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5.2.1 Barometer. The instrument Torricelli designed to measure pressure was called a barometer. The aneroid barometer is the type mostly commonly used by meteorologists and the aviation community.
Essential features of an aneroid barometer (see Figure 5-2) are a flexible metal cell and the registering mechanism. Air is taken out of the cell to create a partial vacuum. The cell contracts or expands as pressure changes. One end of the cell is fixed, while the other end moves the registering mechanism. The coupling mechanism magnifies the movement of the cell driving an indicator hand along a scale graduated in pressure unit.
Figure 5-2. Aneroid Barometer
5.2.2 Atmospheric Pressure Units. Atmospheric pressure is expressed in many ways throughout the world (see Table 5-1). Meteorologists worldwide have long measured atmospheric pressure in millibars (mb or mbar), which denote pressure as a force per square centimeter. However, after the introduction of the International System of Units (SI) in 1960, the hectopascal (hPa) was adopted by most countries and is used in the Aviation Routine Weather Report (METAR)/Aviation Selected Special Weather Report (SPECI) code first developed in 1968. Many meteorologists prefer to use the term they learned during their education and work experience. Therefore, some continue to use the term millibars, while others use hectopascals (which are equivalent). The unit inch of mercury (inHg or Hg) is still used in the United States for altimetry.
5-3 Table 5-1. Units of Pressure Units of Pressure Standard Atmosphere Value at Sea Level Common Use Hectopascals (hPa) 1013.2 hPa METAR/SPECI Millibars (mb or mbar) 1013.2 mb U.S. Weather Charts Inches of mercury (inHg or Hg) 29.92 inHg U.S. Aviation Pounds per square inch (psi) 14.7 psi U.S. Engineering
5.2.3 Station Pressure. The pressure measured at an airport is called station pressure, or the actual pressure at field elevation. Pressure is lower at higher altitudes. Therefore, airports with higher field elevations usually have lower pressure than airports with lower field elevations. For instance, station pressure at Denver is less than at New Orleans (see Figure 5-3).
Figure 5-3. Station Pressure
The next few subparagraphs will examine some factors that influence pressure.
5.2.4 Pressure Variation. Atmospheric pressure varies with altitude and the temperature of the air, as well as with other minor influences, such as water vapor.
5-4
5.2.4.1 Pressure Changes with Altitude. As we move upward through the atmosphere, the weight of the air above us decreases. If we carry a barometer with us, we can measure a decrease in pressure as the weight of the air above us decreases. Figure 5-4 shows the pressure decrease with height in the standard atmosphere.
Figure 5-4. Air Pressure in the Standard Atmosphere
These standard altitudes are based on standard temperatures. In the real atmosphere, temperatures are seldom standard, so we will explore temperature effects on pressure in the following subparagraphs.
5.2.4.2 Temperature Effects on Pressure. Like most substances, air expands as it becomes warmer and contracts as it cools. Figure 5-5 shows three columns of air: one colder than standard, one with standard temperature, and one warmer than standard. Pressure is equal at the bottom and top of each column. Vertical expansion of the warm column has made it taller than the
5-5 column at standard temperature. Contraction of the cold column has made it shorter than the standard column. Since the total pressure decrease is the same in each column, the rate of decrease of pressure with height in warm air is less than standard, while the rate of decrease in pressure with height in cold air is greater than standard.
Figure 5-5. Temperature Effect on Pressure
5.2.5 Sea Level Pressure. Since pressure varies greatly with altitude, we cannot readily compare station pressures between stations at different altitudes. To make them comparable, we adjust them to some common level. Mean sea level (MSL) is the most useful common reference. In Figure 5-6 pressure measured at a station at a 5,000-foot elevation is 25 inches; pressure increases about 1 inch of mercury for each 1,000 feet, or a total of 5 inches. Sea level pressure is approximately 25 + 5, or 30 inches of mercury.
Figure 5-6. Reduction of Station Pressure to Sea Level
Sea level pressure is typically displayed on surface weather charts. Pressure continually changes across the Earth, so a sequence of surface charts must be viewed to follow these changing pressures.
5-6
5.2.5.1 Sea Level Pressure Analyses (Surface Chart). After plotting sea level pressure on a surface chart, lines are drawn connecting points of equal sea level pressure. These lines of equal pressure are isobars. Hence, the surface chart is an isobaric analysis showing identifiable, organized pressure patterns. Four pressure systems are commonly identified: low, high, trough and ridge (see Table 5-2 and Figure 5-7).
Table 5-2. Pressure System Symbols Pressure System Symbol Definition Low
A minimum of atmospheric pressure in two dimensions (closed isobars) on a surface chart, or a minimum of height (closed contours) on a constant pressure chart. Also known as a cyclone.
High
A maximum of atmospheric pressure in two dimensions (closed isobars) on a surface chart, or a maximum of height (closed contours) on a constant pressure chart. Also known as an anticyclone.
Trough
An elongated area of relatively low atmospheric pressure.
Ridge
An elongated area of relatively high atmospheric pressure.
5-7 Figure 5-7. Surface Chart Pressure Patterns
5.2.6 Constant Pressure Surface. A constant pressure surface is a surface along which the atmospheric pressure is everywhere equal at a given instant. For instance, the 500 millibar constant pressure surface has a pressure of 500 millibars everywhere on it. The height (altitude) of a constant pressure surface varies primarily due to temperature; these heights can be measured by a rawinsonde.
5.2.6.1 Rawinsonde Observations. The National Weather Service (NWS) takes routine scheduled upper air observations, usually referred to as soundings.
A balloon carries a rawinsonde instrument (see Figure 5-8), which consists of radio gear and sensing elements. While in flight, the rawinsonde transmits pressure, temperature, and relative humidity data. Wind speed and direction aloft are obtained by tracking the position of the radiosonde in flight using Global Positioning Satellites (GPS). Most stations around the world take rawinsonde observations. However, meteorologists and other data users frequently refer to a rawinsonde observation as a radiosonde observation.
5-8 Figure 5-8. Weather Balloon and Radiosonde
5.2.6.2 Constant Pressure Surface Analysis (Upper Air Chart). These heights measured by the rawinsonde (and other types of instruments) are plotted on a constant pressure chart and analyzed by drawing a line connecting points of equal height. These lines are called height contours. What is a height contour?
First, consider a topographic map with contours showing variations in elevation. These are height contours of the terrain surface. The Earth’s surface is a fixed reference, and variations in its height are contoured.
The same concept applies to height contours on an upper air chart, except the reference is a constant pressure surface. Varying heights of the pressure surface are contoured. For example, a 700 millibar constant pressure analysis is a contour map of the heights of the 700 millibar pressure surface. While the contour map is based on variations in height, these variations are small when compared to flight levels, and for all practical purposes, one may regard the 700 millibar chart as a weather chart at approximately 3,000 meters (10,000 feet) above MSL.
5-9 A contour analysis (see Figure 5-9) shows highs, ridges, lows, and troughs aloft just as the isobaric analysis shows such systems at the surface. These systems of highs/ridges and lows/troughs are called pressure waves. These pressure waves are very similar to waves seen on bodies of water. They have crests (ridges) and valleys (troughs) and are in constant movement.
Figure 5-9. 500 Millibar Constant Pressure Chart
Table 5-3. Common Constant Pressure Charts Chart Pressure Altitude (approximate) Feet (ft) Meters (m) 100 mb 53,000 ft 16,000 m 150 mb 45,000 ft 13,500 m 200 mb 39,000 ft 12,000 m 250 mb 34,000 ft 10,500 m 300 mb 30,000 ft 9,000 m 500 mb 18,000 ft 5,500 m 700 mb 10,000 ft 3,000 m 850 mb 5,000 ft 1,500 m 925 mb 2,500 ft 750 m
5.3 Density. Density is the ratio of any quantity to the volume or area it occupies.
Atmospheric density is defined as ratio of the mass (or weight) of the air to the volume occupied by it, usually expressed in kilograms per cubic meter (see Figure 5-10).
Figure 5-10. Density is Mass (Weight) per Volume DENSITY = MASS (WEIGHT) VOLUME Mass = 1 kg Volume = 1 m3 Density =1 kg = 1.0 kg/m3 1 m
5.3.1 Volume Effects on Density. The density of an air parcel varies inversely with its volume.
Assuming equal mass, an air parcel with a higher density has a smaller volume than an air parcel with a lower density (see Figure 5-11).
Figure 5-11. Volume Effects on Density Mass = 3 kg Mass = 3 kg Volume = 1 m Volume = 2 m Density = 3 kg = 3.0 kg/ma Density = 3 kg = 1.5 kg/m?
1 ma 2 ma Shorter Parcel, Taller Parcel, Higher Density Lower Density 5-10
5-11 The shorter parcel (that is, the parcel with the smaller volume) has a higher density than the taller parcel which contains the larger volume. This is due to the fact that the air molecules within the shorter parcel must be compressed within the smaller volume.
5.3.2 Changes in Density. In general, the density of an air parcel can be changed by changing its mass, pressure, or temperature. Boyle’s law says that the density of an ideal gas (ρ, the Greek letter rho) is given by:
Where M is the molar mass, P is the pressure, R is the universal gas constant, and T is the absolute temperature.
5.3.3 Density Effects on Pressure. Density is directly related to pressure. Assuming constant mass and temperature, an air parcel with a higher pressure is denser than an air parcel with a lower pressure.
As we have already seen, air pressure decreases with height in the atmosphere. Therefore, the density also decreases with height (see Figure 5-12). In the atmosphere, pressure has the greatest effect on density in the vertical direction.
Figure 5-12. Pressure Effects on Density in the Atmosphere
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5.3.4 Temperature Effects on Density. Density is inversely related to temperature. Assuming constant mass and pressure, an air parcel with a higher temperature is less dense than an air parcel with a lower temperature (see Figure 5-13). This is because the warmer air occupies a large volume.
Figure 5-13. Temperature Effects on Density
In the atmosphere, temperature has the most effect on density in the horizontal direction;
that is, with horizontal changes of location (e.g., New York City versus Miami), temperature has the greatest effect on density.
5.3.5 Water Vapor Effects on Density. Density of an air parcel is inversely related to its quantity of water vapor. Assuming constant pressure, temperature, and volume, air with a greater amount of water vapor is less dense than air with a lesser amount of water vapor. This is because dry air molecules have a larger mass (weight) than water vapor molecules, and density is directly related to mass (see Figure 5-14).
Figure 5-14. Water Vapor Effects on Density
5-13
5.4 Altimetry. The altimeter is essentially an aneroid barometer. The difference is the scale.
The altimeter is graduated to read increments of altitude rather than units of pressure.
The standard for graduating the altimeter is the standard atmosphere.
5.4.1 Altitude. Altitude seems like a simple term: it means the vertical elevation of an object above the surface of the Earth. But in aviation, it can have many meanings.
5.4.1.1 True Altitude. Since existing conditions in a real atmosphere are seldom standard, altitude indications on the altimeter are seldom actual or true altitudes. True altitude is the actual vertical distance above MSL.
If an altimeter does not indicate true altitude, what does it indicate?
5.4.1.2 Indicated Altitude. Figure 5-13 shows the effect of mean temperature on the thickness of three columns of air. Pressures are equal at the bottoms and tops of the three layers. Since an altimeter is essentially an aneroid barometer, altitude indicated by the altimeter at the top of each column would be the same. To see this effect more clearly, see Figure 5-15. In the warm air column, a pilot would fly at an altitude that is higher than the indicated altitude. In the cold air column, the pilot would fly at an altitude lower than the indicated altitude.
Figure 5-15. True Versus Indicated Altitude