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
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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.
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• When an isopleth is complete, all data values must be higher than the isopleth’s value on one side of the line and lower on the other.
• A closed loop isopleth must contain an embedded extremum (maximum or minimum).
• When a maximum (minimum) is identified, data values must decrease (increase) in all directions away from it.
• Isopleths can never overlap, intersect, or cross over extrema.
It is impossible for one location to have more than one data value simultaneously.
• Each isopleth must be labeled. A label must be drawn wherever an isopleth exits the data domain. For closed loop isopleths, a break in the loop must be created where a label can be drawn. For very long and/or complex isopleths, breaks should be created where additional labels can be drawn, as necessary.
• Extrema must be labeled. Extrema are often denoted by an “x” embedded within a circle. Beneath the label, the analyzed value of the field must be written and underlined.
• Isopleths and labels should not be drawn over the data point values.
If necessary, breaks in the isopleths should be created so that the data point values can still be read.
Figure 6-3. Analysis Procedure Step 2: Draw the Isopleths and Extrema
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6.3.1.3 Step 3: Identify Significant Weather Features. The third (and final) step is to interpret significant weather features. The conventional labels for extrema are H (high) and L (low) for pressure and height, W (warm) and K (cold) for temperature (they stand for the German words for warm and cold), and X (maxima) and N (minima) for all other elements. Troughs, ridges, and other significant features are often identified as well. Table 6-2 below provides a list of the most common weather chart symbols. Refer to AC 00-45 for details on specific NWS weather charts.
Table 6-2. Common Weather Chart Symbols Feature 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 anti-cyclone.
Trough
An elongated area of relatively low atmospheric pressure or height.
Ridge
An elongated area of relatively high atmospheric pressure or height. May also be used as reference to other meteorological quantities, such as temperature and dewpoint.
In the surface pressure analysis below in Figure 6-4, a high, low, trough, and ridge have been identified.
Figure 6-4. Analysis Procedure Step 3: Interpret Significant Weather Features 1009 1008 1007 1004 1000 996 996 1009 1003 9971 994 993 997 10121012 1008 992 1010 101516013 1005 999 993 995 997 1018 998 993 989 1017 1009 99 1010 (101-1013 1001 999 998 1005 998 998 1008 1010 1015 1011 1006 1002 1000 1000 1000 1000 101 1009 999 • 998 1005/ 1002 1000 1003 1008 1009 1010 1008 1004 10011 1000 1001 1002
6.4 Surface Chart. A surface chart (also called surface map or sea level pressure chart) is an analyzed chart of surface weather observations. Essentially, a surface chart shows the distribution of sea level pressure, including the positions of highs, lows, ridges, and troughs, and the location and character of fronts and various boundaries, such as drylines, outflow boundaries, and sea breeze fronts. Although the pressure is referred to as MSL, all other elements on this chart are presented as they occur at the surface point of observation. A chart in this general form is the one commonly referred to as the weather map.
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6-7 Figure 6-5. Example of a Surface Chart
6.5 Constant Pressure Chart. A constant pressure chart (also called an isobaric chart) is a weather map representing conditions on a surface of equal atmospheric pressure.
For example, a 500 millibar chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 millibars. Constant pressure charts usually contain plotted data and analyses of the distribution of height of the surface (contours), wind (isotachs), temperature (isotherms), and sometimes humidity (isohumes).
The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 millibar level may be at a different height above sea level over Dallas than over New York at a given time, and may also be at a different height over Dallas from one day to the next).
Constant pressure charts are most commonly known by their pressure value.
For example, the 1,000 millibar chart (which closely corresponds to the surface chart), the 850 millibar chart, 700 millibar chart, 500 millibar chart, etc.
6-8 A contour analysis (see Figure 6-6) can reveal highs, ridges, lows, and troughs aloft just as the surface chart shows such systems at the surface. These systems of highs/ridges and lows/troughs are called pressure waves. These pressure waves are similar to waves seen on bodies of water. They have crests (ridges) and valleys (troughs).
Figure 6-6. Example of a 500 Millibar Constant Pressure Chart