CHAPTER 5 (Moran and Morgan, 1997) Humans rarely notice most changes in atmospheric pressure. However, to a meteorologist, air pressure is one of the fundamental weather elements, since even small variations in air pressure over a given distance have been found to drive the atmospheric wind regime. Air pressure represents the force or weight that the atmosphere exerts on any surface of unit area. Air pressure always decreases with height in the atmosphere. Meteorologists use barometers to measure air pressure and its changes over time. The two weather elements of air pressure and air temperature are related, along with air density. If the sea level pressure were considered, changes in air pressure can result from changes in air temperature, water vapor concentration or the wind flow patterns that could produce a region of horizontal convergence or divergence. STUDY NOTES CHAPTER 5 Figure 5.1 -- One method for measuring air pressure involves use of a liquid mercury barometer. Spend a moment considering the mechanism that is utilized by the mercury barometer in making an air pressure measurement. Specifically, this measurement involves a balancing act. The weight of an air column with unit area (hence, pressure) extending to the top of the atmosphere balances the weight of a self-supported column of mercury with a cross-section of unit area inside the tube of the mercury barometer. The length of the red arrow in the tube equals that of the red arrow outside the tube. In reality, the weight of the mercury column inside the tube causes the free surface of the mercury in the lower reservoir to move upward and come into a balance with the atmospheric pressure. An increase of air pressure would push down on the free surface of the mercury in the reservoir, permitting the mercury in the column to move upward by a given amount until a new balance is reached. On the other hand, a reduction in air pressure means that the column of mercury falls until a new balance is attained. Figure 5.2 -- Another method for measuring air pressure uses an aneroid barometer, which ultimately involves a spring balance. Outside air pressure impinges upon the flexible, evacuated canister that has a spring mechanism. As the outside pressure increases, the canister collapses slightly, causing a compression of the spring by a given amount, with the deflection displayed upon the scale by a pointer. A reduction in air pressure causes the spring to expand the canister walls. Look at Table 5.1, which lists conversions between the various pressure units used by meteorologists. Rather than memorizing the exact values, you should remember that sea-level pressure is approximately 1000 millibars (mb), equivalent to approximately 15 pounds per square inch. These units are equivalent to a barometric pressure reading of nearly 76 centimeters (or 30 inches) of mercury. Figure 5.4 -- Consider the vertical variation in air pressure for the lowest 34 km (21 miles) of the atmosphere plotted in a manner that meteorologists traditionally use for plots of vertical profiles. The numerical air pressure values for most of this altitude range appear in Appendix II. This vertical pressure profile is taken from the U.S. Standard Atmosphere, a model used for aircraft design purposes. Note that over the entire atmosphere shown, the pressure varies exponentially since the atmosphere is compressible. In other words, the pressure decreases slowly at first, with a rate of approximately 1 millibar decrease for every 10 meter height rise within the first several kilometers above the earth's surface. At higher altitudes, the pressure decreases more rapidly. We can assume that the pressure decreases by one half for every 5.5 km height rise, from a sea level pressure of approximately 1000 mb to 500 mb at 5.5 km, then by half again to 250 mb at 11 km altitude. By the time we would reach 15 km, the pressure would be approximately 125 mb. Figure 5.5 -- This figure is a sample barograph trace of pressure over time made at Green Bay, WI, with time progressing from left to right. In midlatitudes, the primary cause of pressure variations observed at any station results from migratory low and high pressure systems that pass by the station. The overall range in pressure over this six-day interval was approximately 60 mb (from 965 to 1025 mb). If you are curious as to how these pressure systems look on a surface weather map, you may want to take a quick look at the series of surface weather maps spaced at 24 hour intervals appearing in Figures 11.10, 11.11 and 11.12 (pages 263 through 265). The first weather map shows that on the morning of 1 April 1982, a large high pressure system was over the Upper Midwest approaching Green Bay. At this time, the barograph was experiencing an increase in pressure, reaching a maximum of 1025 mb in the trace near the center of the barograph chart. Once the 1025 mb high moved to the east, a low pressure system moved eastward from the Plains, causing the pressure to fall at Green Bay. The low pressure system, with a central pressure slightly less than 968 mb passed over Green Bay sometime on the afternoon of 2 April, causing the barograph trace to fall to a minimum of 965 mb. As the storm moved away, the pressure at Green Bay once again increased. Figure 5.7 -- Spend a moment looking at this diagram and comparing the motions depicted here with those you learned from the "hand twist model" discussed in Lesson 1. Note that air is being pushed down toward the surface and then diverges or spreads outward over the surface in panel A, which is an oblique view of a high pressure system. As a result of this sinking motion, the force on a unit area of the earth's surface, or equivalently, the air pressure is increased. In panel B, a view of a low pressure system, the air moves inward near the surface or converges and then moves upward away from the surface, forming the surface low. Consequently, the weight exerted per unit area of the earth's surface would be less due to this upward motion of air. Figure 5.8 -- Consider the two samples appearing in this diagram that show how surface pressure features develop and are maintained. Both panels should be viewed as vertical cross-sections, extending from the ground up to the arrows that represent airflow in the upper troposphere, near an altitude of 10 km (30,000 ft.). In panel A, the arrow lengths increase from left to right, suggesting an increase in wind speed in the downwind direction. This speed increase would produce a region of horizontal divergence in the region enclosed by the red dashed circle. As a result, air would move upward from below to displace the lost air, producing a surface low cell (not shown). Conversely, in panel B, the decreasing length of arrows in the downwind direction (from left to right) suggests upper-level horizontal convergence within the circle, meaning that air could sink and produce a surface high pressure cell (not shown). Note that Figure 5.9 is a typical surface weather map with a set of lines called isobars, connecting all points having equal sea-level corrected barometric pressure. These isobars are drawn at the traditional 4 mb interval and permit one to quickly isolate and locate regions of relatively high sea-level barometric pressure over the Great Basin and New England from those containing lower pressure over the Dakotas and Florida. Skim Special Topic (Altimetry) on pages 112-113: Note that this example shows how the known pressure decrease with height can be used to obtain a reasonable estimate of your altitude above sea level, if no other means for altitude determination were available. If you are a pilot or are interested in aviation, consider how the changes in the temperature of the atmospheric column between your aircraft and the ground would affect the pressure altimeter reading. Read Weather Fact (World Extremes in Air Pressure) on page 114. Special Topic (Human responses to changes in air pressure) on pages 116-118 -- Read this topic, noting how the reduction in the partial pressure of oxygen with altitude can cause sickness and even death. Have you ever experienced mountain sickness? Read how one can overcome this lack of oxygen. You should also read how rapid pressure changes could affect your ear. Mathematical Note (The Gas Law) page 122 -- Skim this section, which is designed for those who are mathematically inclined. If you have had a high school chemistry class, you may remember that the ideal gas law was expressed as, PV = nRT. AIR PRESSURE (Moran and Morgan, 1997) Now that we have examined the various controls of air temperature, we turn to another important variable of the atmosphere, air pressure. It is desirable to examine air pressure before considering vapor pressure and adiabatic processes in the next chapter. Air pressure can be thought of as the weight of a column of air with a unit area that extends to the top of the atmosphere. Air pressure (and air density) declines rapidly with increasing altitude and, after reduction to sea level, exhibits important changes from one place to another and with time. Spatial and temporal variations in air pressure arise from changes in air temperature, water vapor concentration, and divergence and convergence of winds. At the Earth's surface, air pressure tends to increase with declining air temperature and decreasing water vapor concentration. Air pressure, temperature, and density are related through the gas law. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: define air pressure. identify the advantages of an aneroid barometer over a mercurial barometer. explain the significance of air pressure tendency for local weather forecasting. describe how air pressure and air density change with altitude. explain how and why meteorologists reduce air pressure readings to sea level. describe how air temperature and water vapor concentration influence the density of air and air pressure at the Earth's surface. show how divergence and convergence of winds can cause changes in air pressure. discuss how surface air pressure varies with different types of air masses. show how the gas law applies to the atmosphere. 5 Air Pressure 108 Defining Air Pressure 109 Air Pressure Measurement 109 Air Pressure Units 111 Variation with Altitude 112 Horizontal Variations 115 Highs and Lows 120 The Gas Law 120 Conclusions 121 Special Topic: Altimetry 112 Weather Fact: World Extremes in Air Pressure 114 Special Topic: Human Responses to Changes in Air Pressure 116 Mathematical Note: The Gas Law 122 Key Terms 122 Summary Statements 122 Review Questions 123 Questions for Critical Thinking 123 Selected Readings 123 3 4