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
7-1 CHAPTER 7. WIND
7.1 Introduction. Wind is the air in motion relative to the surface of the Earth. Although we cannot actually see the air moving, we can measure its motion by the force that it applies on objects. For example, leaves rustling or trees swaying on a windy day indicate that the wind is blowing. Winds are a major factor to both weather and aircraft.
Winds cause the formation, dissipation, and redistribution of weather. Winds also affect aircraft during all phases of flight. This chapter discusses how winds are named and the origin of wind.
7.2 Naming of the Wind. Wind is named according to the direction from which it is blowing. For example, a west wind indicates the wind is blowing from the west to the east. In aviation, 36 points of the compass are normally used to represent the direction from which the wind is blowing. For example, north winds come from 360°, east from 90°, south from 180°, and west from 270°.
7.3 Forces That Affect the Wind. Three primary forces affect the flow of wind: Pressure Gradient Force (PGF), Coriolis force, and friction.
7.3.1 Pressure Gradient Force (PGF). Wind is driven by pressure differences which create a force called the Pressure Gradient Force (PGF). Whenever a pressure difference develops over an area, the PGF makes the wind blow in an attempt to equalize pressure differences. This force is identified by height contour gradients on constant pressure charts and by isobar gradients on surface charts.
PGF is directed from higher height/pressure to lower height/pressure and is perpendicular to contours/isobars. Whenever a pressure difference develops over an area, the PGF begins moving the air directly across the contours/isobars.
Figure 7-1. Direction of Pressure Gradient Force
PGF is directed across contours/isobars towards lower height/pressure.
7-2 Wind speed is directly proportional to the PGF, which itself is directly proportional to the contour/isobar gradient. Closely spaced contours/isobars indicate strong winds, while widely spaced contours/isobars mean lighter wind. From a pressure analysis, you can get a general idea of wind speed from contour/isobar spacing.
Figure 7-2. Magnitude of Pressure Gradient Force
On the left panel, the contours/isobars are widely spaced apart, PGF is weak, and the wind speed is weak. On the right panel, the contours/isobars are more closely spaced, the PGF is stronger, and the wind speed is stronger.
The wind would flow from high to low pressure if the PGF was the only force acting on it. However, because of the Earth’s rotation, there is a second force called the Coriolis force that affects the direction of wind flow.
7.3.2 Coriolis Force. A moving mass travels in a straight line until acted on by some outside force. However, if one views the moving mass from a rotating platform, the path of the moving mass relative to his platform appears to be deflected or curved. To illustrate, consider a turntable. If one used a pencil and a ruler to draw a straight line from the center to the outer edge of the turntable, the pencil will have traveled in a straight line.
However, stopping the turntable, it is evident that the line spirals outward from the center (see Figure 7-3). To a viewer on the turntable, some apparent force deflected the pencil to the right.
Figure 7-3. Illustration of Coriolis Force
7-3 A similar apparent force deflects moving particles on the Earth. Because the Earth is spherical, the deflective force is much more complex than the simple turntable example. Although the force is termed apparent to us on Earth, it is very real.
The principle was first explained by the Frenchman Gaspard-Gustave de Coriolis, and now carries his name—the Coriolis force.
Coriolis force affects all moving objects. The force deflects air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Coriolis force is at a right angle to wind direction and directly proportional to wind speed; that is, as wind speed increases, Coriolis force increases. At a given latitude, double the wind speed and you double the Coriolis force. Why at a given latitude?
Coriolis force varies with latitude from zero at the Equator to a maximum at the poles.
It influences wind direction everywhere except immediately at the Equator, but the effects are more pronounced in middle and high latitudes.
Figure 7-4. Coriolis Force Variations Across the Earth
Coriolis force deflects moving objects to the right of their path in the Northern Hemisphere and to the left of their path in the Southern Hemisphere. Coriolis deflection is maximized at the poles and zero at the Equator.
7-4 Figure 7-5. Coriolis Force Magnitude Variations with Wind Speed
Coriolis force magnitude is directly proportional to wind speed. Wind speed is twice as strong in the right panel;
thus, the Coriolis force is doubled.
7.3.3 Friction Force. Friction between the wind and the terrain surface slows the wind.
The rougher the terrain, the greater the frictional effect. Also, the stronger the wind speed, the greater the friction. One may not think of friction as a force, but it is a very real and effective force always acting opposite to wind direction.
Figure 7-6. Friction Force Magnitude Variations with Terrain Roughness
Friction force magnitude is directly proportional to terrain roughness. Even though the wind speed is the same in both panels, the terrain is rougher in the right panel; thus, the friction force is stronger.
Figure 7-7. Friction Force Magnitude Variations with Wind Speed
Friction force magnitude is directly proportional to wind speed. Wind speed is twice as strong in the right panel;
thus, the friction force is doubled.
The frictional drag of the ground normally decreases with height and becomes insignificant above the lowest few thousand feet. However, this may vary somewhat since both strong winds and rough terrain extend the friction layer to higher altitudes.