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.
6-1 CHAPTER 6. WEATHER CHARTS
6.1 Introduction. A weather chart is a map on which data and analyses are presented that describe the state of the atmosphere over a large area at a given moment in time.
The possible variety of such charts is enormous, but in meteorological history there has been a more or less standard set of charts, including surface charts and the constant pressure charts of the upper atmosphere. Because weather systems are three-dimensional (3-D), both surface and upper air charts are needed. Surface weather charts depict weather on a constant-altitude (usually sea level) surface, while upper air charts depict weather on constant-pressure surfaces.
The National Weather Service (NWS) produces many weather charts that support the aviation community. Refer to the current edition of AC 00-45, Aviation Weather Services, for details.
6.2 Weather Observation Sources. Weather analysis charts can be based on observations from a variety of data sources, including:
• Land surface (e.g., automated surface observing system (ASOS), Automated Weather Observing System (AWOS), and mesonet);
• Marine surface (e.g., ship, buoy, Coastal-Marine Automated Network (C-MAN), and tide gauge);
• Sounding (e.g., radiosonde, dropsonde, pibal, profiler, and Doppler weather radar Velocity Azimuth Display (VAD) wind profile);
• Aircraft (e.g., Aircraft Reports (AIREP), Pilot Weather Reports (PIREP), Aircraft Meteorological Data Relay (AMDAR), and Aircraft Communications Addressing and Reporting System (ACARS)); and
• Satellite.
Figure 6-1. Weather Observation Sources
Land Surface Marine Surface Sounding Aircraft Satellite
6-2
6.3 Analysis. Analysis is the drawing and interpretation of the patterns of various elements on a weather chart. It is an essential part of the forecast process. If meteorologists do not know what is currently occurring, it is nearly impossible to predict what will happen in the future. Computers have been able to analyze weather charts for many years and are commonly used in the process. However, computers cannot interpret what they analyze.
Thus, many meteorologists still perform a subjective analysis of weather charts when needed.
6.3.1 Analysis Procedure. The analysis procedure is similar to drawing in a dot-to-dot coloring book. Just as one would draw a line from one dot to the next, analyzing weather charts is similar in that lines of equal values, or isopleths, are drawn between dots representing various elements of the atmosphere. An isopleth is a broad term for any line on a weather map connecting points with equal values of a particular atmospheric variable.
Table 6-1. Common Isopleths Isopleth Variable Definition Isobar Pressure A line connecting points of equal or constant pressure.
Contour Line (also called isoheight) Height A line of constant elevation above MSL of a defined surface, typically a constant pressure surface.
Isotherm Temperature A line connecting points of equal or constant temperature.
Isotach Wind Speed A line connecting points of equal wind speed.
Isohume Humidity A line drawn through points of equal humidity.
Isodrosotherm Dewpoint A line connecting points of equal dewpoint.
The weather chart analysis procedure begins with a map of the plotted data which is to be analyzed (see Figure 6-2). It is assumed that bad or obviously incorrect data has been removed before beginning the analysis process. At first, the chart will appear to be a big jumble of numbers. But when the analysis procedure is complete, patterns will appear, and significant weather features will be revealed.
6.3.1.1 Step 1: Determine the Optimal Contour Interval and Values to be Analyzed. The first step in the weather chart analysis procedure is to identify the maxima and minima data values and their ranges to determine the optimal contour interval and values to be analyzed. The best contour interval will contain enough contours to identify significant weather features, but not so many that the chart becomes cluttered. Each weather element has a standard contour interval on NWS weather charts, but these values can be adjusted in other analyses as necessary.
6-3 Figure 6-2. Analysis Procedure Step 1: Determine the Optimal Contour Interval and Values to be Analyzed
Every contour value must be evenly divisible by the contour interval. So, for example, if the contour interval is every 4 units, a 40-unit contour is all right, but a 41-unit contour is not. In the surface pressure analysis example shown in Figure 6-2, we will perform an isobar analysis beginning at a value of 992 millibars and use a contour interval of 4 millibars, which is standard on the NWS Surface Analysis Chart.
6.3.1.2 Step 2: Draw the Isopleths and Extrema. The second step is to draw the isopleths and extrema (maxima and minima) using the beginning contour value and contour interval chosen in the first step. It is usually easiest to begin drawing an isopleth either at the edge of the data domain (edge of the chart), or at a data point that matches the isopleth value being drawn. Interpolation must often be used to draw isopleths between data points and determine the extrema. Interpolate means to estimate a value within an interval between known values.
When drawing isopleths and extrema on a weather chart, certain rules must be followed:
• The analysis must remain within the data domain. Analysis must never be drawn beyond the edge of the chart where there are no data points. That would be guessing.
• Isopleths must not contain waves and kinks between two data points. This would indicate a feature too small to be supported by the data. Isopleths should be smooth and drawn generally parallel to each other.