more likely that this image was acquired in July or January? Explain.