Meteorology Today: Air Pressure and Winds (2022)

8 Chapter Air Pressure and Winds L e a r nin g O b j e cti v e s At the end of this section, you should be able to: LO1 Explain how vertical changes in air temperature affect air pressure. LO2 Explain the difference between cons...

8 Chapter Air Pressure and Winds L e a r nin g O b j e cti v e s At the end of this section, you should be able to: LO1 Explain how vertical changes in air temperature affect air pressure. LO2 Explain the difference between constant-pressure charts and constant-height charts. LO3 Discuss Newton’s three laws of motion and how they relate to air movement. LO4 Describe the forces that affect air movement. LO5 Explain the difference between geostrophic and gradient winds. LO6 Discuss how winds blow around high and low pressure centers in the Northern and Southern Hemispheres. DECEMBER 19, 1980, WAS A COOL DAY IN LYNN, Massachusetts, but not cool enough to dampen the spirits of more than 2000 people who gathered in Central Square—all hoping to catch at least one of the 1500 dollar bills that would be dropped from a small airplane at noon. Right on schedule, the aircraft circled the city and dumped the money onto the people below. However, to the dismay of the onlookers, a westerly wind caught the currency before it reached the ground and carried it out over the cold Atlantic Ocean. Had the pilot or the sponsoring leather manufacturer examined the weather charts beforehand, it might have been possible to predict that the wind would ruin the advertising scheme. 199 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. T he scenario on the previous page raises two questions: well up into the atmosphere. In the column, the dots represent (1) Why does the wind blow? and (2) How can one tell air molecules. Our model assumes: (1) that the air molecules its direction by looking at weather charts? Chapter 1 has are not crowded close to the surface and, unlike the real atmo- already answered the first question: Air moves in response sphere, the air density remains constant from the surface up to to horizontal differences in pressure. This is what happens the top of the column, (2) that the width of the column does not when we open a vacuum-packed can: Air rushes from the change with height, and (3) that the air is unable to freely move higher-pressure region outside the can toward the region of into or out of the column. lower pressure inside. In the atmosphere, the wind blows in Suppose we somehow force more air into the column in an attempt to equalize imbalances in air pressure. Does this Fig. 8.1. What would happen? If the air temperature in the mean that the wind always blows directly from high to low column does not change, the added air would make the col- pressure? Not really, because the movement of air is controlled umn more dense, and the added weight of the air in the column not only by pressure differences but by other forces as well. would increase the surface air pressure. Likewise, if a great deal In this chapter, we will first consider how and why atmospheric of air were removed from the column, the surface air pressure pressure varies, then we will look at the forces that influence atmo- would decrease. Consequently, to change the surface air pres- spheric motions aloft and at the surface. Through studying these sure, we need to change the mass of air in the column above the forces, we will be able to tell how the wind should blow in a par- surface. But how can this feat be accomplished? ticular region by examining surface and upper-air charts. Look at the air columns in Fig. 8.2a.* Suppose both col- umns are located at the same elevation, both have the same air temperature, and both have the same surface air pressure. There 8.1 Atmospheric Pressure must, therefore, be the same number of molecules (same mass of air) in each column above both cities. Further suppose that the LO1 surface air pressure for both cities remains the same, while the air above city 1 cools and the air above city 2 warms (see Fig. 8.2b). In Chapter 1, we learned several important concepts about atmo- As the air in column 1 cools, the molecules move more spheric pressure. One is that air pressure is simply the mass of slowly and crowd closer together, so the air becomes more air above a given level. As we climb in altitude above Earth’s sur- dense. In the warm air above city 2, the molecules move faster face, there are fewer air molecules above us; hence, atmospheric and spread farther apart, and the air becomes less dense. Because pressure always decreases with increasing height. Another concept the width of the columns does not change (and if we assume an we learned is that our atmosphere is highly compressible. This invisible barrier exists between the columns), the total number means that the weight of higher layers compresses the atmosphere of molecules above each city remains the same, and the surface beneath, and so most of the molecules in our atmosphere are pressure does not change. Therefore, in the more-dense, colder crowded close to Earth’s surface. Hence, air pressure decreases air above city 1, the height of the column decreases, while it with height, rapidly at first, then more slowly at higher altitudes. increases in the less-dense, warmer air above city 2. So one way to change air pressure is to simply move up or We now have a colder, shorter, more-dense column of air down in the atmosphere. But what causes the air pressure to above city 1 and a warmer, taller, less-dense air column above change in the horizontal? And why does the air pressure change city 2. From this situation, we can conclude that it takes a shorter at the surface? column of cold, more-dense air to exert the same surface pressure as a taller column of warm, less-dense air. This concept has a 8.1a Horizontal Pressure Variations: great deal of meteorological significance. A Tale of Two Cities Atmospheric pressure decreases more rapidly with height To answer these questions, we eliminate some of the complexi- in the cold column of air. In the cold air above city 1 (Fig. 8.2b), ties of the atmosphere by constructing models. Figure 8.1 move up the column and observe how quickly you pass through shows a simple atmospheric model—a column of air, extending the densely packed molecules. This activity indicates a rapid change in pressure. In the warmer, less-dense air, the pressure does not decrease as rapidly with height simply because you Figure 8.1 A model of the atmosphere in which air climb above fewer molecules in the same vertical distance. density remains constant with height. The air pressure at the In Fig. 8.2c, move up the warm, tan column until you come surface is related to the number of molecules above. When air of the same temperature is stuffed into the column, the to the letter H. Now move up the cold, blue column the same surface air pressure rises. When air is removed from the distance until you reach the letter L. Notice there are more mol- column, the surface pressure falls. (In the actual atmosphere, ecules above the letter H in the warm column than above the let- unlike this model, density decreases with height.) ter L in the cold column. The fact that the number of molecules above any level is a measure of the atmospheric pressure leads to an important concept: Warm air aloft is normally associated *We will keep our same assumption as in Fig. 8.1; that is, (1) the air molecules are not crowded close to the surface, (2) the width of the columns does not change, and (3) air is unable to move into or out of the columns. 200 Chapter 8 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Figure 8.2 Illustration of how variations in temperature can produce horizontal pressure forces. (Note that for simplicity, this model assumes that the air density is constant with height, whereas in the actual atmosphere, density decreases with height.) (a) Two air columns, each with identical mass, have the same surface air pressure. (b) Because it takes a shorter column of cold air to exert the same surface pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must rise. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a region of lower pressure. The removal of air from column 2 causes its surface pressure to drop, whereas the addition of air into column 1 causes its surface pressure to rise. (The difference in height between the two columns is greatly exaggerated.) with high atmospheric pressure, and cold air aloft is associated with low atmospheric pressure. In Fig. 8.2c, the horizontal difference in temperature creates a horizontal difference in pressure. The pressure difference estab- lishes a force (called the pressure-gradient force) that causes the air to move from higher pressure toward lower pressure. Consequently, if we remove the invisible barrier between the two columns near the top of column 1 and allow the air aloft to move horizontally, the air will move from column 2 toward column 1. As the air aloft leaves column 2, the mass of the air in the column decreases, and so does the surface air pressure. Meanwhile, the accumulation of air in column 1 causes the surface air pressure to increase. Higher air pressure at the surface in column 1 and lower air pressure at the surface in column 2 causes the surface air to move from city 1 toward city 2 (see Fig. 8.3). As the surface air moves out away from city 1, the air aloft slowly sinks to replace this outwardly spreading surface air. As the surface air flows into city 2, it slowly rises to replace the depleted air aloft. In this manner, a complete circulation of air is established due to the heating and cooling of air columns. As we will see in Chapter 9, this type of thermal circulation is the basis for a wide range of Figure 8.3 The heating and cooling of air columns causes horizontal wind systems throughout the world. pressure variations aloft and at the surface. These pressure variations force the In summary, we can see how heating and cooling columns air to move from areas of higher pressure toward areas of lower pressure. In conjunction with these horizontal air motions, the air slowly sinks above the of air can establish horizontal variations in air pressure both aloft surface high and rises above the surface low. and at the surface. It is these horizontal differences in air pressure that cause the wind to blow. Before moving on to the next sec- surface low pressure. Likewise, bitter cold arctic air in winter tion, you may wish to look at Focus section 8.1, which describes is often accompanied by surface high pressure. Yet, on a daily how air pressure, air density, and air temperature are interrelated. basis, any cyclic change in surface pressure brought on by daily temperature changes is concealed by the pressure changes cre- 8.1b Daily Pressure Variations ated by the warming and cooling of the upper atmosphere. From what we have learned so far, we might expect to see the In the tropics, for example, pressure rises and falls in a regu- surface pressure dropping as the air temperature rises, and vice lar pattern twice a day (see Fig. 8.4). Maximum pressures occur versa. Over large continental areas, especially the southwestern around 10:00 a.m. and 10:00 p.m., minimum near 4:00 a.m. and United States in summer, hot surface air is accompanied by 4:00 p.m. The largest pressure difference, about 2.5 mb, occurs Air Pressure and Winds 201 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Figure 8.4 Diurnal surface pressure changes in the middle latitudes and in the tropics. near the equator. It also shows up in higher latitudes, but with As a reference, Fig. 8.5 compares pressure readings in a much smaller amplitude. This daily (diurnal) fluctuation of inches of mercury and millibars. pressure appears to be due primarily to the absorption of solar The unit of pressure designated by the International System energy by ozone in the upper atmosphere and by water vapor (SI) of measurement is the pascal, named in honor of Blaise Pas- in the lower atmosphere. The warming and cooling of the air cal (1623–1662), whose experiments on atmospheric pressure creates density oscillations known as thermal (or atmospheric) greatly increased our knowledge of the atmosphere. A pascal tides that show up as small pressure changes near Earth’s surface. (Pa) is the force of 1 newton acting on a surface area of 1 square In middle latitudes, surface pressure changes are primarily meter. Thus, 100 pascals equals 1 millibar. The scientific com- the result of large high- and low-pressure areas that move toward munity often uses the kilopascal (kPa) as the unit of pressure, or away from a region. When an area of high pressure approaches where 1kPa 5 10mb. However, a more convenient unit is the a city, surface pressure usually rises. When it moves away, pressure hectopascal (hPa), as usually falls. Likewise, an approaching low causes the air pressure 1 hPa 5 1 mb to fall, and one moving away causes surface pressure to rise. The hectopascal has replaced the millibar as the preferred 8.1c Pressure Measurements unit of pressure in most scientific publications around the world. Instruments that detect and measure pressure changes are called However, millibars are still used in many settings, especially in barometers, which literally means an instrument that measures the United States. (No conversion is needed in moving between bars. You may recall from Chapter 1 that a bar is a unit of pres- millibars and hectopascals.) sure that describes a force over a given area.* Because the bar is Because we measure atmospheric pressure with an instru- a relatively large unit, and because surface pressure changes are ment called a barometer, atmospheric pressure is also referred to normally small, the unit of pressure commonly found on surface as barometric pressure. Evangelista Torricelli, a student of Galileo, weather maps is, as we saw in Chapter 1, the millibar (mb), invented the mercury barometer in 1643. His barometer, similar where 1 mb 5 1/1000 bar or to those in use today, consisted of a long glass tube open at one end and closed at the other (see Fig. 8.6). Removing air from 1 bar 5 1000 mb the tube and covering the open end, Torricelli immersed the A common pressure unit used in aviation is inches of mer- lower portion into a dish of mercury. He removed the cover, and cury (Hg). At sea level, standard atmospheric pressure** is the mercury rose up the tube to nearly 76 cm (or about 30 in.) above the level in the dish. Torricelli correctly concluded that the 1013.25 mb 5 29.92 in. Hg 5 76 cm column of mercury in the tube was balancing the weight of the air above the dish, and hence its height was a measure of atmo- *A bar is a force of 100,000 newtons acting on a surface area of 1 square meter. spheric pressure. The use of mercury barometers has declined A newton (N) is the amount of force required to move an object with a mass of greatly with the development of alternatives that are just as accu- 1 kilogram so that it increases its speed at a rate of 1 meter per second each second. Additional pressure units and conversions are given in Appendix A. rate, along with the growing awareness of the health risks of mer- **Standard atmospheric pressure at sea level is the pressure extended by a column of cury. In several nations and a number of U.S. states and cities, it mercury 29.92 in. (760 mm) high, having a density of 1.36 3 104 kg/m3 , and subject is illegal to manufacture, sell, and/or distribute mercury barom- to an acceleration of gravity of 9.80 m/sec 2. eters, though some exceptions exist for antique barometers. 202 Chapter 8 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 8.6 The mercury barometer. The height of the mercury column is a measure of atmospheric pressure. of the cell is calibrated to represent different pressures, and any Figure 8.5 Atmospheric pressure in inches of mercury and in millibars. change in its size is amplified by levers and transmitted to an indicating arm, which points to the current atmospheric pres- sure (see Fig. 8.7). Weather Watch Notice that the aneroid barometer often has descriptive Although 1013.25 mb (29.92 in.) is the standard atmospheric weather-related words printed above specific pressure values. pressure at sea level, it is not the average sea-level pressure. These descriptions indicate the most likely weather conditions Earth’s average sea-level pressure is 1011.0 mb (29.85 in.). Because when the needle is pointing to that particular pressure reading. much of Earth’s surface is above sea level, Earth’s annual average Generally, the higher the reading, the more likely clear weather surface pressure is estimated to be 984.43 mb (29.07 in.). will occur, and the lower the reading, the better the chances for Why would one use mercury rather than water in a barom- eter? The primary reason is convenience. (Also, water can evaporate in the tube.) Mercury seldom rises to a height above 80 cm (31.5 in.). A water barometer, however, presents a prob- lem. Because water is 13.6 times less dense than mercury, an atmospheric pressure of 76 cm (30 in.) of mercury would be equivalent to 1034 cm (408 in.) of water. A water barometer rest- ing on the ground near sea level would have to be read from a ladder over 10 m (33 ft) tall. Water is sometimes used as a more general indicator of atmospheric pressure in smaller devices called “storm glasses.” The most common type of home barometer—the aneroid barometer—contains no fluid. Inside this instrument is a small, flexible metal box called an aneroid cell. Before the cell is tightly sealed, air is partially removed, so that small changes in exter- nal air pressure cause the cell to expand or contract. The size Figure 8.7 The aneroid barometer. Air Pressure and Winds 203 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Focus on a Special Topic 8.1 The Atmosphere Obeys the Gas Law Air temperature, air pressure, and air density normally close to 500 millibars. If we obtain are all interrelated. If one of these variables the average density at this level, with the aid changes, the other two usually change as well. of the gas law we can calculate the average air The relationship among the pressure, temperature. temperature, and density of air can be expressed Recall that the gas law is written as by p 5T 3r 3C Pressure 5 temperature 3 density 3 constant With the pressure ( p ) in millibars (mb), the This simple relationship, often referred to temperature (T ) in kelvin, and the density (r ) in as the gas law (or equation of state), tells us that Figure 1 Air above a region of surface high kilograms per cubic meter (kg/m3 ), the numerical the pressure of a gas is equal to its temperature pressure is more dense than air above a region of value of the constant (C ) is about 2.87.* times its density times a constant. When we surface low pressure (at the same temperature). (The At an altitude of 5600 m above sea level, ignore the constant and look at the gas law in dots in each column represent air molecules.) where the average (or standard) air pressure symbolic form, it becomes is about 500 mb and the average air density We can see, then, that for surface high- is 0.690 kg/m3, the average air temperature p , T 3r pressure areas (anticyclones) and surface becomes where, of course, p is pressure, T is low-pressure areas (mid-latitude cyclones) to p 5T 3r 3C temperature, and r (the Greek letter rho, form, the air density (mass of air) above these systems must change. As we will see later in this 500 5 T 3 0.690 3 2.87 pronounced “row”) represents air density.* The line , is a symbol meaning “is proportional to.” chapter, as well as in other chapters, surface air 500 5T A change in one variable causes a corresponding pressure increases when the wind causes more 0.690 3 2.87 change in the other two variables. Thus, it will air to move into a column of air than is able to 252.5 K 5 T be easier to understand the behavior of a gas if leave (called net convergence), and surface air To convert kelvins into degrees Celsius, we we keep one variable from changing and pressure decreases when the wind causes more subtract 273 from the Kelvin temperature and observe the behavior of the other two. air to move out of a column of air than is able to obtain a temperature of 220.58C, which is the Suppose, for example, we hold the enter (called net divergence). same as 258F. temperature constant. The relationship then Earlier, we considered how pressure and If we know the numerical values of becomes density are related when the temperature is temperature and density, with the aid of the not changing. What happens to the gas law gas law we can obtain the air pressure. For p,r when the pressure of a gas remains constant? In (assuming temperature is constant). example, in Chapter 1 we saw that the average shorthand notation, the relationship becomes global temperature near sea level is about This expression says that the pressure of (constant pressure) , T 3 r 158C (598F), which is the same as 288 K. If the the gas is proportional to its density, as long as average air density at sea level is 1.226 kg/m3 , its temperature does not change. Consequently, This relationship tells us that when the what would be the standard (average) sea-level if the temperature of a gas (such as air) is held pressure of a gas is held constant, the gas pressure? constant, as the pressure increases the density becomes less dense as the temperature goes up, Using the gas law, we obtain increases, and as the pressure decreases the and more dense as the temperature goes down. density decreases. In other words, at the same Therefore, at a given atmospheric pressure, air p 5T 3r 3C temperature, air at a higher pressure is more that is cold is more dense than air that is warm. p 5 288 3 1.226 3 2.87 dense than air at a lower pressure. If we apply this Keep in mind that the idea that cold air is more p 5 1013 mb concept to the atmosphere, then the air above dense than warm air applies only when we Because the air pressure is related to both a region of surface high pressure is more dense compare volumes of air at the same level, where temperature and density, a small change in than air above a region of surface low pressure, pressure changes are small in any horizontal either or both of these variables can bring about assuming that the temperature and elevation direction compared to the vertical. a change in pressure. are nearly the same in both cases (see Fig. 1). We can use the gas law to obtain *This gas law may also be written as p 3 v 5 T 3 information about the atmosphere. For example, *The constant is usually expressed as 2.87 3 106 erg/gK , constant. Consequently, pressure and temperature at an altitude of about 5600 m (18,400 ft) or, in the SI system, as 287 J/kg K. (See Appendix A for changes are also related to changes in volume. above sea level, the atmospheric pressure is information regarding the units used here.) 204 Chapter 8 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. inclement weather. This situation occurs because surface high- being at different elevations above sea level. This fact becomes pressure areas are associated with sinking air and normally fair even clearer when we realize that atmospheric pressure changes weather, whereas surface low-pressure areas are associated with much more quickly when we move upward than it does when rising air and usually cloudy, wet weather. A steady rise in atmo- we move sideways. As an example, the vertical change in air spheric pressure (a rising barometer reading) usually indicates pressure from the base to the top of the Empire State Building— clearing weather or fair weather, whereas a steady drop in atmo- a distance of a little more than 1 2 km—is typically much greater spheric pressure (a falling barometer reading) often signals the than the horizontal difference in air pressure from New York approach of a cyclonic storm with inclement weather. City to Miami, Florida—a distance of over 1600 km. Therefore, The altimeter and barograph are two types of aneroid barom- we can see that a small vertical difference between two observa- eters. Altimeters are aneroid barometers that measure pressure, tion sites can yield a large difference in station pressure. Thus, but are calibrated to indicate altitude. Barographs are recording to properly monitor horizontal changes in pressure, barometer aneroid barometers. Basically, the barograph consists of a pen readings must be corrected for altitude. attached to an indicating arm that marks a continuous record of Altitude adjustments are made so that a barometer reading pressure on chart paper. The chart paper is attached to a drum taken at one elevation can be compared with a barometer read- rotated slowly by an internal mechanical clock (see Fig. 8.8). ing taken at another. Station pressure observations are normally Digital (electronic) barometers use a device called a trans- adjusted to a level of mean sea level—the level representing the ducer that detects the change in pressure exerted by the atmo- average surface of the ocean. The adjusted reading is called sea- sphere on a precisely engineered surface. The change in pressure level pressure. The size of the adjustment depends primarily is then converted into an electrical signal. Some digital barom- on how high the station is above sea level. The adjustment also eters are small enough to be included in smartphones, while varies with surface temperature. others are designed for research settings. Weather Watch 8.1d Pressure Readings The pressure was on in London, England, on the night of January Obtaining the correct air pressure from a mercury barometer 19, 2020. Around midnight, Heathrow Airport recorded a sea-level involves more than simply reading the height of the mercury pressure of 1049.6 millibars (30.99"). This was the highest sea-level column. Being a fluid, mercury is sensitive to changes in tem- pressure observed in more than 300 years of recordkeeping for the perature; it will expand when heated and contract when cooled. London area. A pressure reading this high would extend to the very Consequently, to obtain accurate pressure readings without the top of the scale of many home barometers. influence of temperature, all mercury barometers are corrected as if they were read at the same temperature. Also, because Earth is not a perfect sphere, the force of gravity is not a constant. 8.2 Surface and Upper-Level Charts Because small gravity differences influence the height of the mercury column, they must be taken into account when read- LO2 ing the barometer. Finally, each mercury barometer has its own “built-in” error, called instrument error, which is caused, in part, Near Earth’s surface, atmospheric pressure decreases on the by the surface tension of the mercury against the glass tube. average by about 10 millibars for every 100 meters of increase in After being corrected for temperature, gravity, and instrument elevation (about 1 in. of mercury for each 1000-ft rise).* Notice error, the barometer reading at a particular location and eleva- in Fig. 8.9a that city A has a station pressure of 952 millibars. tion is termed station pressure. Notice also that city A is 600 meters above sea level. Adding Figure 8.9a gives the station pressure measured at four 10 millibars per 100 meters to its station pressure yields a sea- locations only a few hundred kilometers apart. The different level pressure of 1012 mb (Fig. 8.9b). After all the station pres- station pressures of the four cities are due primarily to the cities sures are adjusted to sea level (Fig. 8.9c), we are able to see the horizontal variations in sea-level pressure—something we were not able to see from the station pressures alone in Fig. 8.9a. When more pressure data are added (see Fig. 8.9c), the chart can be analyzed and the pressure pattern visualized. Isobars (lines connecting points of equal pressure) are drawn at intervals of 4 mb,** with 1000 mb being the base value. Note that the isobars do not pass through each point, but, rather, between many of them, with the exact values being interpolated from the *This decrease in atmospheric pressure with height (10 mb/100 m) occurs when the air temperature decreases at the standard lapse rate of 6.58C/1000m. Because atmo- spheric pressure decreases more rapidly with height in cold (more-dense) air than it does in warm (less-dense) air, the vertical rate of pressure change is typically greater than 10 mb per 100 m in cold air and less than that in warm air. **An interval of 2 mb would put the lines too close together, and an 8-mb interval Figure 8.8 A recording barograph. would spread them too far apart. Air Pressure and Winds 205 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Figure 8.9 The top diagram (a) shows four cities (A, B, C, and D) at varying elevations above sea level, all with different station pressures. The middle diagram (b) represents sea-level pressures of the four cities plotted on a sea-level chart. The bottom diagram (c) shows sea- level pressure readings of the four cities plus other sea-level pressure readings at locations not shown in (a) and (b). Isobars are drawn on the chart (gray lines) at intervals of 4 millibars. data given on the chart. For example, follow the 1008-mb line Another type of chart more commonly used in studying the from the top of the chart southward and observe that there is no weather is the constant pressure chart, or isobaric chart. Instead plotted pressure of 1008 millibars. The 1008-millibars isobar, of showing pressure variations at a constant altitude, these charts however, comes closer to the station with a sea-level pressure of are constructed to show height variations along a constant pres- 1007 mb than it does to the station with a pressure of 1010 mb. sure (isobaric) surface. Constant pressure charts are convenient With its isobars, the bottom chart (Fig. 8.9c) is now called a sea- to use because the height variables they show are easier to deal level pressure chart or simply a surface map. When weather data with in meteorological equations than the variables of pressure. are plotted on the map, it becomes a surface weather map. Given that isobaric charts are routinely used by meteorologists, The isobars in Fig. 8.10 have been smoothed to eliminate let’s examine them in detail. small-scale wiggles produced by data collected at high-altitude Imagine that the dots inside the air column in Fig. 8.12 stations and at stations that might have small observational represent tightly packed air molecules from the surface up errors. Otherwise, the isobars might be significantly distorted. to the tropopause. Assume that the air density is constant An extreme case of this type of error occurs at Leadville, Colo- throughout the entire air layer and that all of the air molecules rado (elevation 3096 m), the highest city in the United States. are squeezed into this layer. If we climb halfway up the air col- Here, the station pressure is typically near 700 mb. This means umn and stop, then draw a sheetlike surface representing this that nearly 300 mb must be added to obtain a sea-level pressure level, we will have made a constant height surface. This alti- reading! A mere 1 percent error in estimating the adjustment tude (5600 m) is where we would, under standard conditions, would result in a 3-mb error in sea-level pressure. measure a pressure of 500 millibars. Observe that everywhere The sea-level pressure chart described so far is called a along this surface (shaded tan in the diagram) there are an constant height chart because it represents the atmospheric pres- equal number of molecules above and below it. This condition sure at a constant level—in this case, sea level. The same type of means that the level of constant height also represents a level chart could be drawn to show the horizontal variations in pres- of constant pressure. At every point on this isobaric surface, sure at any level in the atmosphere; for example, at 3000 meters the height is 5600 meters above sea level and the pressure is (see Fig. 8.11). 500 millibars. Within this simplified air column, we could 206 Chapter 8 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Figure 8.10 Sea-level isobars, drawn every 4 millibars and smoothed to account for small-scale variations related to high-altitude stations and potential observational errors. Figure 8.12 When there are no horizontal variations in pressure, constant pressure surfaces are parallel to constant height surfaces. In the diagram, a measured pressure of 500 millibars is 5600 meters above sea level everywhere. The actual atmosphere extends well above the tropopause level shown here. (Dots in the diagram represent air molecules.) it. Notice in Fig. 8.13 that we have colder air to the north and warmer air to the south. To simplify this situation, we will assume that the atmospheric pressure at Earth’s surface remains constant. Hence, the total number of molecules in the column above each region must remain constant. In Fig. 8.13, the area shaded gray at the top of the col- umn represents a constant pressure (isobaric) surface, where the atmospheric pressure at all points along this surface is 500 millibars. Notice that in the warmer, less-dense air the 500-mb pressure surface is found at a higher (than average) level, while in the colder, more-dense air, it is observed at a much lower (than average) level. From these observations, we can see that when the air aloft is warm, constant pressure surfaces are typically found at higher elevations than normal, and when the air aloft is cold, constant pressure surfaces are typically found at lower elevations than normal. The variations in height of the isobaric surface in Fig. 8.13 are shown in Fig. 8.14. Note that where the constant altitude lines intersect the 500-mb pressure surface, contour lines (lines connecting points of equal elevation) are drawn on the 500-mb Figure 8.11 Each map shows isobars on a constant height chart. The map. Each contour line, of course, tells us the altitude above sea isobars represent variations in horizontal pressure at that altitude. An average level at which we can obtain a pressure reading of 500 mb. In isobar at sea level would be about 1000 mb; at 3000 m, about 700 mb; and at the warmer air to the south, the elevations are high, while in the 5600 m, about 500 mb. cold air to the north, the elevations are low. The contour lines are crowded together in the middle of the chart, where the pres- cut any number of horizontal slices, each one at a different sure surface dips rapidly due to the changing air temperatures. altitude, and each slice would represent both an isobaric and Where there is little horizontal temperature change, there are constant height surface. A contour map of any one of these also few contour lines. Although the contour lines are lines of surfaces would be blank, as there are no horizontal variations constant height, keep in mind that they illustrate pressure as do in either pressure or altitude. isobars, in that contour lines of low height represent a region of If the air temperature should change in any portion of the lower pressure and contour lines of high height represent a region column, the air density and pressure would change along with of higher pressure. Air Pressure and Winds 207 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Although we have examined only the 500-mb chart, other isobaric charts are commonly used. ▼ Table 8.1 lists these charts and their approximate heights above sea level. Upper-level charts are a valuable tool. As we will see, they can be used to analyze wind flow patterns that are extremely important in forecasting the weather. They can also be used to determine the movement of weather systems and to predict the behavior of surface pressure areas. To the pilot of a small aircraft, a constant pressure chart can help determine whether the plane is flying at an altitude either higher or lower than its altimeter indicates. (For more information on this topic, read Focus section 8.2.) Figure 8.16a is a simplified surface map that shows areas of high and low pressure and arrows that indicate wind direction—the direction from which the wind is blowing. The large blue Hs on the map indicate the centers of high pressure, which are also called anticyclones. The large Ls represent cen- ters of low pressure, also known as depressions or mid-latitude Figure 8.13 The area shaded gray in the above diagram represents a surface cyclonic storms because they form in the middle latitudes, out- of constant pressure, or isobaric surface. Because of the changes in air density, the isobaric surface rises in warm, less-dense air and lowers in cold, more-dense air. side of the tropics. The solid dark lines are isobars with units in Where the horizontal temperature changes most quickly, the isobaric surface changes millibars. Notice that the surface winds tend to blow across the elevation most rapidly. isobars toward regions of lower pressure. In fact, as we briefly observed in Chapter 1, in the Northern Hemisphere the winds blow counterclockwise and inward toward the center of the lows Because cold air aloft is normally associated with low and clockwise and outward from the center of the highs. heights and warm air aloft with high heights, on upper-air Figure 8.16b shows a simplified upper-air chart (a 500-mb charts representing the Northern Hemisphere, contour lines isobaric map) for the same day as the idealized surface map (and isobars) usually decrease in value from south to north in Fig. 8.16a. The solid gray lines on the map are contour because the air is typically warmer to the south and colder to lines given in meters above sea level. The difference in eleva- the north. The lines, however, are not straight; they bend and tion between each contour line (called the contour interval) turn, indicating ridges (elongated highs) where the air is warm is 60 meters. Superimposed on this map are dashed red lines, and indicating depressions, or troughs (elongated lows), where which represent lines of equal temperature (isotherms). the air is cold. In Fig. 8.15, we can see how the wavy con- Observe how the contour lines tend to parallel the isotherms. tours on the map relate to the changes in altitude of the isobaric As we would expect, the contour lines tend to decrease in value surface. from south to north. Figure 8.14 Changes in altitude of an isobaric surface (500 mb) show up as contour lines on an isobaric (500 mb) map. Where the isobaric surface dips most rapidly, the contour lines are closer together on the 500-mb map. 208 Chapter 8 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  ▼▼Table 8.1 Common Isobaric Charts and their Approximate Elevation Above Sea Level Approximate Elevation Isobaric Surface (MB) Charts (M) (FT) 1000    120    400 850 1460 4800 700 3000 9800 500 5600 18,400 300 9180 30,100 200 11,800 38,700 100 16,200 53,200 the wind tend to cross the isobars on a surface map, yet blow parallel to the contour lines (or isobars) on an upper-air chart? To answer this question we will now examine the forces that affect winds. 8.3 Newton’s Laws of Motion LO3 Figure 8.15 The wavelike patterns of an isobaric surface reflect the changes in air temperature. An elongated region of warm air aloft shows up on an isobaric map Our understanding of why the wind blows stretches back as higher heights and a ridge; the colder air shows as lower heights and a trough. through several centuries, with many scientists contributing to our knowledge. When we think of the movement of air, how- The arrows on the 500-mb map show the wind direction. ever, one great scholar stands out—Isaac Newton (1642–1727), Notice that, unlike the surface winds that cross the isobars in who formulated several fundamental laws of motion. Fig. 8.16a, the winds on the 500-mb chart tend to flow parallel Newton’s first law of motion states that an object at rest will to the contour lines in a wavy west-to-east direction. Why does remain at rest and an object in motion will remain in motion Figure 8.16 (a) Surface map showing areas of high and low pressure. The solid lines are isobars drawn at 4-mb intervals. The arrows represent wind direction. Notice that the wind blows across the isobars. (b) The upper-level (500-mb) map for the same day as the surface map. Solid lines on the map are contour lines in meters above sea level. Dashed red lines are isotherms in 8C. Arrows show wind direction. Notice that, on this upper-air map, the wind blows parallel to the contour lines. Air Pressure and Winds 209 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Focus on an Observation 8.2 Flying on a Constant Pressure Surface—High to Low, Look Out Below Aircraft that use pressure altimeters typically fly along a constant pressure surface rather than a constant altitude surface. They do this because the altimeter, as we saw earlier, is simply an aneroid barometer calibrated to convert atmospheric pressure to an approximate altitude. The altitude indicated by an altimeter assumes a standard atmosphere where the air temperature decreases at the rate of 658C/100 m (3.68F/1000 ft). Given that the air temperature seldom, if ever, decreases at exactly this rate, altimeters generally indicate an Figure 2 An aircraft flying along a surface of constant pressure (orange line) altitude different from their true elevation. may change altitude as the air temperature changes. Without being corrected for the Figure 2 shows a standard column of temperature change, a pressure altimeter will continue to read the same elevation. air bounded on each side by air with a different temperature and density. On the left side, the consider air temperature, and compute a more Because of the inaccuracies inherent in the air is warm; on the right, it is cold. The orange realistic altitude by resetting their altimeters to pressure altimeter, most high-performance and line represents a constant pressure surface of reflect these conditions. commercial aircraft are equipped with a radio 700 mb as seen from the side. In the standard Even without sharp temperature altimeter, also known as a radar altimeter. This air, the 700-mb surface is located at 10,000 ft changes, pressure surfaces may dip suddenly device measures the altitude of the aircraft by above sea level. (see Fig. 3). An aircraft flying into an area of sending out radio waves that bounce off the In the warm air, the 700-mb surface rises; decreasing pressure will lose altitude unless terrain below. The time it takes these waves to in the cold air, it descends. An aircraft flying corrections are made. For example, suppose a reach the surface and return is a measure of the along the 700-mb surface would be at an pilot has set the altimeter for sea-level pressure aircraft’s altitude. When this device is used in altitude less than 10,000 ft in the cold air, equal above station A. At this location, the plane is conjunction with a pressure altimeter, a pilot can to 10,000 ft in the standard air, and greater flying along an isobaric surface at a true altitude determine the variations in a constant pressure than 10,000 ft in the warmer air. With no of 500 ft. As the plane flies toward station B, surface simply by flying along that surface and corrections for temperature, the altimeter would the pressure surface (and the plane) dips but the observing how the true elevation measured by indicate the same altitude at all three positions altimeter continues to read 500 ft, which is too the radio altimeter changes. (Note that while because the air pressure does not change. high. To correct for such changes in pressure, a the Global Positioning System is very useful in We can see that, if no temperature corrections pilot can obtain a current altimeter setting from horizontal mapping, such as telling you where are made, an aircraft flying into warm air ground control. With this additional information, your vehicle is headed, GPS signals are not will increase in altitude and fly higher than the altimeter reading will more closely match considered as reliable as altimeters in gauging its altimeter indicates. Put another way: The the aircraft’s actual altitude. the altitude of an aircraft.) altimeter inside the plane will read an altitude lower than the plane’s true elevation. Flying from standard air into cold air represents a potentially dangerous situation. As an aircraft flies into cold air, it flies along a lowering pressure surface. If no correction for temperature is made, the altimeter shows no change in elevation even though the aircraft is losing altitude; hence, the plane will be flying lower than the altimeter indicates. This problem can be serious, especially for planes flying above mountainous terrain with poor visibility and where high winds and turbulence can reduce the air pressure drastically. To ensure adequate Figure 3 In the absence of horizontal temperature changes, pressure surfaces can dip clearance under these conditions, pilots fly toward Earth’s surface. An aircraft flying along the pressure surface will either lose or gain altitude, their aircraft higher than they normally would, depending on the direction of flight. 210 Chapter 8 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. (and travel at a constant velocity along a straight line) as long as no force is exerted on the object. For example, a baseball in a pitcher’s hand will remain there until a force (a push) acts upon the ball. After the ball is pushed (thrown), it would continue to move in that direction forever if it were not for the force of air friction (which slows it down), the force of gravity (which pulls it toward the ground), and the catcher’s mitt (which exerts an equal but opposite force to bring it to a halt). Similarly, to start air moving, to speed it up, to slow it down, or even to change its direction requires the action of an external force. This brings us to Newton’s second law. Newton’s second law states that the force exerted on an object equals its mass times the acceleration produced. In symbolic form, this law is written as F 5 ma From this relationship we can see that, when the mass of an object is constant, the force acting on the object is directly related to the acceleration that is produced. A force in its sim- plest form is a push or a pull. Acceleration is the speeding up, the slowing down, and/or the changing of direction of an object. Figure 8.17 The higher water level creates higher fluid pressure at the bottom (More precisely, acceleration is the change in velocity* over a of tank A and a net force directed toward the lower fluid pressure at the bottom period of time.) of tank B. This net force causes water to move from higher pressure toward lower Because more than one force may act upon an object, pressure. Newton’s second law always refers to the net, or total, force that results. An object will always accelerate in the direction of the total force acting on it. Therefore, to determine in which direc- all directions, there is a greater pressure in the pipe directed tion the wind will blow, we must identify and examine all of the from tank A toward tank B than from B toward A. forces that affect the horizontal movement of air. These forces Because pressure is force per unit area, there must also be a include: net force directed from tank A toward tank B. This force causes the water to flow from left to right, from higher pressure toward 1. pressure-gradient force lower pressure. The greater the pressure difference, the stronger 2. Coriolis force the force, and the faster the water moves. In a similar way, hori- 3. friction zontal differences in atmospheric pressure cause air to move. We will first study the forces that influence the flow of air aloft. Then we will see which forces modify winds near the 8.4a Pressure-Gradient Force Figure 8.18 shows a region of higher pressure on the map’s ground. left side, lower pressure on the right. The isobars show how the horizontal pressure is changing. If we compute the amount of pressure change that occurs over a given distance, we have the pressure gradient; thus, 8.4 Forces That Influence the Winds difference in pressure LO4 Pressure gradient 5 distance We already know that horizontal differences in atmospheric If we let the symbol delta (D) mean “a change in,” we can pressure cause air to move and, hence, the wind to blow. Given simplify the expression and write the pressure gradient as that air is an invisible gas, it may be easier to see how pressure Dp differences cause motion if we examine a visible fluid, such as PG 5 water. d In Fig. 8.17, the two large tanks are connected by a pipe. where Dp is the pressure difference between two places Tank A is three-quarters full and tank B is only one-half full. some horizontal distance (d ) apart. In Fig. 8.18 the pressure Because the water pressure at the bottom of each tank is pro- gradient between points 1 and 2 is 4 millibars per 100 kilometers. portional to the weight of water above, the pressure at the bot- Suppose the pressure in Fig. 8.18 were to change and the tom of tank A is greater than the pressure at the bottom of isobars become closer together. This condition would produce tank B. Moreover, because fluid pressure is exerted equally in a rapid change in pressure over a relatively short distance, or what is called a steep (or strong) pressure gradient. However, if *Velocity specifies both the speed of an object and its direction of motion. the pressure were to change such that the isobars spread farther Air Pressure and Winds 211 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Figure 8.18 The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force directed from higher toward lower pressure is the pressure-gradient force. Figure 8.19 The closer the spacing of the isobars, the greater the pressure gradient. The greater the pr