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
5-12
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
5-14 Height indicated on the altimeter also changes with changes in surface pressure. A movable scale on the altimeter permits the pilot to adjust for surface pressure, but he or she has no means of adjusting the altimeter for mean temperature of the column of air below. Indicated altitude is the altitude above MSL indicated on the altimeter when set at the local altimeter setting.
But what is altimeter setting?
5.4.1.2.1 Altimeter Setting. Since the altitude scale is adjustable, a pilot can set his or her altimeter to read true altitude at some specified height. Takeoff and landing are the most critical phases of flight; therefore, airport elevation is the most desirable altitude for a true reading of the altimeter. The altimeter setting is the value to which the scale of the pressure altimeter is set so the altimeter indicates true altitude at field elevation.
To ensure the altimeter reading is compatible with altimeter readings of other aircraft in the vicinity, a pilot must ensure the altimeter setting is current.
He or she must adjust it frequently while in flight, according to the nearest surface weather reporting station. Figure 5-16 shows the trouble a pilot can encounter if not vigilant in adjusting the altimeter during flight. As he or she flies from high pressure to low pressure, the plane is lower than the altimeter indicates.
Figure 5-16. Pressure Change Effects on Altimeter Readings
5-15 Figure 5-17. Temperature Change Effects on Altimeter Readings
Figure 5-17 shows that as a pilot flies from warm to cold air, the altimeter reads too high—he or she is lower than the altimeter indicates. Over flat terrain, this lower-than-true reading is no great problem; other aircraft in the vicinity also are flying indicated rather than true altitude, and everyone’s altimeter readings are compatible. If flying in cold weather over mountainous areas, however, a pilot must take this difference between indicated and true altitude into account. He or she must know that the true altitude assures clearance of terrain, and compute a correction to indicated altitude.
5.4.1.3 Corrected (Approximately True) Altitude. If a pilot could always determine mean temperature of a column of air between the aircraft and the surface, flight computers would be designed to use this mean temperature in computing true altitude. However, the only guide a pilot has to temperature below him is free air temperature at his altitude. Therefore, the flight computer uses outside air temperature to correct indicated altitude to approximate true altitude. The corrected (approximately true) altitude is indicated altitude corrected for the temperature of the air column below the aircraft, the correction being based on the estimated deviation of the existing temperature from standard atmosphere temperature. It is a close approximation to true altitude and is labeled true altitude on flight computers.
It is close enough to true altitude to be used for terrain clearance, provided the pilot has his altimeter set to the value reported from a nearby reporting station.
5.4.1.4 Pressure Altitude. In the standard atmosphere, sea level pressure is 29.92 inches of mercury (1013.2 millibars). Pressure decreases at a fixed rate upward through the standard atmosphere. Therefore, in the standard atmosphere, a given pressure exists at any specified altitude. Pressure altitude is the altitude (above MSL) shown by the altimeter when set to 29.92 inches
5-16 of mercury. In other words, it is the altitude associated with a specific pressure measured by the static port when the altimeter is set to 29.92. Since pressure is the same everywhere regardless of the specific pressure altitude, a constant pressure surface defines a constant pressure altitude. When a pilot flies a constant pressure altitude, he or she is flying a constant pressure surface.
As discussed earlier, constant pressure surfaces have different heights across them. Therefore, when flying at a specific pressure altitude (i.e., constant pressure surface) a pilot’s true altitude will change with distance. However, since pressure altitudes are flown at or above FL180 (in the United States), a pilot will almost always be above the highest terrain features.
5.5 Density Altitude. Density altitude is the pressure altitude corrected for temperature deviations from the standard atmosphere. Density altitude bears the same relation to pressure altitude as true altitude does to indicated altitude.
Density altitude is indirectly related to atmospheric density; as air density increases (decreases), the density altitude decreases (increases). Airports with higher field elevations (e.g., Denver) have lower pressure, lower density, and, therefore, higher density altitudes than airports with lower field elevations (e.g., New Orleans).
Density altitude equals field elevation during standard atmospheric conditions, but conditions are rarely standard. Density altitude is higher (lower) than standard at airports that report lower (higher) than standard pressures (29.92 inches of mercury) and/or higher (lower) than standard temperatures. Temperature is the most important factor since temperature has the greatest effect on density horizontally in the atmosphere. On hot days, the air becomes less dense, causing high density altitudes. On cold days the air is denser, causing lower density altitudes. Dewpoint (water vapor) is also a contributing factor, but its effects are generally negligible.
Density altitude is an index to aircraft performance. Higher (lower) density altitude decreases (increases) performance. High density altitude is a hazard since it reduces aircraft performance in the following three ways:
1. It reduces power because the engine takes in less air to support combustion.
2. It reduces thrust because there is less air for the propeller to work with, or a jet has less mass of gases to force out of the exhaust.
3. It reduces lift because the light air exerts less force on the airfoils.
A pilot cannot detect the effect of high density altitude on his airspeed indicator.
The aircraft lifts off, climbs, cruises, glides, and lands at the prescribed indicated airspeeds; but at a specified indicated airspeed, the pilot’s true airspeed and groundspeed increase proportionally as density altitude becomes higher.
The net results are that high density altitude lengthens a pilot’s takeoff, and landing rolls and reduces his or her rate of climb. Before lift-off, the plane must attain a faster groundspeed, and, therefore, needs more runway; and the reduced power and thrust add
5-17 a need for still more runway. The plane lands at a faster groundspeed and, therefore, needs more room to stop. At a prescribed indicated airspeed, it is flying at a faster true airspeed, and, therefore, covers more distance in a given time, which means climbing at a shallower angle. Adding to this are the problems of reduced power and rate of climb.
Figure 5-18 shows the effect of density altitude on takeoff distance and rate of climb.
Figure 5-18. High Density Altitude Effects on Flight
High density altitude also can be a problem at cruising altitude. When air temperature is higher (warmer) than standard atmosphere, the higher density altitude lowers the service ceiling. For example, if temperature at a pressure altitude of 10,000 feet is 20 °C, density altitude is 12,700 feet. A pilot’s aircraft will perform as though it were at 12,700 feet indicated with a normal temperature of -8 °C.
To compute density altitude, a pilot can set the altimeter to 29.92 inches (1013.2 millibars), read the pressure altitude from the altimeter, obtain the outside air temperature, and then use a flight computer to compute density altitude.