Ch 3 – Pressure, Altitude & Density

Ch 3 – Pressure, Altitude & Density Introduction Pressure and its variations have important applications for aviation, ranging from measurements of altitude and airspeed to the prediction of winds and weather. This chapter focuses on several of these applications. When you complete the chapter, you will have a good physical understanding of atmospheric pressure, altimetry, and density altitude (Lester, 2006).

Ch 3 – Pressure, Altitude & Density Introduction You will develop important background knowledge about the global patterns of atmospheric pressure. This information will prove useful in the next chapter when we examine the causes and characteristics of atmospheric winds (Lester, 2006).

Ch 3 – Pressure, Altitude & Density Section A – Atmospheric Pressure Pressure Measurements Section B – Charting Atmospheric Pressure Station and Sea Level Pressure Sea Level Pressure Patterns Constant Pressure Charts Section C – The Pressure Altimeter METARs and Altimeter Settings Section D - Density

Ch 3 – Pressure, Altitude & Density Section A: Atmospheric Pressure – may also be defined as the weight of a column of the atmosphere with a given cross-sectional area; figure 3-1 Atmospheric Pressure = Weight (Force) / Area = 14.7 lbs/1 in sq = 14.7 lbs/in sq Hydrostatic balance – figure 3-2; the balance between the downward-directed gravitational force and an upward-directed force caused by the decrease of atmospheric pressure with altitude is called hydrostatic balance

Ch 3 – Pressure, Altitude & Density Pressure Measurements Mercurial barometer – one of the most basic devices for the measurement of pressure Aneroid barometer – another pressure instrument, known as the aneroid barometer is more frequently used outside the laboratory has no liquid operates on differences in air pressure between the atmosphere and a closed vessel (an aneroid cell) aneroid means “not wet” Figure 3-4

Ch 3 – Pressure, Altitude & Density Section B: Charting Atmospheric Pressure Station and Sea Level Pressure – surface pressure measurements are most useful if they can be compared with nearby measurements at the same altitude over land areas, the direct comparison of station pressures are usually difficult because weather stations are often at different altitudes; Figure 3-5 the atmospheric pressure measured or estimated at an elevation equal to mean sea level

Ch 3 – Pressure, Altitude & Density this extrapolated sea level pressure (station pressure corrected for elevation) is used by pilots to determine altitude also used in aviation reports to depict the atmospheric pressure of a reporting location

Ch 3 – Pressure, Altitude & Density Sea Level Pressure Patterns Surface analysis charts – a chart which shows pressure as well as other meteorological conditions at the surface of the earth Isobars – lines of constant pressure; figure 3-6 High (H) – location where the sea level pressure is high compared to its surroundings Ridge – an elongated region of relatively high pressure

Ch 3 – Pressure, Altitude & Density Low (L) – a roughly circular area with a lower sea level pressure in the center as compared to the surrounding region Trough – an elongated region of relatively low pressure; these features are to a surface analysis chart what mountains and valleys are to a topographical chart

Ch 3 – Pressure, Altitude & Density Pressure gradient – a difference in pressure over a given distance Semi-permanent pressure systems – Bermuda High, Aleutian Low, Siberian High, Icelandic Low, Pacific High

Ch 3 – Pressure, Altitude & Density ***On a surface analysis chart, the solid lines that depict sea-level pressure patterns are called isobars

Ch 3 – Pressure, Altitude & Density ***On a surface analysis chart, close spacing of the isobars indicates strong pressure gradient

Ch 3 – Pressure, Altitude & Density Constant Pressure Charts – a constant pressure surface is one where the pressure is the same at all points like the ocean’s surface, a constant pressure surface is not necessarily level many upper air weather charts that you may use are called constant pressure analysis charts or simply constant pressure charts; figure 3-7 the interpretation of a constant pressure chart is identical with the sea level pressure chart as far as highs, lows, troughs, ridges and gradients are concerned

Ch 3 – Pressure, Altitude & Density the main difference is one of terms used to describe the elements on constant pressure surfaces, lines of constant height are called contours rather than isobars gradients are height gradients rather than pressure gradients; figure 3-8

Ch 3 – Pressure, Altitude & Density Pressure Altitude – the altitude of a given pressure surface in the standard atmosphere; figure 3-9 pilots can determine pressure altitude by setting the standard sea level pressure, 29.92 inches Hg in the aircraft altimeter

Ch 3 – Pressure, Altitude & Density ***Pressure altitude is the altitude indicated when the altimeter setting is 29.92. If you set your altimeter at 29.92 and fly at 18,289 feet indicated altitude, you will be flying along the 500 mb constant pressure surface

Ch 3 – Pressure, Altitude & Density Section C: The Pressure Altimeter – the altimeter is essentially an aneroid barometer that reads in units of altitude rather than pressure this is possible by using the standard atmosphere to make the conversion from pressure to altitude; Appendix B; Figure 3-10 Indicated altitude – the altitude measured by your altimeter True altitude – the actual altitude of your aircraft above mean sea level Absolute altitude – the altitude of your aircraft above the ground

Ch 3 – Pressure, Altitude & Density Altimeter errors – there are three specific altimeter errors caused by nonstandard atmospheric conditions 1. sea level pressure different from 29.92 inches of mercury 2. temperature warmer or colder than standard temperature atmospheric pressure decreases with altitude more rapidly in cold air than in warm air 3. strong vertical gusts

Ch 3 – Pressure, Altitude & Density ***Altimeter setting is the value to which the barometric pressure scale on the altimeter is set so the altitude indicates true altitude at field elevation

Ch 3 – Pressure, Altitude & Density ***On warm days pressure surfaces are raised and the indicated altitude is lower than true altitude

Ch 3 – Pressure, Altitude & Density Altimeter setting – figure 3-10; variable sea level pressure is usually taken care of by adjusting the altimeter to the proper altimeter setting this is the sea level pressure determined from the station pressure and the standard atmosphere

Ch 3 – Pressure, Altitude & Density METARs and Altimeter Settings Aviation Routine Weather Report (METAR) – altimeter settings for airports worldwide are reported and transmitted regularly with other weather information in a standard coded format these reports are commonly available to pilots; figure 3-13

Ch 3 – Pressure, Altitude & Density Type of Report Aviation selected special weather report (SPECI) – the special METAR weather observation is an unscheduled report indicating a significant change in one or more elements Station Designator and Date/Time of Report – each reporting station is listed by its four-letter International Civil Aviation Organization (ICAO) identifier in the contiguous 48 states, the letter “K” prefixes the three-letter domestic location identifier

Ch 3 – Pressure, Altitude & Density Modifier – when a METAR is created by a totally automated weather observation station, the modifier AUTO will follow the date/time element RMK A02 indicates the weather observing equipment used has the capability of distinguishing precipitation type the modifier COR is used to indicate a corrected METAR which replaces a previously disseminated report no modifier indicates a manual station or manual input at an automated station

Ch 3 – Pressure, Altitude & Density Temperature and Dew-point – the current air temperature and dew-point are reported in two-digit form in degrees Celsius and are separated by a slash Altimeter – reported in inches of mercury in a four digit group without the decimal point and is prefixed by an “A”

Ch 3 – Pressure, Altitude & Density Remarks – the remarks section begins with “RMK” the remark “SLP134” refers to the sea level pressure of 1013.4 hecto Pascals (hPa) the leading 9 or 10 is omitted prefix the number with a 9 or 10 whichever brings it closer to 1,000.0 a remark such as “T00081016” refers to the temperature and dew point in tenths degrees Celsius in this example, the first zero in the sequence indicates a plus value for temperature (+.8C) and the leading “one” in the sequence shows a minus value for dew point (-1.6 C).

Ch 3 – Pressure, Altitude & Density Section D: Density – aircraft performance depends critically on atmospheric density

Ch 3 – Pressure, Altitude & Density ***Pressure altitude and density altitude have the same value at standard temperature in the standard atmosphere

Ch 3 – Pressure, Altitude & Density Density Altitude – the altitude above mean sea level at which a given atmospheric density occurs in the standard atmosphere can be interpreted as pressure altitude corrected for non-standard temperature differences in warmer than standard surface conditions, you would say that the density altitude is “high” that is, operation of your aircraft in a high density altitude condition is equivalent to taking off from a higher airport during standard conditions in a high density altitude situation, the actual density at the surface is found above the airport elevation in the standard atmosphere; figure 3-14

Summary Atmospheric pressure is an essential component of aviation weather basics. An understanding of pressure is the foundation for understanding such diverse and important topics as altimetry, winds, and storms. In this chapter, you have learned about the useful relationship between atmospheric pressure and the weight of the atmosphere and how that relationship allows us to measure pressure and altitude (Lester, 2006).

Summary Details about the distribution of average sea level pressure around the globe, as well as the terminology and methods for the interpretation of atmospheric pressure charts at the surface and aloft should now be part of your growing knowledge of aviation weather (Lester, 2006).

Summary You have gained valuable insight into the effects of atmospheric variations in pressure and temperature on the accuracy of pressure altimeter measurements. You have been introduced to standard weather reports available to pilots. In particular, you have learned where to find locations of the reporting stations, times, pressures, temperatures, and altimeter settings in those reports.

Summary Finally, you have become familiar with the concept of density altitude and its impact on aircraft performance (Lester, 2006).