Frequently-asked-questions (FAQs) and appropriate answers are found below. They are organized according to chapter of Aviation Weather, 3rd Edition, unless otherwise stated. In cases of overlapping material, some questions and answers may appear in two or more FAQ locations.

Part II Atmospheric Circulation Systems
Chapter 7 Scales of Atmospheric Circulations
Chapter 8 Airmasses, Fronts, and Cyclones
Chapter 9 Thunderstorms
Chapter 10 Local Winds

Part III Aviation Weather Hazards
Chapter 11 Wind Shear
Chapter 12 Turbulence
Chapter 13 Icing
Chapter 14 Instrument Meteorological Conditions (IMC)
Chapter 15 Additional Weather Hazards

Part IV Applying Weather Knowledge
Chapter 16 Aviation Weather Resources
Chapter 17 Weather Evaluation for Flight

Appendixes
Appendix A: Conversion Factors
Appendix B: Standard Atmosphere
Appendix C: Dewpoint and Humidity Tables
Appendix D: Standard Meteorological Codes and Graphics for Aviation
Appendix E: Glossary of Weather Terms
Appendix F: Internet Resources and Printed References
Appendix G: Review Question Answers

1. Q. Why do I have to learn this preliminary stuff? Why not go directly to the "important" weather topics for flight?

A. The importance of the "Basics" is that it lays the foundation for understanding important meteorological processes. Without the Basics, understanding is replaced by rote memorization of a few facts, restricting your ability to deal with real weather situations. The goal of safe flight is replaced by the goal of simply passing exams. Remember, the weather will get you unless you get it first. Take your time to learn it the right way.

2. Q. OK, I understand the need of the "Basics" as a foundation, but some of this stuff is pretty dry. How do I move through it in the shortest time for the greatest benefit?

   A. The answer has two parts.

   1) Be organized in your reading and studying. You can do this by following the "How The System Works" outline in the front of the text (page vi in the 3rd Edition). Each chapter is laid out in the same way so, after a couple of chapters, you should have the study process down pat.

   2) Connect with aviation. If you can relate the study material to your primary interest (aviation), then you can make concrete mental connections that help you both to retain and to understand information. In other words, apply your knowledge at every opportunity.

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1. Q. In Figure 1-7, p 1-8, do the edges of the colored layers have anything to do with the layers discussed in the text?

A. Boundaries of the colored layers correspond roughly with the troposphere and stratosphere (see Figure 1-8, page 1-9). The reddish (lowest) layer, in which cloud silhouettes are visible, is the troposphere. The yellow layer includes at least the lower stratosphere. The colors of both the red and yellow layers are caused by the scattering of solar radiation by dust and other relatively large particles. The blue-violet-black layer above indicates the gradually decreasing presence of oxygen molecules with altitude. Oxygen molecules are particularly effective in the scattering of blue light.

2. Q. In figures 1-8 and 1-10, the temperature in the troposphere decreases with altitude. This does not agree with some of my experience. Especially during some morning takeoffs, I have often noticed that the air temperature increases with altitude. What am I missing here?

   A. Each diagram that you mention in the text is a plot of the International Standard Atmosphere (ISA). ISA is a long-term AVERAGE of temperature soundings at many locations around the globe at different times of day and night. Whereas ISA is always the same, individual temperature soundings (your experience for morning flights) are a measure of actual conditions at a particular place and time; so they may DEVIATE significantly from ISA. The accuracy of readings from aircraft instrumentation designed on the basis of ISA (e.g., pressure altimeter) is affected by non-standard temperature conditions. More details on this topic are discussed in chapter 3.

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1. Q. Also concerning Figure 2-7: Are soundings still made by balloon? It seems pretty archaic in these days of satellites and computers.

   A. Yes, twice-a-day (0000Z and 1200Z) soundings are made from hundreds of land and ship stations around the world. These observations are supplemented daily by thousands of aircraft reports, satellite soundings, and a few networks of ground-based sounders that use remote-sensing techniques including radar, sound, and LIDAR.

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Chapter 3 Pressure, Altitude, and Density

1. Q. I still get confused about the difference between Altimeter Setting (A) and Sea Level Pressure (SLP). The only differences I see in coded METAR reports is that SLP is given in millibars (mb = hPa, hectopascals) while A is reported in inches. When I convert the units, they are both pretty close. Why not just report altimeter setting?

A. While SLP and A are both atmospheric pressure indicators, they are used for different purposes and are not necessarily equal. SLP is an important measurement for meteorologists in their efforts to diagnose and predict atmospheric conditions. It is determined by a standard procedure using observed values of pressure (station pressure) and temperature, among other things. In contrast, A is determined for aviation uses via a simpler (less accurate) procedure using observed pressure and the standard atmosphere for temperature. The simplicity of the procedure allows it to be integrated into the mechanism of your altimeter so it reads out in altitude rather than pressure. This difference between SLP and A can be significant. For example, a difference of only 0.10" (~3mb) is about 100 foot error on your altimeter at sea level. So "pretty close" just doesn't cut it. However, if all pilots use the pressure altimeter properly (proper setting and attention paid to possible altimeter errors), then safe vertical separation from the ground and from other aircraft can be maintained.

2. Q. It bothers me to have to learn METAR in such a piecemeal way (For example, see Figure 3-13, p 3-15). I already know a bit of METAR.

   A. If you already know some METAR, you may want to hone your skills by going directly to Appendix D where METAR is laid out in its entirety. You can practice by decoding METAR examples and questions in Chapters 3, 4, and 6. Also, the Aviation Weather Laboratory Manual also has several exercises suitable for self-testing.

3. Q. Concerning Figure 3-14 and Density Altitude, I would like to see more practical detail, something that will give me actual numbers for real aircraft configurations.

   A. As noted in the text, your aircraft flight handbook will give more specifics for the particular equipment you are flying. Also there are now a number of electronic aids (e.g., Flight Computers) that give you precise numbers. Between the simplicity of Figure 3-14 and the detailed, black box output of a flight computer, the Koch Chart does a very good job in graphically emphasizing to impact of density altitude on takeoff roll and climb rate. An example of the Koch Chart is given below.



4. Q. Density Altitude: Isn't relative humidity also important in the consideration of the density altitude hazard?

   A. High temperature effect is the major contributor to critical Density Altitude conditions. However, if extremely high humidities are also present with high temperatures, the added water vapor in the air can significantly increase Density Altitude.

5. Q. Why does METAR and TAF stuff have to be so difficult to understand? In these times of high speed communications, can’t plain language be used?

   A. There are plain language “translations” available, although you may not necessarily get all the detail of a coded METAR report. Most NWS offices have both coded and decoded METAR information on their websites. The best plain language decoder (for aviation purposes) that I have seen is at the website of the Aviation Digital Data Service (ADDS: adds.aviationweather.noaa.gov).

   Actually, with practice, METAR and TAF codes are fairly easy to understand. Of course, the key word is “practice.” One of the nice things about the ADDS METAR decoded report is that you also get the coded version at the bottom of the page. This allows you to “test” yourself on the interpretation of coded data every time you request a decoded METAR. More practice for METAR and TAF can be found in the chapter question sections throughout Aviation Weather (3rd Ed.) and in Appendix D. Note International METARs and TAFs (appendix D) are slightly different than U.S. versions. The Aviation Weather Laboratory Manual (2007) has METAR decoding practice in Exercises 3, 4, 6, 7, 10 (Frontal Passage), 15, and 17. TAF practice decoding is found in Exercises 16 and 17. Material in Aviation Weather Services (AC 00-45F) may also be helpful.

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1. Q. I don't understand Figure 4-1. What does it mean?

A. Figure 4-1 shows that the total air motion for a parcel of air at a particular point in space can be considered as the vector sum of a horizontal component (we call this the “wind”) and the vertical component (“vertical motion”). Wind is a “vector quantity” as opposed to a “scalar quantity.” As a vector, wind has both a magnitude (speed) and a direction. In contrast, Temperature is a “scalar;” it is completely describable in terms of temperature (its magnitude) alone.

While the direction of the wind vector can vary through the 360 degrees of the compass, the direction of the vertical motion vector can only be upward or downward. The difference between wind and vertical motion for an air parcel is similar to the difference between groundspeed (and heading) and rate-of-climb for an airplane.

2. Q. Figure 4-9 shows that pressure gradient force and Coriolis force are balanced. How can these forces be exactly equal and opposite with a wind blowing?

   A. When there is a balance of forces, it is the acceleration that is zero, not necessarily the velocity. If the air is at rest OR if it is moving with a constant speed and direction, it is not undergoing any acceleration; that is, the acceleration is zero.

3. Q. All of this “Force Stuff” is great to fill a class discussion, but what are the practical (aviation) applications of, for example, the “force due to friction?”

   A. It would take a lot of pages to list all of the “practical applications” of the concept of “friction” with regard to the atmosphere. Here is an example. Every time you descend to land on a windy day, turbulence increases as you approach the ground. This is a direct effect of friction. The depth of that turbulence layer increases with the wind speed and the roughness of the surface. Friction generally causes the wind direction to back (decrease) while descending through the last couple of thousand feet above the ground … the list goes on. All of this is useful information for the pilot who is preparing for a landing … or searching for a downed aircraft … or laying a retardant on a forest fire. Review Section F on page 4-16 and pages 12-5 through 12-8. If you have access to Turbulence, A New Perspective for Pilots, Read Sections A and B in Chapter 2.

4. Q. Why does METAR and TAF stuff have to be so difficult to understand? In these times of high speed communications, can’t plain language be used?

   A. (4/24/03) There are plain language “translations” available, although you may not necessarily get all the detail of a coded METAR report. Most NWS offices have both coded and decoded METAR information on their websites. The best plain language decoder (for aviation purposes) that I have seen is at the website of the Aviation Digital Data Service (ADDS: http://adds.aviationweather.noaa.gov/).

   Actually, with practice, METAR and TAF codes are fairly easy to understand. Of course, the key word is “practice.” One of the nice things about the ADDS METAR decoded report is that you also get the coded version at the bottom of the page. This allows you to “test” yourself on the interpretation of coded data every time you request a decoded METAR. More practice for METAR and TAF can be found in the chapter question sections throughout Aviation Weather (3rd Ed.) and in Appendix D. Note International METARs and TAFs (appendix D) are slightly different than U.S. versions. The Aviation Weather Laboratory Manual (2007) has METAR decoding practice in Exercises 3, 4, 6, 7, 10 (Frontal Passage), 15, and 17. TAF practice decoding is found in Exercises 16 and 17.

5. Q. Is there a plotting model strictly for METAR reports that is similar to the inset in Figure 4-10 and to Appendices D-15 and D-16? This would help in the interpretation of plotted METAR reports throughout the text.”
   
   A. Yes. In the Aviation Digital Data Service (ADDS) at http://adds.aviationweather.noaa.gov/metars/description2.php provides a METAR plotting model. It is repeated below. Note there may be plotting variations for other countries.

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1. Q. I have heard meteorologists refer to a "steep" lapse rate as being very unstable. However, in Figure 5-15 of the Aviation Weather text, aren't the lines representing the "absolutely unstable" temperature soundings flatter than the "stable" ones?

A. The "steep" lapse rate describes a decrease in temperature with altitude that is greater than, for example, the dry adiabatic rate. The "steep" curves appear flatter in the temperature plot because of the way the diagram has been constructed for convenience of interpretation. In order to put the "up" direction in the vertical on the diagram, it is not constructed in the usual mathematical sense. The sounding diagrams are plotted with altitude (the independent variable) along the vertical axis and temperature (the dependent variable) along the horizontal axis. This causes the mathematically "steep" temperature curves (those showing the greatest decrease in temperature with altitude) to appear flat and visa-versa. Just remember that the greater the decrease in temperature with height, the “steeper” the lapse rate and the greater the chance that that layer of the atmosphere is unstable.

2. Q. How can a parcel of air be accelerated downward and moving upward at the same time under stable conditions?

   A. This question is one of several that revolve around the concept of atmospheric stability. In general, stability is not a difficult concept; but its application to meteorology, especially the first time around, may seem to be difficult. The understanding actually becomes easier if you can get a handle on the concept of buoyancy. A good example is found in the answer to the above question:

   If a parcel of air is deflected upward, such as when horizontal air flow (“wind”) intersects a mountain or a sloping frontal boundary, the parcel’s pressure will decrease as its altitude increases. This occurs because the parcel expands to adjust for the lower pressure aloft. If, at any level, the upward moving parcel becomes colder than its surroundings, the parcel is in a stable environment. Another way of saying this (for a dry air parcel) is that the adiabatic lapse rate (how fast the parcel temperature decreases with altitude) is greater than the measured lapse rate (how fast the temperature of the surroundings changes with increasing height) See Figure 5-17 in the text.

   Under these conditions, the air colder parcel will be ACCELERATED downward (it is negatively buoyant). What actually happens to the parcel depends on the magnitudes of the upward push and the downward acceleration, the parcel may continue to move upward, but at a slower speed (the case raised in the question above); OR it may become zero; OR the parcel may reverse its vertical direction and move downward.

   Important point: although the question here had to do with stability, the answer has more to do with clarifying a more a common confusion. Speed and acceleration are different. Think of an aircraft flying at a given speed in a particular direction. If the aircraft slows down, it is still going forward but its ACCELERATION is negative (toward its tail). More generally, Velocity and Acceleration are vector quantities and both magnitude and direction must be taken into account. Acceleration can be a change in speed (+ or -) AND/OR direction over time.

   An interesting brain exercise is to describe the temperature conditions in which a DOWNWARD displaced air parcel is accelerated DOWNWARD. This is an unstable situation which will also result in an UPWARD displaced parcel being accelerated UPWARD.

3. Q. How big is a "parcel" of air?
   
   A. A parcel is simply a "blob" of air; that is, its dimensions are somewhat arbitrary. If this doesn’t work for you, try the following: “a parcel of air is bigger than a bread box but smaller than a freight car.” The main thing about the parcel is that it small enough so its properties are everywhere the same. This means that a single value of temperature, moisture, and pressure (and therefore, density) will be representative of the parcel as a whole.

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1. Q. I learned early on that "sublimation" meant the change of state of EITHER water vapor directly to ice OR ice directly to water vapor. Where did the word "deposition" come from?

A. What you learned is correct. However, the word, "sublimation," is ambiguous when used to describe both processes. For clarity, cloud physicists introduced the word "deposition" for the water vapor-to-ice state change while retaining "sublimation" for the change from ice to water vapor. The text follows the latter "depostion/sublimation" convention. For a pilot concerned with understanding precipitation and icing processes, the use of the two terms is helpful.

2. Q. Sometimes I see METAR reports with five digit groups at the end. Sometimes there is a slash instead of a digit. I don’t understand what these added groups mean and why I don’t see them at some stations.

   A. Numerical information other than that discussed in the text is often added to METAR reports at non-FAA observation sites. For example, it is not unusual to see a five-digit cloud code group of the form “8/LMH” added to METAR reports at 3- and 6-hour intervals. The “8/” identifies the group and the “L,” “M,” and “H” correspond with numbers from 0 to 9, where each number identifies the predominant low (“L”), middle (“M”), and high (“H”) cloud type. These types are expansions of the list of cloud types given in Figure 6-17 in Aviation Weather. There are several other additive groups that provide more detailed information than that normally provided by a METAR. The NWS/NOAA Federal Meteorological Handbook, FMH-1 (Chapter 12, Coding), will give you more information. Try the following URL.
   http://www.ofcm.gov/fmh-1/fmh1.htm
   If it doesn’t work, search the net on the key words “FMH 1”.

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